Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

False killer whales (Pseudorca crassidens Cetacea: Delphinidae) along

the Pacific coast of Central America and Mexico: Long-term movements,

association patterns and assessment of fishery interactions

Annie B. Douglas1; https://orcid.org/0000-0003-2755-2510

Frank Garita Alpízar1; https://orcid.org/0000-0003-3713-7804

Alejandro Acevedo-Gutiérrez2, 3; https://orcid.org/0000-0002-9128-826X

Sabre D. Mahaffy1; https://orcid.org/0000-0001-8255-192X

Kristin Rasmussen4; https://orcid.org/0000-0003-4758-5010

Ester Quintana-Rizzo5, 6; https://orcid.org/0000-0002-8957-0506

Joëlle De Weerdt7, 8; https://orcid.org/0000-0003-4054-6609

Daniel M. Palacios9; https://orcid.org/0000-0001-7069-7913

Damián Martínez-Fernández10; https://orcid.org/0000-0002-8498-7919

Camila Lazcano-Pacheco11; https://orcid.org/0000-0002-3725-8078

Christian Daniel Ortega Ortiz12; https://orcid.org/0000-0002-5691-9388

Nicola Ransome13, 14; https://orcid.org/0000-0002-3130-3966

Astrid Frisch-Jordán15; https://orcid.org/0000-0003-4937-8023

Francisco Villegas-Zurita16; https://orcid.org/0000-0003-1614-997X

John Calambokidis1; https://orcid.org/0000-0002-5028-7172

Robin W. Baird1; https://orcid.org/0000-0002-9419-6336

1. Cascadia Research Collective, 218 ½ W 4th Ave, Olympia, WA 98501 USA; abdouglas@cascadiaresearch.org, frank-

garita@gmail.com, mahaffys@cascadiaresearch.org, rwbaird@cascadiaresearch.org,

calambokidis@cascadiaresearch.org

2. Marine Mammal Research Program, Texas A&M University, Galveston, TX 77551, USA;

3. Department of Biology, Western Washington University, Bellingham, WA 98225, USA; aceveda@www.edu

4. Panacetacea, 1554 Delaware Ave, Saint Paul, MN 55118, USA; Krill@aol.com

5. Simmons University, Department of Biology, 300 Fenway, Boston, MA 02115, USA; tetequintana@comcast.net

6. Emmanuel College, Department of Biology, 400 Fenway, Boston, MA 02115, USA

7. Association ELI-S, Allée, de Verdalle 39, 33470 Gujan-Mestras, France; eliscientific@gmail.com

8. Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium; joelle.de.weerdt@vub.be

9. Marine Mammal Institute and Department of Fisheries, Wildlife, and Conservation Sciences, Oregon State University,

2030 SE Marine Science Dr., Newport, OR 97365 USA; Daniel.Palacios@oregonstate.edu

10. Federación Costarricense de Pesca, San José, Costa Rica; damian.martinezcr@gmail.com

11. Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara. Zapopan, Jalisco, México;

camilalazcano.lp@gmail.com

12. Facultad de Ciencias Marinas, Universidad de Colima, Kilometro 20 carretera Manzanillo-Barra de Navidad, C.P.

28860. Manzanillo, Colima, México; christian_ortega@ucol.mx

13. Harry Butler Institute, Environmental and Conservation Sciences, College of Science, Health, Engineering and

Education, Murdoch University, Western Australia, Australia; nicola.ransome@murdoch.edu.au

14. La Orca de Sayulita, Sayulita, Nayarit, Mexico

15. Ecología y Conservación de Ballenas, A.C., México; fibbcatalogo@yahoo.com

16. Instituto de Ecología, Universidad del Mar, Oaxaca, México; fvillegas@angel.umar.mx

Received 31-VII-2022. Corrected 21-X-2022. Accepted 11-IV-2023.

https://doi.org/10.15517/rev.biol.trop..v71iS4.57189

SUPPLEMENT • SMALL CETACEANS

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

ABSTRACT

Introduction: Worldwide, false killer whales (Pseudorca crassidens) are infrequently encountered, yet long-term

studies have shown strong site fidelity as well as long-term associations among individuals in several locations.

Detailed studies of this species have primarily been conducted around tropical oceanic islands or in the subtropi-

cal southern hemisphere.

Objectives: We assess movements and association patterns among false killer whales along the Pacific coasts of

the USA, Mexico, Guatemala, Nicaragua, Costa Rica including Isla del Coco, and Panama, representing one of the

longest-running (albeit non-continuous) studies of this species. We also examine photos for evidence of interac-

tions with fisheries, a known source of mortality to false killer whales.

Methods: From Central America, we selected 212 individuals (50 encounter groups) out of 244 individuals (56

encounters) for inclusion in analyses based on photo quality and distinctiveness. Photos were collected on dedi-

cated surveys from 1991-1994 and dedicated and opportunistic surveys from 1998-2022. Other than the effort

off the oceanic Isla del Coco (1993-1994), surveys were undertaken in continental shelf waters. Additionally, we

selected by photo quality and distinctiveness 124 (33 encounter groups) out of 189 individuals from southern

California and Mexico for inclusion in these analyses. Association patterns were analyzed in SOCPROG and

movements were analyzed in R.

Results: Of the 328 total individuals, 158 (48.2 %) were encountered more than once, and 114 (34.8 %) were

re-sighted after a year or more. The longest individual sighting history spanned 26.2 years with six re-sightings

over that period between southern Costa Rica and Panama. Association and movement analyses revealed that

individuals identified off southern Costa Rica and Panama linked into a single social network, with extensive

movements between the two countries. Three individuals encountered off northern Costa Rica were re-sighted

off northern Nicaragua, and individuals encountered off Nicaragua were encountered off Guatemala and central

mainland Mexico. Nine matches were found among false killer whales between central mainland Mexico and

Central America. There were no matches between the mainland coastal waters and the 33 individuals encoun-

tered around Isla del Coco. Dorsal fin disfigurements consistent with interactions with line fisheries ranged from

0 to 21 % for individuals within social clusters identified by community division.

Conclusions: The infrequency of sightings combined with a high re-sighting rate of individuals and groups from

the same area, suggests multiple small populations with large home ranges that include coastal waters. Small

populations are sensitive to environmental changes, and as the human population grows, so do the demands on

fisheries and ecotourism, which could directly impact the different populations. Additional effort in offshore

areas is needed to determine the population status of false killer whales in pelagic waters, how often false killer

whales using coastal waters move into pelagic waters, and the relationship between whales in the two habitats.

Key words: small cetacean; photo-identification; fisheries; dorsal fin disfigurement; social network; social

organization.

RESUMEN

Falsas orcas (Pseudorca crassidens Cetacea: Delphinidae) de la costa del Pacífico de Centroamérica

y México: movimientos, patrones de asociación y evaluación de interacciones pesqueras

Introducción: Alrededor del mundo, las falsas orcas (Pseudorca crassidens) son encontradas con poca frecuencia,

aunque estudios a largo plazo han demostrado una fuerte fidelidad al sitio, así como asociaciones a largo plazo

entre individuos. Estudios detallados de esta especie se han realizado principalmente alrededor de islas oceánicas

tropicales o en la región subtropical del hemisferio sur.

Objetivo: Se evaluaron los movimientos y patrones de asociaciones a largo plazo entre falsas orcas, a lo largo de las

costas del Pacífico de E. U., México, Guatemala, Nicaragua, Costa Rica (incluyendo a la Isla del Coco) y Panamá,

lo que representa uno de los más extensos (aunque no continuos) estudios de esta especie. Además, se analizaron

las fotos de aletas dorsales en busca de evidencia de interacciones con la pesca, una fuente de mortalidad conocida

para las falsas orcas.

Métodos: Seleccionamos a 212 (50 encuentro grupos) de 244 individuos (56 encuentros) de Centroamérica para

incluirlos en los análisis basados en la calidad y el carácter distintivo de las fotografías. Utilizamos fotos recopi-

ladas en muestreos dedicados de 1991 a 1994 y encuentros dedicados y oportunistas de 1998 a 2022. Aparte del

esfuerzo alrededor de la isla oceánica, Isla del Coco (1993-1994), se realizaron estudios en aguas de la platafor-

ma continental. Además, seleccionamos basados en la calidad y el carácter distintivo de las fotografías 124 (33

encuentro grupos) de 189 individuos del sur de California y México para incluirlos en los análisis. Patrones de

asociaciones fueron analizados en SOCPROG y los movimientos fueron analizados con el programa R.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

INTRODUCTION

False killer whales, Pseudorca crassidens

(Owen, 1846) primarily inhabit pelagic tropical

and warm temperate waters worldwide, with

the highest density in the tropics (Ferguson &

Barlow, 2003). There are a few well-document-

ed populations that are island-associated or

encountered in nearshore waters (Baird et al.,

2008; Palmer et al., 2017; Zaeschmar, 2014).

Although this species is highly surface-active

and tends to travel in large groups (20–100

individuals), false killer whale encounters are

infrequent even in areas where they are resident

year-round (e.g., Hawai‘i; Baird, 2016) or pres-

ent seasonally (e.g., New Zealand; Zaeschmar,

2014). In the eastern tropical Pacific (ETP),

extensive large and small vessel surveys con-

ducted in the Exclusive Economic Zone of

Costa Rica from 1979 to 2001 documented

only nine encounters of false killer whales

(May-Collado et al., 2005) and sightings were

relatively few and sparsely distributed through-

out the ETP (Martínez-Fernández et al., 2011;

Quintana-Rizzo & Gerrodette, 2009; Quintana-

Rizzo, 2012; Wade & Gerrodette, 1993). Based

on extensive survey work of the eastern Pacific

Ocean from 1985-2005, no false killer whales

were encountered north of Mexico (Hamilton

et al., 2009), although there have been occa-

sional documented encounters off California

and even farther north into British Columbia

(Baird et al., 1989; Norris & Prescott, 1961).

The first photographic identification (pho-

to-ID) study of this species was conducted

from 1991-1995 by Acevedo-Gutiérrez et al.

(1997) in the coastal waters of Golfo Dulce,

southern Costa Rica, and off Isla del Coco,

an island approximately 500 km southwest of

Costa Rica. Acevedo-Gutiérrez et al. (1997)

found that individuals were re-sighted over

two years in Golfo Dulce and three years off

Isla del Coco, and stable associations between

some individuals were evident. Based in the

same area of southern Costa Rica (not includ-

ing Isla del Coco) but using a separate photo-

ID catalog and dataset, Sánchez Roblado et

al. (2020) estimated that 92 false killer whales

used this area. Although these earlier studies

were spatially limited, they indicated that there

is a small population encountered occasionally

off southern Costa Rica, with no documented

Resultados: Del total de 328 individuos encontrados en Centroamérica, 158 (48.2 %) fueron observados más de

una vez y 114 (34.8 %) se volvieron a avistar después de un año o más. El historial de avistamientos individuales

más largo abarcó 26.2 años con seis re-avistamientos durante ese período entre el sur de Costa Rica y Panamá.

Los análisis de asociación revelaron que todos los individuos identificados en el sur de Costa Rica y Panamá,

se vincularon a una sola red social, con amplios movimientos entre los dos países. Tres individuos encontrados

frente al norte de Costa Rica fueron avistados frente al norte de Nicaragua, y los individuos encontrados frente

a Nicaragua fueron encontrados frente a la región central continental de México. Hubo traslape de nueve indi-

viduos entre México y Centroamérica. No hubo traslape entre los individuos avistados en el continente y los 33

individuos identificados alrededor de la Isla del Coco. Las desfiguraciones de la aleta dorsal, consistentes con

interacciones con artes de pesca que usan líneas variaron de 0 a 21 % para los individuos dentro de los grupos

identificados por división de la comunidad.

Conclusiones: La poca frecuencia de avistamientos combinada con muchos re-avistamientos de individuos y

grupos en la misma área, sugiere que las falsa orcas representan muchas poblaciones pequeñas con áreas de dis-

tribución grandes que incluyen aguas costeras. Las poblaciones pequeñas son sensibles a los cambios ambientales

y, a medida que crece la población humana, también lo hacen las demandas sobre la pesca y el ecoturismo, lo que

podría afectar directamente a estas poblaciones. Se necesitan más estudios en las áreas alejadas de la costa para

determinar el estado de conservación de las falsas orcas en regiones pelágicas, la frecuencia con la que las falsas

orcas que usan aguas costeras se trasladan a aguas pelágicas y la relación entre ellas en los dos hábitats.

Palabras clave: pequeños cetáceos; foto-identificación; pesca; desfiguración de la aleta dorsal; red social; orga-

nización social.

Nomenclature: SMT1: Supplementary material Table 1; SMF1: Supplementary material Figure 1.

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

interchange with Isla del Coco (Fig. 1). An

entirely separate photo-ID study identified 14

individuals but documented no re-sightings in

Guatemala (Quintana-Rizzo, 2012).

The question of how far these individuals

are ranging along Mexico, Central America,

and into offshore waters is unclear. Genetic

and photo-ID studies in the eastern Hawaiian

Islands show a distinct insular/island-associ-

ated population of false killer whales with

high site fidelity (Baird et al., 2008, Baird

et al., 2012), and a genetically differentiated

broadly ranging offshore population (Ander-

son et al., 2020; Chivers et al., 2007; Fader et

al., 2021; Martien et al., 2014). Home ranges

for groups and individuals from the main

Hawaiian Islands’ insular population tend to be

extensive, but predictable (Baird et al., 2012),

while individuals from the pelagic population

appear to be much wider ranging (Anderson

et al., 2020; Fader et al., 2021). The maximum

travel distance for a satellite tagged insular

main Hawaiian Islands false killer whale was

421 km (Baird et al., 2010), while it was 2 263

km for an individual from the pelagic popula-

tion (E. Oleson personal communication, 27

July, 2022). Although far less is known about

populations elsewhere, Palmer et al. (2017)

reported that the maximum travel distance

from a satellite-tagged false killer whale in the

Arafura and Timor Seas off Australia was about

880 km from 104 days of tag transmission. Off

New Zealand, Zaeschmar (2014) reported a

maximum travel distance of 647 km based on

photo-ID. From photo recapture studies in the

Hawaiian Islands, the greatest span of years for

an individual was 33 years (S. Mahaffy 6 May,

2022, personal communication).

The ongoing studies on false killer whales

in the Hawaiian Islands have shown that there

are three discrete yet partially overlapping pop-

ulations (Chivers et al., 2007, Chivers et al.

2010; Baird et al., 2008, Baird et al., 2010, Baird

et al., 2013; Martien et al., 2014). Individuals

within a population maintain strong bonds over

decades, hunt cooperatively, and share prey

with hunting partners (Baird, 2016; Martien et

al., 2019). Martien et al. (2019) found that both

male and female main Hawaiian Islands insular

false killer whales remain in their natal social

groups throughout their lives and that between

34 to 64 % of matings occurred in the same

Fig. 1. Map of sighting locations (red circles) of false killer whales (Pseudorca crassidens) with acceptable quality photographs.

Line width between sightings reflects the number of re-sightings of individuals between areas. Map created using R Statistical

Software (R Core Team, 2021).

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

social group. This differs from other highly

social species that may practice natal dispersal

or exogamy to avoid inbreeding. As with other

top predators, false killer whale population

numbers are fairly low, they are slow to mature,

have low birth rates, and females remain active

in their family groups past their reproduc-

tive years (Baird, 2018a). False killer whales

are active during the day and night and have

been observed feeding on various species of

fish including neritic, demersal, bathydemersal,

reef, and pelagic game fish (Baird, 2016; Herz-

ing & Elliser, 2016) and an analysis of stomach

contents of stranded animals has shown that

oceanic and neritic-oceanic squids make up

a large part of their diet as well (Alonso et

al., 1999). False killer whales’ cooperative and

adaptable hunting styles as well as their propen-

sity to share prey within their group and occa-

sionally with divers and boaters (Baird, 2018b)

tend to put this species in conflict with small

and large fishing boat operators where human

and false killer whale fishing areas overlap.

Globally, false killer whales and other dol-

phin species are known to take the bait and

catch off hook and line fisheries, and in the

Hawai‘i-based deep-set longline fishery, false

killer whales are the cetacean species most fre-

quently recorded as hooked as bycatch (Brad-

ford et al., 2014; Forney & Kobayashi, 2005).

One study of depredation in the Hawai‘i-based

deep-set longline fishery with 21 % observer

coverage reported that ~6 % of hauls from

2004-2018 experienced odontocete depreda-

tion, most of it thought to be from false killer

whales (Fader et al., 2021). The Inter-American

Tropical Tuna Commission [IATTC] manages

large longline vessels (>24 m) in the area from

500 N to 500 S from the coast of the Americas

to the 1500 W meridian of the eastern Pacific

Ocean, with 1 123 longline vessels currently

authorized to fish in this area (IATTC, 2022).

Importantly, from the perspective of docu-

menting fishery interactions, vessels less than

24 m are not managed by the IATTC and

are not required to carry observers (IATTC,

2011). The Organization of the Fisheries and

Aquaculture Sector of the Central America

Isthmus [OSPESCA] reports more than 5 000

vessels in the coastal and Pacific longline fleet

(OSPESCA, 2012). The IATTC has repeatedly

recommended at least 20 % observer coverage

on longline vessels in this area, yet coverage has

remained at only 5% (IATTC, 2019), and since

April 2020, due to the COVID pandemic, the

requirement of any observer coverage may be

waived upon request (National Oceanic and

Atmospheric Administration [NOAA], 2022).

Thus, indirect methods of assessing whether

individual false killer whales have survived

fishery interactions may be the only way to

determine the magnitude of such interactions

on groups or populations.

In the case of false killer whales, when an

individual ingests a hook or is hooked in the

mouth and is able to break free from the line,

scars around the mouthline and dorsal fin are

often the only external evidence that an animal

has been hooked and survived (Baird & Gor-

gone, 2005; Baird et al., 2014, Baird et al., 2017).

In the absence of observer coverage, Baird et al.

(2014) conducted a photo review of dorsal fin

disfigurements and scarring of individual false

killer whales encountered in Hawaiian waters.

They found that 7.5 % of the individuals from

the main Hawaiian Islands population bore

scars consistent with fisheries interactions and

had higher rates of fishery-related injuries than

the offshore or northwestern Hawaiian Islands

populations (Baird et al., 2014).

The purpose of the current study was to

examine the movements of false killer whales

along the Pacific coast of North and Central

America, ranging from southern California to

Panama. We combined the results of indepen-

dent photo-ID efforts from six different coun-

tries, providing an assessment of association

patterns and site fidelity, as well as examining

individuals for evidence of prior fishery inter-

actions. We hope that our findings will add to

the current body of knowledge on false killer

whales, as well as inspire future collaborative

research with this species in these regions.

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Research Collective (CRC) as well as inde-

pendent researchers. Other efforts included

systematic cetacean surveys conducted in Gua-

temala year-round 2008-2009 and between

December-April from 2018 onward, and Costa

Rica 2005-2006. In the majority of cases, effort

was conducted from “pangas” or tour boats

with an outboard motor, and were restricted to

returning to the launch location at the end of

the survey day. Photos and encounter details

from 2010 to 2022 were collected and shared

with CRC from directed surveys and whale

and dolphin watch operations working along

Nicaragua, Costa Rica, and Panama expressly

for this project. The majority of CRC surveys

were nonsystematic and attempted to cover a

large coastal area with the primary objective of

discovering humpback whales. Non-humpback

whale cetacean encounters were approached

for sighting position, species identification,

group size estimation, and photo documenta-

tion depending on species and time of day.

Sighting positions were based on an onboard

GPS (directed surveys), an estimated position

based on the photographers’ description, or

a “general” position based on where the ves-

sel launched and returned. During false killer

whale encounters from directed surveys, efforts

were made to photograph all individuals from a

group, regardless of age class, or distinctiveness.

As with the Central America collection, the

southern California-Mexico photo-ID catalog

and sighting data were collected by both direct-

ed research efforts and opportunistic sightings

from ecotourism businesses. Mexico data were

collected from 2004, 2007 to 2008, and 2011

to 2020 (Lazcano-Pacheco et al., 2023). Survey

effort and collection methods are described by

Ortega-Ortiz et al. (2014).

Data analysis: Groups were defined as

all individuals encountered on the same day

within a region, and IDs obtained during that

day were pooled as part of a single encounter.

This is a broader definition for a group than is

typically used for association analysis of odon-

tocetes, however, as with Baird et al. (2008), we

view that the choice is justified for this species

MATERIALS AND METHODS

Study area: The ETP is characterized by a

strong shallow thermocline, relatively high sea

surface temperatures and strong winds (Heile-

man, 2008). The southern part of Mexico and

northern part of Central America form one side

of the eastern Pacific warm pool, which consti-

tutes an open-ocean biogeographic province

with a distinct biological community (Fiedler

& Talley 2006). The study area is part of a

marine mega-ecosystem characterized by gulfs,

bays, coastal lagoons, and extensive intertidal

areas and barriers (Gocke et al., 2001; Lizano

& Alfaro, 2004). It includes part of the Costa

Rica Dome (CRD), an open-ocean upwelling

region caused by a seasonally changing com-

bination of interconnected features including

the Intertropical Convergence Zone, coastal

jets and eddies, and geostrophic balance at the

eastern extreme of the 10° N thermocline ridge

(Mora-Escalante et al., 2020). The CRD sup-

ports a higher density of marine fauna includ-

ing cetaceans than other parts of the Central

American marine ecosystem (Fiedler & Talley,

2006; Lavín et al., 2006), and likely influences

the high productivity of the Pacific Central

American coast (Heileman, 2008). For the pur-

pose of examining movements, the study area

was broken down into a number of regions

reflecting discrete study sites: southern Califor-

nia, southern Baja California, the central and

southern Mexican mainland coasts, Guatemala,

Nicaragua, northern and southern Costa Rica,

Panama and Isla del Coco.

Data collection: Research efforts varied

by year, season and among regions (Table 1;

Fig. 1). False killer whale photos were col-

lected from 1991-1992 on directed surveys for

cetaceans along southern Costa Rica and 1993-

1994 off Isla del Coco (Acevedo-Gutiérrez

et al., 1997). From 1998 to 2009 false killer

whale encounters were documented during

directed humpback whale surveys conduct-

ed during the dry season (December-March)

along the Pacific coast of southern Costa Rica

and northern Nicaragua (Table 1) by Cascadia

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Table 1

Number of groups encountered (number of identifications) of false killer whales (Pseudorca crassidens) by year and region, and total number of individuals by region, restricted to

identifications with fair to excellent quality photographs and slightly to very distinct dorsal fins. Group was defined as all individuals encountered on the same day in the same general

area. Number of groups in which group size estimates with acceptable photos are available are also included.

Ye a r S.

California

Baja

California

Cent. Mainland

Mexico

S. Mainland

Mexico

Guatemala Nicaragua N. Costa Rica S. Costa Rica Panama Isla del Coco Tota l

1991 1 (10) 1 (10)

1992 1 (7) 1 (7)

1993 2 (11) 2 (11)

1994 5 (40) 5 (40)

1998 2 (5) 2 (5)

2000 1 (2) 1 (2)

2004 1 (7) 1 (7)

2005 1 (4) 4 (24) 5 (28)

2006 3 (36) 3 (36)

2007 1 (1) 1 (1)

2008 1 (5) 2 ( 9) 1 (7) 1 (1) 5 (22)

2009 1 (6) 6 (102) 7 (108)

2010 3 (12) 3 (12)

2011 1 (8) 1 (8)

2012 3 (17) 1 (1) 3 (4) 7 (22)

2013 2 (11) 1 (13) 2 (20) 5 (44)

2014 4 (36) 4 (36)

2015 1 (1) 2 (18) 2 (9) 5 (28)

2016 1 (10) 2 (6) 2 (13) 5 (29)

2017 1 (1) 1 (9) 2 (10)

2018 4 (7) 1 (1) 2 (20 ) 7 (28)

2019 1 (9) 1 (4) 1 (9) 3 (22)

2020 2 (13) 1 (15) 3 (28)

2021 1 (3) 2 (23) 3 (26)

2022 1 (7) 1 (7)

Sum of groups 9 2 19 3 4 7 1 26 5 7 83 (577)

Sum of groups that included

group size estimates

5 2 17 3 4 7 1 24 5 7 75

Mean group size (SD) 34.5 (16.2) 42.7 (52.7) 54.5 (60.1) 17.7 (6.8) 12.5 (7.7) 13.6 (8.1) 17 (13.0) 41 (15.6) 18.1 (10.9) 28.2 (34.7)

Median (range) 32.5 (15-60) 42.7 (5-80) 35 (5-200) 20 (10-23) 14 (2-20) 10 (6-25) 13 (1-50) 25 (25-60) 14 (4-34) 18 (1-200)

Sum identifications 66 11 99 21 16 56 15 204 38 51 577

Sum individuals 37 10 77 16 16 45 15 95 31 33 375

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

based on the infrequency of false killer whale

encounters and the small groups encountered,

often traveling in the same direction and spread

out over many kilometers (see e.g., Bradford et

al., 2014). This choice is also justified based on

the many positive re-sightings of individuals

between subgroups encountered on the same

day. To report the mean and median group size

when only a range (min/max) of group size

was available, we chose the median of the two

numbers. When multiple encounters occurred

on the same day of the same group of animals,

we chose the largest group size number, with

the assumption that smaller estimates were

derived from counts of subgroups encountered.

Following the photo-ID protocol described in

Baird et al. (2008), a Central America catalog

was constructed from photographs of individu-

als taken off Guatemala, Nicaragua, Costa Rica

and Panama. Photos from each encounter were

sorted by individual into folders, assigned tem-

porary IDs and given separate scores for photo

quality (1 – poor, 2 – fair, 3 – good, 4 – excel-

lent) and distinctiveness (1 – indistinct, 2 –

slightly distinct, 3 – distinct, 4 – very distinct).

Photo quality was based on focus, angle, and

proportion of the dorsal fin visible, and distinc-

tiveness was based on the absence or presence

of notches on the leading and trailing edge of

the fin, and/or the dorsal fin shape. Poor quality

or indistinct individuals were only compared

to individuals encountered on the same day,

within region, while all fair to excellent quality

photos of slightly distinct to very distinct IDs

were compared to all IDs in the catalog. When

possible, photos of unusual scars on the body

and along the mouthline were noted for each

individual, and scored with the likelihood that

the injury could have been associated with a

fishery interaction (Baird et al., 2014, Baird et

al., 2017). Once the temporary ID had been

compared to the catalog, the best left and/or

right dorsal fin of each individual was assigned

a unique ID number, or if it was found in the

historical catalog the ID was collapsed into the

existing record of that individual. Every posi-

tive match found between sightings was con-

firmed by at least two experienced matchers.

The southern California-Mexico catalog was

created with similar practices as described

above with quality and distinctiveness scores

for each individual. Once completed, the Cen-

tral America catalog was compared with the

southern California-Mexico false killer whale

catalog, with two experienced matchers con-

firming all positive matches between catalogs.

In an effort to avoid false-negative matches

between these two catalogs, a second experi-

enced matcher (SDM) compared 18 % of the

IDs in the southern California-Mexico catalog

that had not been found by the initial matcher

(ABD) (Elliser et al., 2022), and no additional

matches were found. For all catalogs, only those

IDs with photo quality and distinctiveness cat-

egories of two (fair) or better were compared

between catalogs – these are defined as “accept-

able quality” identifications. Our decision to

include “fair” quality photos and slightly-dis-

tinctive IDs was made to retain a reasonable

sample size for analysis and interpretation, and

while it is more permissive than many studies

with larger catalogs or for less remote locations,

it is not without precedent (Baird et al., 2021;

Elliser et al., 2022).

Linear-geographical distances between

all possible pairs of encounter locations both

within regions and among all regions were

calculated for all encounters where acceptable

identification photos and latitude and longi-

tude were available using R Statistical Software

(R Core Team, 2021). To control for pseudo-

replication, when more than one individual

was identified from a particular encounter,

that encounter location was only used once in

the calculations. If there was more than one

encounter in an area on the same day (that

were pooled as a single encounter), the first

location was used. Combinations of encounters

were generated using the combinations function

within the gtools package (Warnes et al., 2020).

Straight line geographical distances were calcu-

lated using the st_distance function within the

sf package (Pebesma, 2018). Distances between

all encounter combinations for each individual

sighted on two or more occasions were also

calculated. Because of the sociality of false killer

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

whales, there were several instances where

multiple individuals were re-sighted together

more than once. Hence, when summarizing

distances across individual re-sightings, we

only used a single set of calculated distances

between pairs of individual sightings to avoid

pseudoreplication.

Association analyses of photo-identified

individuals were undertaken in SOCPROG

2.9 with MATLAB 9.5 (Whitehead, 2009), and

social network metrics were calculated and

illustrated in Netdraw 2.176 (Borgatti, 2002). To

provide a quantitative measure of the frequency

of co-occurrence of individuals, while control-

ling for effort (Whitehead, 2008), we used the

half-weight index of association (HWI). White-

head (2008) and Cairns & Schwager (1987)

recommend the use of HWI in situations where

it is likely that not all individuals within a sam-

pling period are identified or when individuals

of a pair are more likely to be observed sepa-

rately than when together.

We used SOCPROG to assess whether

the false killer whales in our study could be

divided into meaningful social clusters based

on levels of association between individuals

using community-based modularity (Newman,

2004; Whitehead, 2009). This method divides

the population into clusters in a way that maxi-

mizes associations within clusters rather than

between them. A modularity value greater than

0.3 is considered to indicate the useful division

of a population (Newman, 2004). We checked

all cluster assignments to make sure that they

made logical sense based on our knowledge of

these data and the eigenvector or final bifurca-

tion involving the individual. The eigenvector

value corresponds to the certainty in the assign-

ment of an individual in the cluster in which

the individual was placed, with values near zero

indicating uncertainty. We tested whether indi-

viduals showed preferential associations with

companions, using the preferred/avoided asso-

ciation test in SOCPROG (Bejder et al., 1998;

Whitehead, 2009). The null hypothesis of this

test is that individuals will associate with the

same probability with all other individuals in

the population without individual preference.

Based on similar studies (Baird et al., 2008), we

tested our data against 20 000 randomly per-

muted variations, so that the resultant P value

was determined by the proportion of 20 000

permutations that had higher Standard Devia-

tion (SD) values of the association indices than

the SD of the association indices found in our

data. For these analyses, we restricted our data

to all individuals seen two or more times. We

refer to groups of three or more individuals

linked by association in the social network as

separate components, and these components

could be comprised of one or more social clus-

ters based on the association analyses.

To evaluate possible fishery interactions,

the primary catalog curator (ABD) reviewed

the best left and/or right dorsal fin photo of

each individual in both the Central America

and southern California-Mexico catalogs for

evidence of fishery interactions. Based on the

presence of linear cuts, dorsal fin disfigure-

ment (e.g., deep cut on the leading edge, miss-

ing dorsal fin, bent dorsal fin) or scarring of

the area immediately in front or behind the

dorsal fin (Baird et al., 2014), ABD chose pho-

tos of individuals for further evaluation. Each

individual was assigned a score of one (not

consistent), two (possibly consistent), or three

(consistent) with a fishery interaction by ABD

and two additional reviewers (SDM, RWB)

with experience in reviewing dorsal fin injuries

in relation to fishery interactions. The reviews

were conducted independently and sent to

ABD who averaged the scores for each indi-

vidual whale. To assess differences in fishery

interactions among areas or social clusters, we

considered individuals with an average score of

>2.6 as having injuries consistent with a fishery

interaction (i.e., at least two reviewers would

have to have scored it 3 (consistent) and the

third reviewer would have to score 2 (possibly

consistent)), following Baird et al. (2014).

RESULTS

Acceptable quality photos were obtained

from 83 encountered groups, resulting in 577

identifications and 328 individuals (Table 1;

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Fig. 1). Group sizes were available for 75 of the

83 encounters, with a mean group size of 28.2

(SD = 34.7, range = 1-200, median = 18) (Table

1; SMT1). The smallest groups encountered

were in Guatemala with a mean of 12.5 (SD =

7.7, range = 2 –20, n = 4 groups), and the largest

groups were encountered in central mainland

Mexico, with a mean of 54.5 (SD = 60.1, range

= 5-200, n = 17 groups) (Table 1). The number

of individuals identified in each encountered

group (n = 83) ranged from one to 40 (mean =

7.6, SD = 10.0, median = 5). Of the 328 individ-

uals identified, 158 (48.2 %) were encountered

more than once, with the span of years between

first and last sightings ranging from one to 9

577 days (26.2 years). One hundred and four-

teen individuals (34.8 %) were seen over peri-

ods of a year or greater, and the maximum

times an individual was documented was 10.

Association analyses revealed nine sepa-

rate components containing three or more

individuals in the social network, as well as five

individuals that were not linked by association

to any others (Fig. 2A). Matches were found

between individuals encountered off southern

California and Baja California (Fig. 1; Table 2),

but no individuals from either of those regions

were encountered farther south, and all but two

of the 39 individuals encountered off southern

California and Baja California were linked by

association in the same component of the social

network (Fig. 2A). Of the 328 individuals in the

social network, the largest component included

183 identifications of 119 individuals (37.4 %)

from 29 encounters, with individuals docu-

mented from mainland Mexico to northern

Costa Rica. Within this component, individuals

encountered off central mainland Mexico were

also sighted in Guatemala, and as far south

as southern Nicaragua. Nicaragua encounters

occurred either close to the northern or south-

ern borders of the country, with four individu-

als encountered in both the north and south.

Nine of 45 individuals encountered off Nica-

ragua were also seen off northern Costa Rica

(Table 2). Four smaller groups of individuals

and two lone individuals photographed along

the coast from mainland Mexico to Nicaragua

Table 2

Re-sightings of false killer whales (Pseudorca crassidens) within and among regions. Individuals that are re-sighted in multiple regions appear in the total count for each region

Region S.

California

Baja

California

Central Mainland

Mexico

S. Mainland

Mexico Guatemala Nicaragua N.

Costa Rica

Costa Rica Panama Isla del

Coco

S. California 12

Baja California 7 1

Cent. Mainland MX 0 0 21

S. Mainland MX 0 0 9 5

Guatemala 0 0 1 2 0

Nicaragua 0 0 5 5 4 9

N. Costa Rica 0 0 0 0 0 9 0

S. Costa Rica 0 0 0 0 0 0 0 59

Panama 0 0 0 0 0 0 0 10 6

Isla del Coco 0 0 0 0 0 0 0 0 0 16

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Fig. 2. Social network of false killer whales (Pseudorca crassidens), with individuals presented as nodes and lines between

nodes indicating individuals encountered in the same area on the same day, restricted to individuals with acceptable quality

and distinctiveness scores. Node color indicates the region where it was first encountered. Number of sightings by region:

Southern California (n = 9); Baja California (n = 2); central mainland Mexico (n = 19); southern mainland Mexico (n = 3);

Guatemala (n = 4); Nicaragua (n = 7); northern Costa Rica (n = 1); southern Costa Rica (n = 26); Panama (n = 5); Isla del

Coco (n = 7). Large nodes represent individuals with injuries consistent with fishery interactions. A. All individuals from

California to Panama and Isla del Coco (1991 to 2022), (n = 328). The greatest number of individuals linked by association

(upper left – n = 119) are from mainland Mexico to northern Costa Rica. The second largest number linked by association

(upper right – n = 116) represents individuals from southern Costa Rica and Panama. Node shape indicates individuals

encountered in multiple regions (circle) or single region (square). B. False killer whales from southern Costa Rica to Panama

(1991 to 2022), restricted to individuals seen 2 or more times. Node shape represents clusters: hourglass – Cluster 3; up

triangle – Cluster 4; diamond – Cluster 9; square – Cluster 13; down triangle – Cluster 17 (Clusters with evidence of fishery

interaction are listed in Table 4). Location first seen is indicated by color as per A. Social network metrics were calculated

and illustrated in Netdraw 2.176.

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

were not linked to the largest component of the

social network (Fig. 2A). The second largest

number of individuals linked by association

(242 identifications of 116 individuals over 31

encounters) was from southern Costa Rica and

Panama (Fig. 2B). All individuals documented

in those two areas were linked in the same com-

ponent of the social network, with ten individu-

als encountered in both regions (Table 2). Out

of the 116 southern Costa Rica and Panama

individuals, 52.6 % (61) individuals have been

re-sighted on more than one day, and 40.5 %

(47) individuals have been re-sighted for more

than one year.

No matches were found between the 33

individuals documented (from seven encoun-

ters) around Isla del Coco and individuals

encountered in coastal waters, although re-

sightings of individuals were reported within

the region (Table 2). The majority (21 of 33,

63.6 %) of individuals from Isla del Coco were

linked by association in one component of the

social network, and 11 (33.3 %) were linked in

another (Fig. 2A).

The average straight-line distance among

re-sightings of individuals (mean = 303.3 km,

SD = 505.4, median = 51.2) was far less than

the average distance among encounters (mean

= 1 796.7 km, SD = 1 551.2, median = 1 594.1).

Greatest straight-line distances between re-

sightings of individuals were between Mexico

and Central America (mean = 794.5 km, SD

= 626, median = 639.5), with 2 265 km being

the maximum straight-line travel distance for

an individual (CRC_CA_Pc234_MX_Pc176)

sighted off central mainland Mexico on 1 Feb-

ruary 2020 and re-sighted off southern Nica-

ragua 6 August 2021 (Table 3, Fig. 1). Average

re-sighting distances of individuals between

southern Costa Rica and Panama were far less

than between the other regions (mean = 79.9

km, SD = 88.5).

Social network cluster analysis and com-

munity structure: Using community division

by modularity and social network analysis, we

found that the 328 distinct individuals iden-

tified could be assigned into 22 clusters by

association (modularity = 0.77, maximum

modularity type 1 controlling for gregarious-

ness). Tests for preferred/avoided associations

among individuals were significant (P = 0.999),

so that we could reject the null hypothesis that

associations were random. Repeated associa-

tions were documented most consistently in the

southern Costa Rica-Panama social network,

with average and maximum mean association

HWI values among individuals of 0.09 (SD =

0.04) and 0.79 (SD = 0.18), respectively (Fig.

2B). Individuals within the Costa Rica-Panama

social network from Cluster 3 (average and

maximum mean association HWI values 0.11

(SD = 0.03) and 0.80 (SD = 0.17)) had the

longest association between individuals (CRC_

CA_Pc027 and CRC_CA_Pc028) spanning

12.1 years, with five sightings of these whales

Table 3

Straight-line distances between all possible pairs of encounters where individual false killer whales (Pseudorca crassidens)

were photo-identified, and distances among re-sightings of individuals. For the comparison of distances of re-sighted

individuals, the grand mean/median values are shown.

Mean distance

(SD) (km)

Median

distance (km)

Maximum

distance (km)

All countries – all possible pairs 1 796.7 (1 551.2) 1 594.1 5 096.3

Re-sighted individuals 303.3 (504.4) 51.2 2 265.1

S. California & Baja California – all possible pairs 628.7 (784.0) 50.9 1 655.8

Re-sighted individuals 543.5 (745.7) 34.4 1 655. 8

Cent. Mainland Mexico-N. Costa Rica – all possible pairs 1 162.5 (874.0) 1 095.0 2 710.1

Re-sighted individuals 794.5 (626.5) 639.5 2 265.1

S. Costa Rica & Panama – all possible pairs 79.8 (83.3) 48.6 246.1

Re-sighted individuals 79.8 (88.5) 42.1 238.6

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

encountered together. Except for the first and

last encounters of CRC_CA_Pc027, in 2005 and

2019, CRC_CA_Pc027 and CRC_CA_Pc028

have been encountered together every time that

one has been sighted. Long term associations

have been observed in the southern California

and Baja California region as well, with two

individuals (IDs MX_085 and MX_90) encoun-

tered together five times between 2013-2021

(9.0 years). Additionally, individuals MX_085

and MX_166 have been encountered together

five times between 2014-2022 (8.0 years).

Insufficient information on survey effort

is available to quantify seasonal sighting rates.

However, it is worth noting that thirty-eight

(76.0 %) of Central America encounters

occurred during the Boreal winter (November-

March), while 90.0 % of southern California-

Mexico encounters occurred during the same

period. All of the southern California and one

of the two Baja California encounters occurred

in March, the second Baja California encounter

occurred in May.

Fishery interactions: Seventeen individu-

als were initially selected for review of injuries

consistent with fisheries interactions, 12 from

the Central America catalog and five from the

southern California-Mexico catalog. Of these,

our three reviewers agreed that nine had inju-

ries consistent with fisheries interaction (aver-

age score > 2.6): one off southern California,

two off central mainland Mexico, one individu-

al seen off Guatemala, Nicaragua, and northern

Costa Rica, and five from southern Costa Rica

(see Fig. 3 for examples). A single individual

encountered off Isla del Coco bore injuries that

may have been related to fisheries, but received

an average score of 2.3. Individuals with fish-

ery-related injuries were found in five of the

clusters identified through community divi-

sion, with the greatest percentage of individuals

with fisheries-related injuries (21.4 %) found in

southern Costa Rica (Cluster 17) (Table 4), and

the second greatest percentage from Cluster

12 encountered off central mainland Mexico,

with 7.1 % of individuals having fishery related

injuries. One of the individuals from Cluster 21

(Fig. 3G) had injuries consistent with fishery

interactions, although it is also possible the

wound was caused by a propeller injury.

DISCUSSION

Our analyses of false killer whales individ-

ually identified from 1991 to 2022 from south-

ern California to Panama show high levels of

site fidelity, particularly off southern Costa Rica

and Panama, and strong associations among

individuals, with maximum HWI association

values among individuals exceeding 0.50. Gero

et al., (2008) and Durrell et al., (2004) note

associations are considered strong when the

HWI between associates was at least twice the

mean index of all the dyads in the unit or cluster

being considered, which was the case in Cluster

3 from southern Costa Rica and Panama. From

re-sightings of individuals photographed off

central mainland Mexico and Nicaragua, we

documented travel distances greater than those

observed from satellite-tagged pelagic false

killer whales in Hawaiian waters (Anderson et

Table 4

Number and percentage of individual false killer whales (Pseudorca crassidens) by cluster (determined through community

division) with injuries consistent with fishery interactions.

Cluster Regions documented Number of individuals in

cluster

Number (%) with injuries consistent with

fishery interactions

6S. California & Baja California 37 1 (2.7 %)

12 Cent. Mainland Mexico 28 2 (7.1 %)

10 Cent. Mainland Mexico - N. Costa Rica 33 1 (3.0 %)

17 S. Costa Rica 14 3 (21.4 %)

3S. Costa Rica & Panama 69 2 (2.9 %)

Tota l 182 9 (4.9 %)

14 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Fig. 3. Examples of scarring and injury of dorsal fins determined to be consistent with fisheries interaction (FI) from

southern Costa Rica, (A) CRC_CA_Pc23, 1992, (B) CRC_CA_Pc102, 1991, (C) CRC_CA_Pc057, 2009, (D) CRC_CA_Pc032,

2006, and Central Mainland Mexico (E) MX_074, 2013, (F) MX_084, 2013. (G) CRC_CA_Pc188 from northern Costa Rica

is an example of an injury that we determined to be consistent with fisheries interaction, although whether the injuries were

caused by a line or propeller is unknown.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

al. 2020; E. Oleson personal communication,

27 July, 2022) especially considering that we

calculated straight-line distances, and did not

account for non-linear movements and inter-

vening land masses. False killer whales encoun-

tered off southern Costa Rica and Panama

appear to have a much smaller range of travel,

smaller even than the insular false killer whales

tagged in the Hawaiian Islands (Baird et al.,

2012). It is important to note, however, that

there was very limited effort off the continental

shelf, so we are unable to assess offshore move-

ments. The infrequency of sightings of false

killer whales on the continental shelf could be

an indication that individuals documented in

our study spend a considerable proportion of

their time in offshore waters, that local abun-

dance is low, or a combination of both factors.

There were no matches between individuals

documented off Isla del Coco in 1993 or 1994

and the mainland. While the mainland photo-

graphic sample was largely obtained from six to

more than twenty years later, there were match-

es from the Acevedo-Gutiérrez et al. (1997)

effort off mainland Costa Rica from prior to the

Isla del Coco effort.

We found that the proportion of indi-

vidual false killer whales with evidence of

surviving prior fishery interactions for at least

one cluster was higher than those found in

the endangered main Hawaiian Islands insular

population (Baird et al., 2014). Individuals

from Cluster 17, which had the greatest per-

centage of fisheries interactions (21.4 %), have

only been encountered off southern Costa Rica.

They were encountered by Acevedo-Gutiérrez

et al. (1997) in Golfo Dulce in 1991 and 1992

and the most recent sightings of any of these

animals occurred off Drake Bay in 2006. In

a similar study of fishery-related injuries in

Hawai‘i, the evaluators found significant dif-

ferences in fishery interaction rates by popu-

lation and cluster, with 12.8 % of individuals

from Cluster 3 from the main Hawaiian island

population determined to have fishery-related

scarring. Baird et al. (2014) found a significant

bias towards females with injuries consistent

with fisheries interactions, which, the authors

note, could reduce the potential population

growth rate to a greater extent than if fishery

interaction was unbiased by sex. Although we

identified the sex of some individuals based on

close attendance of small calves, performing a

comparable test with our data was beyond the

scope of this study. It is important to note that

false killer whales can incur severe injuries that

may appear to be fishery related from sources

other than fisheries interaction. For example,

Ortega-Ortiz et al. (2014) describe at least one

individual with injuries obtained from interac-

tions with a billfish or sailfish.

There are a number of assumptions and

biases in this study that we would like to note.

Many of the encounters in this study came from

community scientists or were collected oppor-

tunistically, and thus not all individuals within

larger groups were necessarily photographed.

Thus, there are a lot of isolated individuals

or small clusters in the social network that in

reality should be linked to other individuals.

For example, the 2018 encounter of the only

individual not linked to a cluster from Gua-

temala had an estimated group size of two.

Therefore, we can safely assume that there is at

least one and likely a number of clusters that

use our larger study area that are unaccounted

for in these data. Group size estimates were

collected with different methodologies, and we

acknowledge that false killer whale group size

is difficult to assess, as individuals are gener-

ally fast moving in a spread-out group (Baird

et al., 2008; Bradford et al., 2014). Sighting

positions are estimated in most non-directed

survey efforts, and this can impact the cal-

culated distance between sightings, although

this should not influence the large difference

between encounter locations and re-sighting

locations documented. Distinct individuals are

more likely to be photographed and recognized

over time, even with lesser quality photographs,

so there are likely some missed matches within

our catalog, particularly given the long dura-

tion of our study. The two individuals with the

longest sighting history (CRC_CA_Pc063 seen

over 26 years, and CRC_CA_Pc009 seen over

19 years) were both very distinct. Although

16 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

we are not estimating abundance here nor

survival, we would like to note that our stated

method of including “fair” quality photos as

well as only considering recaptures with high

certainty, could result in higher abundance

estimates and lower re-sighting rates (Ashe &

Hammond, 2022).

We recommend additional effort in off-

shore areas as well as satellite tagging to clar-

ify population structure and relationship to

animals in coastal areas, especially in light

of the lack of connection between Central

America and Isla del Coco. Additional accept-

able ID photos from any region would be wel-

come, especially offshore waters, the Galapagos

Islands, Isla del Coco, and South America. One

of our authors (DMP) recalls false killer whale

encounters a handful of times in the Galapagos

during the warm-water season. From records

collected from 1923 to 2003 there is a single

record of a mass stranding of six false killer

whales in the Galapagos Islands (Palacios et al.,

2004). False killer whales are also encountered

farther south, with an encounter noted off

central Ecuador in 2003 (Baird, 2010; Castro,

2004). Photos of two individuals from the

Galapagos were not included in this study due

to poor quality; however, these photos are avail-

able to compare to any future identifications

that we receive.

Field effort in most of the study sites was

seasonally biased. However, southern Califor-

nia has whale and dolphin watch excursions

year-round, therefore the apparent seasonal

presence of false killers along southern Cali-

fornia in March is likely not random. This con-

sistency of presence could allow for planning a

successful photo-ID and tagging effort on this

group of false killer whales, which could shed

light on these animals’ whereabouts over time.

Throughout the study area, individuals whose

livelihoods depend on whale and dolphin watch

tourism would benefit if these animals’ patterns

were better understood, and most importantly,

we hope that future studies could help inform

fisheries so that they could avoid encounters

that are detrimental to false killer whales as well

as the humans whose livelihoods depend on

the same food source. Currently, to learn more

about how and why false killer whales are inter-

acting with longline vessels, the fisheries have

options of either increasing observer coverage

and/or the installment of electronic monitoring

systems (EMS) on fishing vessels. Due to the

stated difficulty of finding observers willing

to take part in the observer program, as well

as the difficulties of dealing with the COVID

pandemic, Costa Rica and other countries have

appealed to the IATTC to provide funds for

EMS (Villanueva, 2018).

In addition to recommending future sur-

vey work, we urge greater cooperation among

researchers and community scientists in shar-

ing historical catalogs/photo collections. Our

data collection has been a slow process, span-

ning 31 years with sometimes a few photos

and a general location from a tour vessel rep-

resenting the only encounter for a year or a

region. Additional catalogs/photo collections

exist throughout this study area, which we

were not able to access, and we hope that with

careful planning and cooperation these data

will become available in the near future. As

with other odontocetes, false killer whales are

viewed as sentinel species in their environment,

and studies in the Hawaiian Islands have shown

they have high levels of lipophilic contaminants

(Kratofil et al., 2020). Greater cooperation

among researchers with these data will lead to a

greater understanding of how false killer whales

use these waters and the depth of long-term

associations with individuals and their habitat.

Ethical statement: the authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

Author Contribution: ABD conceptual-

ized the manuscript, coordinated data acquisi-

tion, performed analysis, interpreted the data

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

results, and drafted the initial and revised ver-

sions of this manuscript. FGA, AAG, SDM, KR,

EQR, JDW, DMP, DMF, CLP, CDOO, JC, and

RWB contributed with data, and interpretation

of the results for each study site, reviewed and

approved the various drafts and final version

of the manuscript. NR, AFJ and FV contrib-

uted data and reviewed the final version of

the manuscript.

See supplementary material

a09v71s4-MS1

ACKNOWLEDGMENTS

CRC would like to thank and acknowl-

edge photo contributors throughout Central

America, especially R. M. Arias, E. Falcone, F.

A. Madrigal, C. Aguilera, P. Godoy, Oceanic

Society, Road Scholar participants especially

A. Hermann, J. Harris and M. V. Anderson,

the Drake Bay Wilderness Lodge, Costa Rica,

and Marina Puesta del Sol, Nicaragua. From

Southern California-Mexico we acknowledge

O. Guzón, J. Urbán, L. Kretchschmar, D. Kalez,

D. Frank, M.Tyson, C. Jaeger, K. Campbell, S.

Velasco, K. Audley, C. Mayer, A. Schulman-

Janiger, M. Stumpf, and Captain Dave’s Dolphin

and Whale Watching Safari. Surveys conducted

in Guatemala were run by EQR and funded

by Fondo Nacional de Ciencia Tecnología,

awarded by the Consejo Nacional de Ciencia y

Tecnología, through the Secretaria Nacional de

Ciencia y Tecnología (Fodecyt 85-2007 Proj-

ect), Cetacean Society International, Sarasota

Dolphin Research Institute; PADI Foundation;

Idea Wild; Defensores de la Naturaleza Founda-

tion. Surveys in Nicaragua were run by Asso-

ciation ELI-S and funded by Cetacean Society

International, Rufford Foundation, Vrije Uni-

versiteit Brussel. JDW would like to acknowl-

edge E. Pouplard and V. Pouey-Santalou who

helped with photo-ID and boat-based surveys.

Surveys off Panama were conducted by Panace-

tacea and funded by the Islas Secas Foundation.

EQR would like to acknowledge several key

people who helped with the cetacean photo-ID

surveys including L. Girón, G. Méndez, and J.

Morales, and the enthusiasm and support from

P. Negreros, V. García, L. Palmieri, S. Rosales,

and O. Zamora. Surveys conducted by AAG

were possible via funding from the Marine

Mammal Research Program, Texas A&M Uni-

versity at Galveston, and the Whale Conserva-

tion Institute in Lincoln, MA; in Golfo Dulce:

Earthwatch-Center for Field Research; at Isla

del Coco: Parque Nacional Isla del Coco and

The Netherlands Embassy in Costa Rica; and

fellowships from Texas A&M University, Inter-

national Women’s Fishing Association, and

Houston Underwater Society; logistic support

was provided in Golfo Dulce by the Golfito

Research Station of the Universidad de Costa

Rica and at Isla del Coco by Parque Nacional

Isla del Coco, Okeanos Aggressor, and Under-

sea Hunter; the Servicio de Parques Nacionales,

Costa Rica, issued permits #01-91 and #08-95

to work at the island; Y. Camacho, K. Dudzik,

L. Gonzalez, K. Lebo, E. Lundin, and the R/V

Odyssey crew assisted during sightings. NR

conducted surveys in Central Mainland Mexico

funded by Cetacean Society International. FV

acknowledges Mamiferos Marinos de Oaxaca

Biodiversidad y Conservación AC for support-

ing data collection in southern Mexico. Final-

ly, thank you to three anonymous reviewers

whose comments and suggestions substantially

improved this manuscript.

REFERENCES

Acevedo-Gutierrez, A., Brennan, B., Rodriguez, P. & Tho-

mas, M. (1997). Resightings and behavior of false

killer whales (Pseudorca crassidens) in Costa Rica.

Marine Mammal Science, 13, 307–314. https://doi.

org/10.1111/j.1748-7692.1997.tb00634.x

Alonso, M. K., Pedaza, S. N., Schiavini, A. C. M., Goodall,

R. N. P., & Crespo, E. A. (1999). Stomach contents of

false killer whales (Pseudorca crassidens) stranded on

the coasts of the Strait of Magellan, Tierra del Fuego.

Marine Mammal Science, 15, 712–724. https://doi.

org/10.1111/j.1748-7692.1999.tb00838.x

Anderson, D., Baird, R. W., Bradford, A. L., & Oleson. E.

M. (2020). Is it all about the haul? Pelagic false killer

whale interactions with longline fisheries in the cen-

tral North Pacific. Fisheries Research, 230, 105665.

https://doi.org/10.1016/j.fishres.2020.105665

18 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Ashe, E., & Hammond, P. S. (2022). Effect of matching

uncertainty on population parameter estimation

in mark-recapture analysis of photo-identification

data. Mammalian Biology, 102, 781-792 https://doi.

org/10.1007/s42991-022-00236-4

Baird, R. W. (2010). Pygmy killer whales (Feresa attenuata)

or false killer whales (Pseudorca crassidens)? Identifi-

cation of a group of small cetaceans seen off Ecuador

in 2003. Aquatic Mammals, 35, 326–327. https://doi.

org/10.1578/AM.36.3.2010.326

Baird, R. W. (2016). The Lives of Hawai‘i’s Dolphins and Wha-

les: Natural History and Conservation. University of

Hawai‘i Press. https://doi.org/10.1515/9780824865931

Baird, R. W. (2018a). Pseudorca crassidens

[e.T18596A145357488]. The IUCN Red List of

Threatened Species.. https://doi.org/10.2305/IUCN.

UK.2018-2.RLTS.T18596A145357488.en

Baird, R.W. (2018b). False Killer Whales. In B. Würsig, J.

G. M. Thewissen & K. Kovacs (Eds.), Encyclopedia

of Marine Mammals (3rd ed., pp. 347-349). Elsevier.

Baird, R. W., & Gorgone, A. M. (2005). False killer whale

dorsal fin disfigurements as a possible indicator of

long-line fishery interactions in Hawaiian waters.

Pacific Science, 59, 593–601. https://doi:10.1353/

psc.2005.0042

Baird, R. W., Gorgone, A. M., McSweeney, D. J., Webster,

D. L., Salden, D. R., Deakos, M. H., Ligon, A. D.,

Schorr, G. S., Barlow, J., & Mahaffy, S. D. (2008).

False killer whales (Pseudorca crassidens) around

the main Hawaiian Islands: long-term site fidelity,

inter-island movements, and association patterns.

Marine Mammal Science, 24, 591–612. https://doi.

org/10.1111/j.1748-7692.2008.00200.x

Baird, R. W., Hanson, M. B., Schorr, G. S., Webster, D. L.,

McSweeney, D. J., Gorgone, A. M., Mahaffy, S. D.,

Holzer, D., Oleson, E. M., & Andrews, R. D. (2012).

Range and primary habitats of Hawaiian insular

false killer whales: informing determination of cri-

tical habitat. Endangered Species Research 18, 47–61.

https://doi.org/10.3354/esr00435

Baird, R. W., Langelier, K. M., & Stacey, P. J. (1989). First

records of false killer whales, Pseudorca crassidens,

in Canada. Canadian Field Naturalist, 103, 368–371.

Baird, R. W., Mahaffy, S. D., Gorgone, A. M., Cullins, T.,

McSweeney, D. J., Oleson, E. M., Bradford, A. L., Bar-

low, J., & Webster, D. L. (2014). False killer whales and

fisheries interactions in Hawaiian waters: evidence for

sex bias and variation among populations and social

groups. Marine Mammal Science, 31, 579–590. https://

doi.org/10.1111/mms.12177

Baird, R. W., Mahaffy, S. D., Gorgone, A. M., Beach, K.

A., Cullins, T., McSweeney, D. J., Verbeck, D. S., &

Webster, D. L. (2017). Updated evidence of inte-

ractions between false killer whales and fisheries

around the main Hawaiian Islands: assessment of

mouthline and dorsal fin injuries. Document PSRG-

2017-16 submitted to the Pacific Scientific Review

Group.https://cascadiaresearch.org/publications/

Bairdetal2017_FKW_PSRG/

Baird, R. W., Mahaffy, S. D., & Lerma, J. K. (2021). Site fide-

lity, spatial use, and behavior of dwarf sperm whales

in Hawaiian waters: using small-boat surveys, photo-

identification, and unmanned aerial systems to study

a difficult-to-study species. Marine Mammal Science,

38, 326–348. https://doi.org/10.1111/mms.12861

Baird, R. W., Schorr, G. S., Webster, D. L., McSweeney, D. J.,

Hanson, M. B., & Andrews, R. D. (2010). Movements

and habitat use of satellite-tagged false killer whales

around the main Hawaiian Islands. Endangered Spe-

cies Research, 10, 107–121. https://doi.org/10.3354/

esr00258

Bejder, L., Fletcher, D., & Bräger, S. (1998). A method for

testing association patterns of social animals. Ani-

mal Behaviour, 56, 719–725. https://doi.org/10.1006/

anbe.1998.0802.

Borgatti, S. P. (2002). NetDraw: Graph Visualization

Software [Computer software]. Harvard: Analytic

Technologies.

Bradford, A. L., Forney, K. A., Oleson, E. M., & Bar-

low, J. (2014). Accounting for subgroup structure

in line-transect abundance estimates of false killer

whales (Pseudorca crassidens) in Hawaiian waters.

PLOS ONE, 9 e90464. https://doi.org/10.1371/journal.

pone.0090464

Cairns, S. J., & Schwager, S. J. (1987). A comparison of

association indices. Animal Behaviour, 35, 1454–1469.

https://doi.org/10.1016/S0003-3472(87)80018-0

Castro, C. (2004). Encounter with a school of pygmy killer

whales (Feresa attenuata) in Ecuador, southeast tro-

pical Pacific. Aquatic Mammals, 30, 441–444. https://

doi.org/10.1578/AM.30.3.2004.441

Chivers, S. J., Baird, R. W., Martien, K. M., Taylor, B. L.,

Archer, E., Gorgone, A. M., Hancock, B. L., Hedrick,

N. M., Matilla, D., McSweeney, D. J., Oleson, E. M.,

Palmer, C. L., Pease, V., Robertson, K. M., Robbins,

J., Salinas, J. C., Schorr, G. S., Schultz, M., Theileking,

J. L., & Webster, D. L. (2010). Evidence of genetic

differentiation for Hawai‘i insular false killer whales

(Pseudorca crassidens)[ NOAA Technical Memoran-

dum NMFS-SWFSC-458]p.National Oceanic and

Atmospheric Administration.

Chivers, S. J., Baird, R. W., McSweeney, D. J., Webster, D.

L., Hedrick, N. M., & Salinas, J. C. (2007). Genetic

variation and evidence for population structure in

eastern North Pacific false killer whales (Pseudorca

crassidens). Canadian Journal of Zoology, 85, 783–794.

https://doi.org/10.1139/Z07-059

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

Durrell, J. L., Sneddon, I. A., O’Connell, N. E., & White-

head, H. (2004). Do pigs form preferential associa-

tions? Applied Animal Behaviour Science, 89, 41–52.

https://doi.org/10.1016/j.applanim.2004.05.003

Elliser, C. R., van der Linde, K. & MacIver, K. (2022). Adap-

ting photo-identification methods to study poorly

marked cetaceans: a case study for common dolphins

and harbor porpoises. Mammalian Biology, 102, 811-

827. https://doi.org/10.1007/s42991-021-00194-3

Fader, J. E., Baird, R. W., Bradford, A. L., Dunn, D. C., For-

ney, K. A., & Read, A. J. (2021). Patterns of depreda-

tion in the Hawai‘i deep-set longline fishery informed

by fishery and false killer whale behavior. Ecosphere,

12, e03682. https://doi.org/10.1002/ecs2.3682

Ferguson, M. C., & Barlow, J. (2003). Addendum: Spatial

distribution and density of cetaceans in the eastern

tropical Pacific Ocean based on summer/fall research

vessel surveys in 1986-96. NOAA Administrative

Report LJ-01-04 (Addendum): 99 Ferguson and Bar-

low 2001-SWFSC-AR. National Oceanic and Atmos-

pheric Administration.

Fiedler, P., & Lavín, M. (2006). Introduction: A review of

eastern tropical Pacific oceanography. Progress in

Oceanography, 69, 94–100. https://doi.org/10.1016/j.

pocean.2006.03.006

Fiedler, P. C., & Talley, L. D. (2006). Hydrography of the

eastern tropical Pacific: A review. Progress in Ocea-

nography, 69, 143–180. https://doi. org/10.1016/j.

pocean.2006.03.008

Forney, K. A., & Kobayashi, D. (2007). Updated estimates of

mortality and injury of cetaceans in the Hawaii-based

longline fishery, 1994-2005 [NOAA Technical Memo-

randum NMFS-SWFSC-412]. National Oceanic and

Atmospheric Administration.

Gero, S., Engelhaupt, D., & Whitehead, H. (2008). Hete-

rogeneous social associations within a sperm whale,

Physeter macrocephalus, unit reflect pairwise rela-

tedness. Behavioral Ecology and Sociobiology, 63,

143–151. http://www.jstor.org/stable/40645502

Gocke, K., Cortés, J., & Murillo, M.M. (2001). The annual

cycle of primary productivity in a tropical estuary:

The inner regions of the Golfo de Nicoya, Costa Rica.

Revista de Biología Tropical, 49, 289–306.

Hamilton, T. A., Redfern, J. V., Barlow, J., Ballance, L. T.,

Gerrodette, T., Holt, R. S., Forney, K. A., & Taylor, B.

L. (2009). Atlas of cetacean sightings for Southwest

Fisheries Science Center cetacean and ecosystem

surveys: 1986-2005 [NOAA Technical Memorandum

NMFS NOAA-TM-NMFS-SWFSC-440].National

Oceanic and Atmospheric Administration.

Heileman, S. (2008). XIV-48 Pacific Central-American

Coastal Large Marine Ecosystem. In K. Sherman, &

G. Hempel (Eds). The UNEP large marine ecosystem

report: a perspective on changing conditions in LMEs

of the world’s regional seas. UNEP Regional Seas

Report and Studies No. 182. United Nations Environ-

ment Programme.

Herzing, D. & Elliser, C. R. (2016). Opportunistic Sightings

of Cetaceans in Nearshore and Offshore Waters

of Southeast Florida. Journal of Northwest Atlantic

Fishery Science, 48, 21–31. https://doi:10.2960/J.v48.

m709.

Inter-American Tropical Tuna Commission. (2011). Reso-

lution (amended) on the establishment of a list of

longline fishing vessels over 24 meters (LSTLFVs)

authorized to operate in the eastern Pacific Ocean.

Inter-American Tropical Tuna Commission. https://

www.iattc.org/getattachment/68e29c16-c476-462f-

a8ba-a4e71d2e16fb/C-11-05%20Positive%20list%20

of%20longline%20vessels

Inter-American Tropical Tuna Commission. (2019). Reso-

lution on scientific observers for longline vessels.

Inter-American Tropical Tuna Commission. https://

www.iattc.org/GetAttachment/614c5692-74c5-

40a7-a8b0-148ec0e52206/Observers%20on%20

longliners

Inter-American Tropical Tuna Commission. (2022).

Inter-American Tropical Tuna Commission.

https://www.iattc.org/en-US/Management/Vessel-

Kratofil, M. A., Ylitalo, G. M., Mahaffy, S. D., West, K.

L., and Baird, R. W. (2020). Life history and social

structure as drivers of persistent organic pollutant

levels and stable isotopes in Hawaiian false killer

whales (Pseudorca crassidens). Science of the Total

Environment, 733, 138880. https://doi.org/10.1016/j.

scitotenv.2020.138880

Lazcano-Pacheco, C., Castillo-Sánchez, A. J., Ortega-Ortiz,

C. D., Martínez-Serrano, I., Villegas-Zurita, F., Frisch-

Jordán, A., Guzón-Zatarain, O. R., Audley, K., Ranso-

me, N., Bolaños-Jiménez, J., Urbán-R., J., & Douglas,

A. B. (2023). Ecological aspects of false killer whales

(Pseudorca crassidens) from Mexican Pacific and

Southern California waters. Marine Mammal Science,

39, 1344–1355. https://doi.org/10.1111/mms.13056

Lavín, M.F., Fiedler, P.C., Amador, J.A., Ballance, L.T.,

Färber-Lorda, L., & Mestas-Nuñez, A.M. (2006). A

review of eastern tropical Pacific oceanography: Sum-

mary. Progress in Oceanography, 69, 391–398. https://

doi.org/10.1016/j.pocean.2006.03.005

Lizano, O, & Alfaro, J. (2004). Algunas características de las

corrientes marinas en el Golfo de Nicoya, Costa Rica.

Revista de Biología Tropical 52, 77–94. https://doi.

org/10.15517/RBT.V52I2.26579

Martien, K. K., Chivers, S. J., Baird, R. W., Archer, F. I.,

Gorgone, A. M., Hancock-Hanser, B. L., Mattila, D.,

McSweeney, D. J., Oleson, E. M., Palmer, C., Pease,

V. L., Robertson, K. M. Schorr, G. S., Schultz, M. B.,

Webster, D. L., & Taylor, B. L. (2014). Nuclear and

20 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57189, diciembre 2023 (Publicado Nov. 01, 2023)

mitochondrial patterns of population structure in

North Pacific false killer whales (Pseudorca crassi-

dens). Journal of Heredity, 105, 611–626. https://doi.

org/10.1093/jhered/esu029

Martien, K. K., Taylor, B. L., Chivers, S. J., Mahaffy, S.

D., Gorgone, A. M., & Baird, R. W. (2019). Fidelity

to natal social groups and mating both within and

between social groups in an endangered false killer

whale (Pseudorca crassidens) population. Endangered

Species Research, 40, 219–230. https://www.int-res.

com/abstracts/esr/v40/p219-230/

Martínez-Fernández, D., Montero-Cordero, A., & May-

Collado, L. (2011). Cetáceos de las aguas costeras del

Pacífico norte y sur de Costa Rica. Revista de Biología

Tropical, 59, 283–290.

May-Collado, L., Gerrodette, T., Calambokidis J., Ras-

mussen, K., & Irena, S. (2005). Patterns of cetacean

sighting distribution in the Pacific Exclusive Econo-

mic Zone of Costa Rica based on data collected from

1979-2001. Revista de Biología Tropical, 53, 249–263.

Mora-Escalante, R. E., Lizano, O. G., Alfaro, E. J., &

Rodríguez, A. (2020). Distribución de temperatu-

ra y salinidad en campañas oceanográficas recien-

tes en el Pacífico Tropical Oriental de Costa Rica.

Revista de Biología Tropical, 68, 177–197. https://doi.

org/10.15517/RBT.V68IS1.41180

National Oceanic and Atmospheric Administration. (2022).

International Fisheries; Pacific Tuna Fisheries; Purse

seine observer exemptions in the Eastern Pacific

Ocean. Federal Register. https://public-inspection.

federalregister.gov/2022-06337.pdf

Newman, M. E. J. (2004). Analysis of weighted networks.

Physical Review E, 70, 056131. https://doi.org/10.1103/

PhysRevE.70.056131

Norris, K. S., & Prescott, H. P. (1961). Observations on

Pacific cetaceans of California and Mexican waters.

University of California Publications in Zoology, 63,

291–401.

Ortega-Ortiz, C., Elorriaga-Verplancken, F. R., Olivos-

Ortiz, A., Liñán-Cabello, M. A., & Vargas-Bravo, M.

H. (2014). Insights into the feeding habits of false

killer whales (Pseudorca crassidens) in the Mexi-

can Central Pacific. Aquatic Mammals, 40, 386–393.

https://doi.org/10.1578/AM.40.4.2014.386

Palacios, D., Salazar, S., & Day, D. (2004). Cetacean remains

and strandings in the Galápagos Islands, 1923-2003.

Latin American Journal of Aquatic Mammals, 3, 127–

150. https://doi.org/10.5597/lajam00058

Palmer, C., Baird, R. W., Webster, D. L., Edwards, A. C.,

Patterson, R., Withers, A., Withers, E., Woinarski, J.

C. Z., & Groom, R. (2017). A preliminary study of the

movement patterns of false killer whales (Pseudorca

crassidens) in coastal and pelagic waters of the Nor-

thern Territory, Australia. Marine and Freshwater

Research, 68, 1726–1733 https://doi.org/10.1071/

MF16296

Pebesma, E. (2018). Simple features for R: Standardized

support for spatial vector data. The R Journal, 10,

439–446. https://doi.org/10.32614/RJ-2018-009

Quintana-Rizzo, E., and Gerrodette, T. (2009). First study

of the abundance and distribution of cetaceans in

the Guatemalan Exclusive Economic Zone[Report].

Chicago Board of Trade Endangered Species Fund,

Chicago Zoological Society.

Quintana-Rizzo, E. (2012). Estado y ecología de las pobla-

ciones de cetáceos en el Océano Pacífico de Guatema-

la. Consejo Nacional de Ciencia y Tecnología. https://

fondo.senacyt.gob.gt/portal/index.php/catalogo/15-

codigo/421-85-2007-ciencias-de-la-tierra-el-oceano-

y-el-espacio

R Core Team (2021). R: A language and environment

for statistical computing [Computer software]. R

Foundation for Statistical Computing. https://www.R-

project.org/ Sánchez Robledo, E., Oviedo, L., Herra

Miranda, D., Pacheco Polanco, J. D., Goodman, S., &

Guzman, H. M. (2020). The abundance of false killer wha-

les (Pseudorca crassidens) (Artiodactyla: Delphinidae)

in coastal waters of Golfo Dulce and Osa Peninsula,

Costa Rica. Revista de Biología Tropical, 68, 580–589.

https://doi.org/10.15517/RBT.V68I2.37196

Villanueva, M. M. (2018). Observadores científi-

cos a bordo de palangreros (Costa Rica). Inter-

American Tropical Tuna Commission. https://

www.iattc.org/GetAttachment/52c3d9b8-e0f5-

4393-9377-0e532728250b/Observadores%20

cient%C3%ADficos%20a%20bordo%20de%20

palangreros%20(Costa%20Rica)

Wade, P. R. & Gerrodette, T. (1993). Estimates of cetacean

abundance and distribution in the eastern tropical

Pacific. Report International Whaling Commission,

43, 477–493.

Whitehead, H. (2008). Analyzing animal societies: quanti-

tative methods for vertebrate social analysis. Univer-

sity of Chicago Press.

Whitehead, H. (2009). SOCPROG programs: analyzing

animal social structures. Behavioral Ecology and

Sociobology, 63, 765–778.

Zaeschmar, J. R. (2014). False killer whales (Pseudor-

ca crassidens) in New Zealand waters. (Master’s.

thesis, Massey University]. Massey Universi-

ty, University of New Zeland. https://mro.massey.

ac.nz/bitstream/handle/10179/6902/01_front.

pdf?sequence=1&isAllowed=y