Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

Population size and demographic parameters of pantropical spotted dolphin

(Stenella attenuata graffmani) (Cetartiodactyla: Delphinidae)

in Golfo Dulce, Costa Rica

Lenin Oviedo Correa1*; https://orcid.org/0000-0001-8015-1367

David Herra-Miranda1; https://orcid.org/0000-0003-2056-5060

Juan Diego Pacheco-Polanco1; https://orcid.org/0000-0003-3592-0950

1. Laboratorio de Ecología de Mamíferos Marinos Tropicales, Centro de Investigación de Cetáceos-Costa Rica,

Puntarenas, Costa Rica; leninovi1@gmail.com (*Correspondence); davidceic@gmail.com; dpachecop@gmail.com

Received 30-VIII-2022. Corrected 17-III-2023. Accepted 12-IV-2023.

ABSTRACT

Introduction: The coastal form of pantropical spotted dolphins (Stenella attenuata graffmani) is commonly

found along the Pacific coast of Costa Rica. Within Golfo Dulce, a fiord-like-embayment bordering the Osa

Peninsula, pantropical spotted dolphins are sympatric with inshore bottlenose dolphins (Tursiops truncatus) and

these marine predators provide an important source of revenue for local communities through boat-based tours.

Objective: Here we estimated the population size and demographic parameters of the coastal pantropical spotted

dolphins in Golfo Dulce.

Methods: The study area was surveyed using non-random boat surveys. Upon encounter, dolphins were indi-

vidually photo-identified using natural marks in their dorsal fins to estimate population abundance and survival

using three emigration scenarios.

Results: A total of 280 dolphins were photoidentified, 65 % of which were observed only once. A total of 30

models were produced, and only two were considered to be parsimonious. Both models explain seasonal appar-

ent survival and its variation due to heterogeneity in capture-recapture probability, one under no emigration

(ΔQAICc = 0.00) and the other under random emigration (ΔQAICc = 1.72). We deemed the latter to be a more

realistic model as it better reflects our in-situ observations. Under this preferred model the population size of

pantropical spotted dolphins in Golfo Dulce varied from 187.30 individuals (CI: 168.67 – 208.02, CV: 0.11) to

367.88 individuals (CI: 341.51 – 396.31, CV: 0.07), with no significant differences in abundance due to seasonality

and very high apparent survival (S = 0.98, CI: 0.68 – 0.99, SE: 0.02).

Conclusions: The number of identified dolphins and the proportion of individuals seen only once suggest the

fluid movement of the population in and out of the gulf. However, the population size and demographic estimates

are characterized by several identified individuals regularly recaptured inside the gulf. This group of dolphins

appears to favor the inner basin as a critical foraging habitat. Given the increase of anthropogenic impacts within

Golfo Dulce, future management and conservation efforts will require the recognition of an ecologically discrete

population unit of coastal pantropical spotted dolphins within the gulf.

Key words: Golfo Dulce; Stenella attenuatta graffmani; population size; survival; robust design; Costa Rica;

photoidentification.

https://doi.org/10.15517/rev.biol.trop..v71iS4.57287

SUPPLEMENT • SMALL CETACEANS

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

INTRODUCTION

The pantropical spotted dolphin, Stenella

attenuata (Gray, 1846) is distributed throughout

the Eastern Tropical Pacific (ETP) where two

genetically and morphologically distinct popu-

lations or ecotypes are recognized, oceanic and

coastal (Perrin 1975, Perrin 2001, Perrin 2018).

The oceanic ecotype lives in pelagic waters

where, along with spinner dolphins, Stenella

longirostris(Gray, 1828), it tends to associate

with yellowfin tuna, resulting in high mortality

due to the tuna purse seine fishery (Ballance et

al., 2021; Cramer et al., 2008; Scott et al., 2012).

The coastal ecotype is distributed at < 200 km

from the coast of Central America where it is

exposed to various human activities including

commercial whale watching (Montero-Cordero

& Lobo 2010; Pacheco-Polanco 2016); water

quality and coastal habitat modification (Pache-

co-Polanco 2016); and marina development

projects near critical coastal cetacean habitats

(Herra-Miranda et al., 2016).

Of the two ecotypes, the oceanic eco-

type’s population and demographic status are

best known. Since the late 1970s, the U.S.

government agency National Oceanographic

Atmospheric Administration (NOAA) has gen-

erated management recommendations based

on estimates of abundance, population size,

survival, migration, and recruitment of oce-

anic pantropical spotted dolphins in the ETP

(e.g., Gerrodette & Forcada 2005; Gerrodette

et al., 2008; Wade et al., 2007). These efforts

are the responsibility of NOAA, which is one

of three U.S. federal agencies responsible for

RESUMEN

Tamaño poblacional y parámetros demográficos del delfín manchado pantropical

(Stenella attenuata graffmani) (Cetartiodactyla: Delphinidae) en el Golfo Dulce, Costa Rica.

Introducción: La forma costera del delfín manchado pantropical (Stenella attenuata graffmani) se encuentra

comúnmente a lo largo de la costa Pacífica de Costa Rica. En el Golfo Dulce, una bahía similar a un fiordo en la

Península de Osa, los delfines manchados pantropicales son simpátricos con los delfines nariz de botella costeros

(Tursiops truncatus) y estos depredadores marinos proporcionan una importante fuente de ingresos para las

comunidades locales a través de actividades de avistamientos ecoturísticos.

Objetivo: Se estimó el tamaño de la población y los parámetros demográficos de los delfines manchados pantro-

picales costeros para el Golfo Dulce.

Métodos: El área de estudio se estudió mediante muestreos no aleatorios desde embarcaciones. Tras el encuentro,

los delfines fueron foto-identificados individualmente usando marcas naturales en sus aletas dorsales, para esti-

mar la abundancia y supervivencia de la población usando tres escenarios posibles de emigración.

Resultados: Un total de 280 delfines fueron foto-identificados, 65 % de los cuales fueron observados una sola

vez. Se elaboraron un total de 30 modelos, y sólo dos se consideraron parsimoniosos. Ambos modelos explican

la supervivencia estacional aparente y su variación debido a la heterogeneidad en la probabilidad de captura-

recaptura, uno bajo emigración nula (ΔQAICc = 0.00) y el otro bajo emigración aleatoria (ΔQAICc = 1.72).

Consideramos que este último es un modelo más realista, ya que refleja mejor nuestras observaciones in situ.

Bajo este modelo seleccionado, el tamaño de la población de delfines manchados pantropicales en el Golfo Dulce

varió de 187.30 individuos (CI: 168.67 – 208.02, CV: 0.11) a 367.88 individuos (CI: 341.51 – 396.31, CV: 0.07),

sin diferencias significativas en la abundancia debido a la estacionalidad y una supervivencia aparente muy alta

(S = 0.98, CI: 0.68 – 0.99, SE: 0.02).

Conclusiones: El número de delfines identificados y la proporción de individuos vistos una sola vez sugieren un

movimiento fluido de la población dentro y fuera del golfo. Sin embargo, el tamaño de la población y las esti-

maciones demográficas se caracterizan por varios individuos identificados y re-capturados regularmente dentro

del golfo. Este grupo de delfines parece favorecer la cuenca interior como hábitat crítico de alimentación. Dado

el incremento de los impactos antropogénicos dentro del Golfo Dulce, los futuros esfuerzos de manejo y con-

servación requerirán el reconocimiento de una unidad poblacional ecológicamente discreta del delfín manchado

pantropical costero dentro del golfo.

Palabras clave: Golfo Dulce; Stenella attenuatta graffmani; tamaño poblacional; sobrevivencia; diseño robusto;

Costa Rica; fotoidentificación.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

implementing the mandate of the Marine

Mammal Protection Act. This mandate estab-

lishes that marine mammal populations that are

endangered by the impact of human activities

must be recovered, and to this end, periodic

quantitative assessments of their population

dynamics must be made (Marine Mammal Pro-

tection Act, 1972). However, such efforts have

not been made for the coastal ecotype. Pres-

ently, there are few estimates of the population

size of coastal spotted dolphins at the Central

American region level (Gerrodette & Palacios,

1996; Palacios & Gerrodette 1996) or even by

country, and they do not include demographic

characterizations of the populations studied.

The coastal pantropical spotted dolphin

(S. attenuata graffmani) is probably the most

representative cetacean of the Pacific coast

of Costa Rica. Several aspects regarding its

ecology have been documented since the late

1990s, in particular, its presence and distri-

bution (Acevedo & Buckhart, 1998; Cubero-

Pardo, 1998; Cubero-Pardo, 2007a; Holst et al.,

2017; Martinez-Fernandez et al., 2011, Mar-

tinez-Fernandez et al., 2014; May-Collado et

al., 2005;; Oviedo, 2007, Oviedo, 2008; Oviedo

et al., 2009, Oviedo et al, 2015), fine-scale

relative abundance estimation (May Collado

& Forcada, 2012), genetic identity (Escorza-

Treviño et al., 2005; Leslie et al., 2019; Leslie &

Morin, 2018), behavior (Cubero-Pardo, 2007b;

May-Collado & Morales, 2005; Oviedo, 2007,

Oviedo, 2008; Oviedo et al., 2018), interac-

tion with fisheries (Palacios-Alfaro, 2006) and

tourism activities (Montero-Cordero & Lobo,

2010). However, the size of the population in

the Pacific of Costa Rican and their demogra-

phy have not yet been assessed.

This study focuses on estimating the popu-

lation size, and some demographic parameters

(apparent survival, capture-recapture probabil-

ity, and emigration) for the coastal pantropical

spotted dolphins in Golfo Dulce. This gulf is a

coastal marine habitat with tropical fiord char-

acteristics, where the species has semi-pelagic

habits and is sympatric with the inshore eco-

type of the bottlenose dolphin (Tursiops trun-

catus). In Golfo Dulce, the spotted dolphins

show habitat partitioning, facilitating the coex-

istence of both marine predators (Oviedo, 2018;

Oviedo et al., 2018). Previous demographic

assessments have shown that dolphins of coast-

al habits show seasonal variability in abun-

dance, affecting demographic characteristics

and possibly reproductive strategies in males

and females (Bolaños-Jiménez et al., 2022). For

these reasons, this study will consider the pos-

sible effects of dry (November-May, as reported

in Oviedo et al., 2018) and rainy (June-October,

as reported in Oviedo et al., 2018) seasons on

the demography and population size of this

delphinid in Golfo Dulce. There remains a need

for a regional quantitative assessment of popu-

lation dynamics and demographic information

for this dolphin ecotype.

MATERIALS AND METHODS

Study Area: Golfo Dulce is a stratified

estuary of tectonic origin, located in the South

Pacific region of Costa Rica, centered at 8°33’N

and 83°14’W (Svendsen et al., 2006). It has a

215 m deep internal basin with a 60 m sill that

restricts ocean circulation (Morales-Ramírez

et al., 2015; Svendsen et al., 2006); a length of

50 km; a width of 10-15 km; and a total area

of approximately 750 km² (Von Wangelin &

Wolff, 1996; Wolff et al.,1996). The climate is

tropical and humid with a rainy season from

June to early November, generating an average

monthly rainfall of 100–700 mm. The main

freshwater supply comes from the Coto Colo-

rado, Tigre, Esquinas, and Rincón rivers, form-

ing estuaries and mangrove zones in their areas

of influence. They also affect the gulf’s circula-

tion pattern, resulting in a stratified current

structure (Spongberg & Davis, 1998). Due to its

physiographic and hydrological characteristics,

Gulf Dulce can be divided into three sub-areas:

1) a deep inner basin with a maximum depth

of about 215 m, an anoxic layer below 100

meters (Brenes & León, 1988) and restricted

surface circulation; 2) a flat outer basin with an

average depth of 70 m (Hebbeln et al., 1996),

which begins 20 km from the mouth of the

Gulf; and 3) a third area corresponding to the

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

transitional-oceanic zone at the mouth of the

Gulf (Oviedo et al., 2009, Oviedo et al., 2015),

where this gulf communicates with the Pacific

Ocean. In this external oceanic portion, depths

close to 1 000 m are reached at a relative dis-

tance of 6 km (Fig. 1).

Field data collection: Pantropical spotted

dolphin surveys were conducted from June

2011 to April 2014 in the three sub-areas

described above. The surveys were conducted

from a 7 m long boat with a 115 HP four-stroke

outboard motor, zig-zagging from the point

of origin (Bahía Rincón or Puerto Jiménez)

to spatially cover as much of each sub-area as

possible. The surveys were conducted between

7:00 am and 4:00 pm. During each survey, four

observers were on board, one of which served

as the main photographer, generally support-

ed by another secondary photographer, they

photographed as many dolphins as possible in

each observed group using digital DSLR cam-

eras (Canon 7D/70D) equipped with 400 mm

telephoto lenses.

The definition of a group used in this

study is that of Karczmarski et al. (2005): a

spatial aggregation of animals engaged in simi-

lar (often the same) activities and interacting

with each other on short enough time scales

that there is little (or no) change in group

membership. During each sighting, the boat

approached within approximately 100 m of

the group, and then the geographic position

of the boat was recorded, as was its position

relative to the group, the time of the encounter,

group size, and composition, and the behavior

records (initial and 10-minute) for the encoun-

ter. Specifically, group size was recorded, while

group composition was classified according to

the presence of adults, juveniles, and calves.

Fig. 1. The study area in Golfo Dulce: 1) Photoidentification sampling during the rainy seasons of 2011, 2012, and 2013.

Blue lines correspond to the routes traveled during data collection. 2) Photoidentification sampling during the dry seasons

of 2011–2012, 2012–2013, and 2013–2014. Green lines correspond to the routes traveled during data collection. Red

circles: encounters and photographic sampling locations with the pantropical spotted dolphin (S. a graffmani). Gray circles:

encounters with other cetaceans.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

The identification of the different age classes

was based on Perrin (1975), Perrin (2001), and

Perrin (2018), where the increase of spots on

the body determines the maturity of the indi-

viduals. Accordingly, juveniles and calves with

a length of 1/3 of the length of the mother have

a coloration pattern composed of two shades of

gray and a few ventral spots. In contrast, adults

are more pigmented with white spots that

merge dorsally.

The stable association of an adult with a

small calf or medium size juvenile, though not

always spotted in the infant position (Mann &

Smut, 1998, Mann & Smut, 1999), was used as

a cue to assign the female gender. On several

occasions clear photographs of the genital slits

supported sex identification. Highly acrobatic

displays or bouts of social behavior such as

copulation facilitated photographic evidence

to differentiate male and female individuals.

Behavior was described as feeding, resting,

socializing, traveling, and milling, following the

definitions of Marfurt et al., (2022) and Macha-

do et al., (2019). Once the initial data collection

was completed, the group-follow protocol for

photoidentification was initiated.

Photoidentification: The traditional

method of individual identification in dolphins

uses photographs of the dorsal fin, which allows

the modeling of presence/absence data by

mark-recapture analysis (Bolaños-Jimenez et al.

2022; Brooks et al., 2017; Hupman et al., 2018;

Parra et al., 2006; Wilson et al., 1999; Wursig

& Jefferson, 1990; Zanardo et al. 2016). In one

photography session, an individual may end up

being photographed several times, depending

on group size and behavior. After the photo-

graphs were collected, they were categorized

based on their quality and distinctiveness and

then used to build an identification catalog

with images of each individual dolphin in Golfo

Dulce. The selection process ensured that each

sampling occasion was backed by a type-photo

of every individual, if available. The type-photo

was then associated with the presence record

on a specific date and location. Photographic

quality was scaled as 1-100, with a minimum

use criterion of ≥ 70. A quality of more than

70 meant that the fin occupied no less than a

quarter of the photo, was focused and defined,

and was as perpendicular as possible to avoid

distortion by angles (parallax) (Karczmarski

et al., 2005). Distinctiveness refers to the pres-

ence of individual marks. Each high-quality

photograph of the dorsal fin was assigned a

distinctiveness (D) value between 0-4. Zero is

when a dorsal fin is smooth, with no appar-

ent markings, and four is a dorsal fin that is

conspicuously distinctive, e.g., a malformed

or partially mutilated fin. Individual dolphin

identification in this study was based on the

presence and distribution of marks (nicks and

notches) on the dorsal fin profile. Other natu-

ral markings such as discolorations, scars, and

spotting patterns were not ruled out but played

a secondary role. Only dorsal fin photographs

with quality ≥ 70 and distinctiveness from

1 to 4 were considered in this analysis. The

construction of the catalog was carried out in

the Discovery program (Galey & Karczmarski,

2012), which allows the processing and selec-

tion of the photographs based on the criteria

described above. It also associated the sighting

data with the identified dolphin and facilitated

the construction of a matrix with the capture

history of each individual (the basic input for

the mark-recapture analysis).

Analysis of population size and demog-

raphy: The capture-recapture matrices were

used to estimate the population size and char-

acterize the demography of pantropical spot-

ted dolphins in Golfo Dulce, using a “robust

design” model (Pollock et al., 1990). This model

involves primary and secondary levels of sam-

pling. The robust design integrated a series of

short-time closed models at the secondary sam-

pling level, where the effect of sampling time

and individual heterogeneity on the probability

of capture was controlled. The demographic

closure models were connected by a frame

model at the primary sampling level, which

released the demographic closure, assuming a

population where births, deaths, as well as the

entry of immigrants and the exit of emigrants

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

were considered temporally. This allowed us

to infer dynamics associated with metapopula-

tions (migratory movements in the absence of

physical geographic barriers), following Kend-

all & Bjorkland (2001). For the secondary level,

the sampling occasions were unified by month

within the seasons, while the primary occasions

were focused explicitly on the seasons (Rainy:

June-October, Dry: November-May).

The above demographic aspects were met,

pursuant with the validation of several basic

assumptions of the mark-recapture analysis,

under the robust design approach: (a) marks

are not lost; although they can be modified,

they are sufficiently distinctive to guarantee

individual identification; b) samples are instan-

taneous at secondary levels; c) given a sampling

occasion, all individuals in the population have

the same probability of capture; d) survival of

marked individuals does not vary from one

capture occasion to the next at primary levels

(seasons) of demographic openness; e) capture

occasions at secondary levels (months) are

with demographic closure within each pri-

mary level: and f) captures are independent

among individuals without aggregation and

overdispersion effect.

All models considered were analyzed using

the Mark software interface in the “R” environ-

ment (R-Mark Laake et al., 2013). The following

demographic parameters of pantropical spotted

dolphins in Golfo Dulce were estimated:

(Sj) apparent survival (does not refer to biolo-

gical survival, but demographic survival

by presence) at the first season j for j>1.

(γ’’) probability of emigrating before season j,

being present in j-1, for j>1

(γ’) probability of emigrating before season j,

being absent in j-1, for j>2

(pij) probability of the first capture in sample i

of season j for i ≥ 1

(cij) probability of recapture in sample i of

season j for i > 1

(Nj) the population size of marked individuals.

For this study, we set pij = cij, since pho-

toidentification, being non-invasive and not

promoting adverse reaction, did not directly

affect the probability that a previously identi-

fied dolphin is recaptured (Parra et al., 2006).

Three classes of temporal emigration models

were evaluated: 1) Markovian emigration (γ’

γ’’), where the probability of a dolphin being

present at Golfo Dulce would be conditional on

presence or absence on the previous sampling

occasion (Kendall & Nichols, 2002; Kendall

et al., 1997); 2) random emigration (γ’= γ’’),

where the probability of a dolphin being pres-

ent in the gulf is independent of presence or

absence on the previous sampling occasion; and

3) no emigration (γ’ = γ’’ = 0), where there is no

movement out of the gulf.

Before the construction and evaluation

of models by robust design, the fitting of the

capture history matrix was corroborated, using

a goodness-of-fit test in the Release GOF pro-

gram. The lack of fit to temporality led to the

estimation of a variance inflation factor (ĉ),

which levelled the dispersion of the data. Thirty

models were constructed and selected using the

Akaike selection criterion (AICc). Considering

the dispersion implied by the variance inflation

factor (ĉ) the QAICc was used under the cri-

terion that models with a difference expressed

by Delta-QAICc greater than 10 lacked support

(Burham & Anderson, 2004), and those less

than two should not be discarded. To obtain

the total population size, the number of marked

individuals ( ) was related to the proportion of

marked individuals in the sample ( ) (Wilson

et al., 1999):

The variance was estimated as follows:

Where n is the total number of individual

dorsal fins for which was estimated. The coef-

ficient of variation for the total population CV

( total ) could be defined as the sum of the

coefficients of variation of and :

The standard confidence interval could

unrealistically set the lower bound to zero.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

Therefore, we followed Burnham et al., (1987)

using a log-normalized confidence interval,

such that the lower limit is given by L = /r,

and the upper limit by U = r.

For a 95% confidence interval, r would be

given by:

Where (1+(CV( total )) 2 is an approximation

of var(ln total ).

The variation of the total population size as

a function of the seasons was explored using the

non-parametric test Kruskal Wallis at a signifi-

cance level (alpha) of 0.05.

RESULTS

There were 201 surveys for photograph-

ic capture of pantropical spotted dolphins in

Golfo Dulce from 2011 to 2014, equating to

1 102 hours over 25 months and a travel effort

of 18 730 km (Fig. 1). Details are shown in

Table 1.

A total of 26 352 photographs were taken

and analyzed, resulting in the identifica-

tion of 280 individuals, 65 % of which were

recorded only once. The rest of the dolphins

cataloged showed an important level of recap-

ture (98 individuals). The discovery curve, or

the cumulative curve of entries into the cata-

log by sampling occasion, shows three trends

(Fig. 2) that could explain the dynamics above:

a non-asymptotic curve that incorporates all

identifications, including those with individu-

als only captured once, and distinctiveness

equal to one. Another curve becomes asymp-

totic only with distinctiveness greater than one,

and finally, a nearly constant curve of individu-

als cataloged with very distinctive fins (D > 3).

For pantropical spotted dolphins the good-

ness-of-fit test showed significantly high dis-

persion (χ2 = 22.81, df = 8, p = 0.001). This

is mainly due to the significant effects of the

probability that capture-recapture is affected

by individual heterogeneity, which is duly

addressed by the robust design models. Due

to the degree of dispersion, a variance infla-

tion factor was estimated (ĉ = 22.81/8 = 2.85).

Thirty models were generated from the capture

history of individuals cataloged under the crite-

ria of quality > 70 and distinctiveness ≥ 1.

The best-fitted models were those of

apparent survival by sex group and the varia-

tion in the probability of capture due to het-

erogeneity during both seasons; among these,

the best fit was that of no emigration (ΔQAICc

= 0.00), followed by the random migration

model (ΔQAICc = 1.72). Based on these two

scenarios the population size differed in coef-

ficient of variation (Table 2). For the no-emi-

gration models, the coefficient of variation

remained between 7 – 10 % (Table 3), and for

the random-emigration models, the coefficient

of variation was between 7 – 12 % (Table 4). If

the spotted dolphin population in Golfo Dulce

is considered under a pattern of no movement

out of the study area, the population size of the

Table 1

Details of the search effort and the identification photographs included in the pantropical spotted dolphin catalog, including

level of distinctiveness: D= 1, D≥ 2, D> 3.

Season No. Months No. Surveys No. Hours No. Cataloged IDs (New)

D = 1 D ≥ 2 D ≥ 3

Rainy 2011 3 28 143 17 22 24

Dry 2011_2012 5 18 94 74 (57) 59 (37) 25 (1)

Rainy 2012 4 44 263 89 (15) 65 (6) 25 (0)

Dry 2012_2013 4 29 155 111 (22) 71 (6) 25 (0)

Rainy 2013 5 56 291 134 (23) 72 (1) 25 (0)

Dry 2013_2014 4 26 156 182 (48) 73 (1) 25 (0)

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

Fig. 2. Discovery curves for the coastal pantropical spotted dolphin in Golfo Dulce: A) The blue curve represents all cataloged

individuals, including those recorded only once. B) The orange curve contains only the individuals with dorsal fins with D>1.

C) The gray curve contains the individuals with dorsal fins classified under a distinctiveness level D>3. The bottom panel

shows representative photographs of distinctiveness levels, from left: one (blue), two (orange), and 4 (gray).

Table 2

Selected best-fitted models generated from the capture history of pantropical spotted dolphins in Golfo Dulce under the

Robust Design. S= apparent survival, P= capture probability, Het= heterogeneity, SC (CI) = apparent survival (confidence

interval) for females, SF (CI)= apparent survival (confidence interval) for males, p-hat = capture probability, ψ ’ = ψ ”

emigration probability. The notation ‘(.)’ implies that a given parameter was kept constant.

Models

Classification Criteria Demographic Parameters

No.

Parameters QAICc ΔQAICc QAICc

Weighted S

(CI) S

(CI) p-hat ψ’=ψ’’

S (. Sex) P(Het) No-Emigration 11 266.94 0.00 0.64 0.98 (CI: 0.81-0.99) ≈ 1 (CI: 0.99-1) 0.10 0.00

S (. Sex) P(Het) Random Emigration 12 268.67 1.72 0.27 0.98 (CI: 0.68-0.99) ≈ 1 (CI: 0.99-1) 0.12 0.16

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

species varies between 217 to 422 dolphins.

For the random emigration model, the popula-

tion size ranges from 187 to 368 individuals.

However, these differences are not significant

(Kruskal Wallis: χ2 = 2.08, df = 1, p = 0.150),

and similarly, there is no statistical difference in

the population size of pantropical spotted dol-

phins in Golfo Dulce between seasons (Kruskal

Wallis: χ2 = 0.05, df = 1, p =0.827).

DISCUSSION

The demographic analysis of coastal

pantropical spotted dolphins in Golfo Dulce

through capture-recapture modeling resulted in

the selection of two models, one where emigra-

tion was null and another one that suggested

fluid dynamics of entries and exits to the popu-

lation of pantropical spotted dolphins in Golfo

Dulce, independent of the season evaluated.

No matter the model, we found the population

size of spotted dolphins in this gulf does not

exceed 400 individuals. These results indicated

that at least a portion of the population, equat-

ing to more than 90 individuals, remains for

extended periods as demonstrated by a high

level of recaptures and suggesting site fidelity

throughout the year, particularly in the inner

basin (Acevedo & Buckhart, 1998; Cubero-

Pardo, 1998; Cubero-Pardo, 2007a; Oviedo,

2007; Oviedo et al., 2015, Oviedo et al., 2018).

The study further indicates that the conditions

associated with seasonality do not affect the

population size and demographic patterns of

spotted dolphins in Golfo Dulce. The discovery

curves (Fig. 2) suggest that dolphins with low

dorsal fin distinctiveness may not be readily

recaptured. Hupman et al. (2018) found low

Table 3

Population size of the pantropical spotted dolphin in Golfo Dulce under the no-emigration robust design model. ⊖ =

proportion of individuals tagged per season, N-Marked= number of marked individuals, N-Total= Total population size,

CI = Confidence Interval, CV Coefficient of Variation, C = dolphins identified as females, F = dolphins identified as male.

Seasons Cluster ⊖N-Marked N-Total CI CV

Rainy 2011 C80.51 203.82 189.95-218.71 0.07

F0.79 58.51 148.13 136.30-160.99 0.08

Total 351.95 326.24-379.70 0.07

Dry 2011-2012 C81.38 235.88 220.95-251.82 0.07

F0.69 64.38 181.61 173.35-200.88 0.07

Total 422.49 394.30-452.71 0.07

Rainy 2012 C52.21 132.18 121.07-144.30 0.09

F0.79 38.21 96.73 87.22-107.28 0.10

Total 228.91 208.29-251.59 0.09

Dry 2012-2013 C47.48 137.62 126.30-149.96 0.09

F0.71 55.16 99.94 90.30-110.62 0.10

Total 237.57 415.42-461.15 0.09

Rainy 2013 C55.16 139.65 128.26-152.04 0.09

F0.80 42.16 106.73 96.80-117.69 0.10

Total 246.38 225.06-269.73 0.09

Dry 2013-2014 C42.42 122.96 112.27-134.66 0.09

F0.68 32.42 93.97 84.63-104.34 0.10

Total 216.93 196.90-239 0.09

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

dorsal fin distinctiveness in oceanic dolphins,

such as Delphinus sp., to be associated with a

low level of recaptures of identified individuals.

For the study population, apparent sur-

vival derived from the best-fitted models was

relatively high. It only decreased slightly for

females. When considering the no-immigration

model, the survival of female pantropical spot-

ted dolphins in Golfo Dulce would be above

reported biological survival levels (Brooks et

al., 2017; Taylor et al., 2007) and is supported

by the low relative width of the confidence

interval. However, it should be considered that

the model based on non-immigration only

describes a partial reality, focusing only on

those individuals that develop long-term site

fidelity. The model that considered random-

type migration seemed closer to what was

recorded in the capture histories of photo-

identified individuals. Under this approach,

the survival of females did not change in mag-

nitude, but it did change in precision, as the

width of the confidence interval was larger.

Additionally, under this model, a comparatively

low probability of emigration was established,

which would support the dynamics of visiting

individuals and resident individuals sharing

the same seascape, as has been documented

in coastal dolphin populations in Australia

(Brooks et al., 2017), though with no apparent

effect of seasonality.

The physiography of Golfo Dulce as a

basin, or semi-enclosed internal sea, supported

expectations of some demographic closure in

the spotted dolphin population. Indeed, even

during in situ sampling, several individuals evi-

denced a notable degree of recaptures, although

there was no physical barrier to effectively pre-

vent their departure from Golfo Dulce. In addi-

tion to the differences in individual behavior

Table 4

Population size of the pantropical spotted dolphin in Golfo Dulce, under the robust design random emigration model. ⊖

= proportion of individuals tagged per season, N-Marked= number of marked individuals, N-Total= Total population size,

CI = Confidence Interval, CV Coefficient of Variation, C = dolphins identified as females, F = dolphins identified as male.

Season Cluster ⊖N-Marked N-Total CI CV

Rain 2011 C70.67 178.91 165.88-192.97 0.08

F0.79 48.67 123.22 112.35-135.14 0.09

Total 302.13 278.23-328.10 0.08

Dry 2011-2012 C71.96 208.58 194.51-223.67 0.07

F0.69 54.96 159.30 147.00-172.64 0.08

Total 367.88 341.51-396.31 0.07

Rain 2012 C45.8 115.95 105.43-127.40 0.09

F0.79 31.8 80.51 71.78-90.29 0.12

Total 196.46 177.31-217.69 0.11

Dry 2012-2013 C41.89 121.42 110.77-133.09 0.09

F0.71 28.89 83.74 74.87-93.66 0.11

Total 205.16 185.64-226.75 0.10

Rain 2013 C48.85 123.67 112.95-135.41 0.09

F0.80 35.86 90.76 81.57-100.98 0.10

Total 214.43 194.52-236.40 0.09

Dry 2013-2014 C37.31 108.14 98.11-119.20 0.10

F0.68 27.31 79.16 70.55-88.81 0.12

Total 187.30 168.67-208.02 0.11

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

affecting the probability of capture (hetero-

geneity), there is nevertheless the possibility

of fluid transience, which rules out the non-

immigration that framed the best-fitted model

(probability of capture as a function of indi-

vidual heterogeneity, constant survival, and no

emigration). Consequently, the model where

the probability of capture was fitted as a func-

tion of individual heterogeneity with constant

survival, under random emigration was consid-

ered the most parsimonious (Delta QAICc ≤ 2).

The seasonal survival estimate defined the

true survival of pantropical spotted dolphins in

Golfo Dulce, as well as the respective trend of

random emigration, based on the high prob-

ability that the survival of an adult remains

constant over relatively short periods when

compared with its life span (≈ 45 years is the

oldest age of a reproductive female) (Taylor et

al., 2007). Notwithstanding a catastrophic event

affecting pantropical spotted dolphin survival

over the study period, emigration patterns are

expected to be associated with individual het-

erogeneity and apparent survival estimates.

The seasonal survival estimate is relatively high

(0.99 +/- 0.01), but similar to that expected for

other adult and sub-adult (non-calf) dolphins.

For example, spinner dolphins (S. longirostris)

from Hawaii have a survival of 0.97 +/- 0.05

(Tyne et al., 2014), and Mediterranean Sea com-

mon and striped dolphins, Delphinus delphis,

Linneaeus, 1758-Stenella coeruleoalba (Meyen,

1833) of 0.94 +/- 0.05 (Santostasi et al., 2016).

Pantropical spotted dolphin population

size apparently fluctuated between seasons, but

this pattern was not significant. The popula-

tion size of pantropical spotted dolphins in

Golfo Dulce is a relatively discrete number

considering that groups larger than 1 000 indi-

viduals have been documented outside the

gulf (Authors’ unpublished data 2005-2022).

Fluctuations in population size are expected to

be primarily influenced by the availability of

resources in Golfo Dulce, especially given the

presence of another delphinid, the inshore bot-

tlenose dolphin (T. truncatus), which, as anoth-

er marine predator, exerts additional pressure

on available prey resources (Cubero Pardo,

2007a; Oviedo, 2007; Oviedo et al., 2018).

Based on the above, potential prey avail-

ability in Golfo Dulce would be the primary

determinant for the aggregation pattern, site

fidelity, and movement of this species in the

study area, as proposed by Gowans et al. (2007)

for several dolphin species with an intermedi-

ate home range pattern. Those authors argue

that variations in home range patterns reflect a

gradient of resource availability from predict-

able to extremely variable. Golfo Dulce may

be a relatively predictable resource locality for

pantropical spotted dolphins in terms of prey

availability, and considering the availability

of females for breeding, and refuge from pre-

dation. Relatedly, a study by Marin-Alpizar

(2011) found small pelagic fish of the family

Hemiramphidae to be present in this gulf year-

round. If this fish is an important source in the

diet of spotted dolphins, it may support not

only the resident dolphin population but also

be available to transient dolphins. To expand

our understanding of the habitat-use patterns

of these dolphins, future studies should incor-

porate spatial and temporal variations of their

prey into their models.

It is important to recognize that individual

differences in behavior, as well as the possibil-

ity of each dolphin leaving the Golfo Dulce,

influenced variation in survival and probabil-

ity of capture. To minimize the associated

bias during field sampling, efforts were made

to provide an equal distribution of individual

photographic capture probability within each

spotted dolphin encounter. Still, the grouping

pattern of the dolphins in this gulf, especially

in group sizes of ≥ 100 individuals, may have

affected photographic coverage. Additionally,

the sample size could also have been affected by

our strict selection and analysis protocol. Given

the increased human impacts in the study

area (Herra-Miranda et al., 2016; Pacheco et

al., 2016), effective conservation and manage-

ment of pantropical spotted dolphins and other

coastal/inshore cetaceans within Golfo Dulce

require the recognition of discrete populations

as management units. Therefore, to facilitate

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57287, diciembre 2023 (Publicado Nov. 01, 2023)

the detection of population-level changes over

time, it is important to effectively determine

the demographic parameters, as estimated here.

Ethical statement: the authors declare that

we all agree with this publication and made sig-

nificant contributions; that there is no conflict

of interest of any kind; and that we followed

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 contributions: LOC conceptual-

ized the study. LOC, DHM, and JDPP designed

boat surveys, carried out field data collection

and data analysis, coordinated data manage-

ment, and wrote this manuscript.

ACKNOWLEDGMENTS

This study was made possible by the valu-

able support of a major citizen science frame-

work promoted by the following institutions: 1)

International Student Volunteers (2011-2013);

we appreciate the support of Wagner Quirós, 2)

Earthwatch Institute (2013-2023) especially the

following internal funding scheme: Arunas A

& Pamela A Chesonis Family Foundation and

Gaye Hill & Jeff Urbina (2013-2014) and Gaye

Hill & Jeff Urbina (2015-2016). Special thanks

to each volunteer that accompanied us on each

field survey and observation. Special thanks

to our CEIC colleagues: to “El Chontal” fam-

ily Jorge Medina and Azucena Herra-Miranda

for providing us with our base at Rincón de

Osa. A special thanks to our captain “Taboga”,

Dr. David Aurioles, Dr. Hector M. Guzman,

and Dr. Leszek Karczmarski for their valuable

academic support. We appreciate the additional

comments from our colleagues Dr. Brooke

Bessesen and Msc. Phoebe Edge.

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