Nauplii production of white shrimp , Litopenaeus vannamei ( Decapoda : Penaeidae ) , on-board of commercial trawlers

Resumen: Los laboratorios no han logrado producir suficientes nauplios de buena calidad por lo que sigue siendo necesario recolectar hembras maduras en alta mar. Sin embargo, no siempre se recolectan hembras en la condición inseminada. En este estudio, se evaluó la recolecta de hembras y producción de nauplios abordo de un barco camaronero durante un viaje comercial como una alternativa para el larvicultivo de Litopenaeus vannamei en el Golfo de Tehuantepec. Se efectuaron arrastres en la tarde hasta el anochecer. El mejor tiempo de arrastre fue de 20 rnin y produjo una captura media de 37 ± 8 camarones adultos con una sobrevivencia de 72 ± 3 %. Se observaron desoves desde las 11:00 PM hasta las 02:00 AM, con un máximo a las 12:00 PM. Se siguieron " 14 desoves en cinco días, durante los cuales se produjeron 2.2 x 106 nauplios con una tasa media de fertilidad y de eclosión de 83 y 75 %, respectivamente. Este trabajo presenta una técnica que puede proveer de simientes para el larvicultivo a partir de los productos reproductivos de camarones capturados por la flota camaronera comercial.

The Mexican Pacific shrimp fishery is among the most important fisheries in Latin America (Magallón-Barajas 1987) and the Mexican shrimp culture production ranks second I n the continent.Oceanic fishing season lasts from September to March.In these months, sexual maturity is reached and usually commercial shrimp boats trawl on ripe shrimp sub-populations, as in the Gulf of Tehuantepec, where reproduction occw-s year-round.This practice decreases the reproductive output potential and leads to slow stock depletion (Salinas-Orta, pers.obs.).Reproductive output can be saved if commercial shrimpers recover a minimum amount of females discarded on its decks and spawned Jater under known rearing techniques, In tbis way, eggs can hatch and naupliar stages can survive in small-scale hatcheries located on the coast.From this point of view, small-scale hatcheries could circumvent the search for ready-to spawn females (Bray et al. 1982, Bray andLawrence 1992).None of the many attempts to produce nauplii onboard commercial shrimp boats has reported successful trials (Beltrán-Félix 1987).Failure refIects ignorance of reproductive behavior under stress and the lack of equipment adapted to the conditions at sea.This study aims to assess the spawning and hatching potential of white sQ¡-imp Litopenaeus vannamei on-board a shrimp trawler equipped with seawater treatment and aeration systems during a commercial cruise in the Gulf of Tehuantepec.
The study was carried out in the inner Gulf ofTehuantepec (14°50' _16° N, 92°-96°40' W) between August 14 and 28, 1994, the onset of maximum reproduction in this area.A comn'lercial shrimp trawler equipped with 25.5 m wide doubler igged nets with 5 cm mesh (Type Phantom) and a 365 HP engine was used.Five 400-1 square plastic tanks covered by PVC can vas s were arranged on the upper deck (8 m 2 ) for nauplii rearing.Three 400-1 square plastic tanks were placed on the main deck as female conditioning tanks.Seawater was pumped directly from the ocean and passed though a 5-l1m mechanical filter, and an activated carbon and UV filter.Air was supplied through fine pore diffusers from a 1/2 HP blower.During the experiments, the sea water was 28-32 oc and 34 ppt.Water renewal in tanks was done twice daily, when pertinent.
Trawlings at depths ranging from 16 to 25 m were made every day after sunset (17:00-20:00), as recommended by Bray and Lawrence (1984).When a fishing ground with abundant adult shrimps was found (20-30 kg per hour of trawling), five consecutive trawls lasting 10 to 40 mio were made during 5 days (N=20) to assess catching rate and survival after one hour of trawling, since these times do not prod u ce severe injuries to shrimps (Beltrán-Félix 1987).According to Bray and Lawrence (1984), females with the fulI spermatophore attached on the thelycum or mated females (MF) with body weights ranging from 35 to 45 g were segregated during five consecutive days of cruising, counted, and then confined in conditioning tanks.After one hour, twenty randomly selected MF from 20 min trawling time were contained in spawning tanks on the upper deck, to evaluate the time of spawning.
When the technique of MF rearing was evaluated, egg and nauplii rearings were done similar to Wyban and Sweeney (1991) and Treece and Fox (1993).Spawnings with fertility rate (the proportion of no r mal eggs related to the total egg produced by each MF) higher than 80% were folIowed to naupliar stages (N=14).Development of eggs and nauplii was checked hourIy and daily with a disecting microscope, respectively, using criteria presented by Treece and Fox (1993).
Triplicate confinement trials were made to assess the survival of females transported at different densities in 400-1 tanks (using the equipment described aboye) during 24 a n d 48 hr navigation periods.Confinement tests were designed to evaluate the followings densities, usualIy used in sourcing: High (145 shrimps/m 2 ), Medium ( 127 shrimps/m 2 ) and Low (91 shrimps/m 2 ).
The trawling times of shrimping nets towed in the present study produced a mean catch ranging from 23.8 ± 8 to 101 ± 17 adult shrimps per trawling.The highest catch was 40 min and the smallest was 10 mino The mean survival decreased as trawling time increased; 10 and 20 min trawls produced the highest survival (Fig. 1).Mated females (MF) represented 40% of the individual catch, therefore, minimum amounts of MF needed for spawning were achieved.Therefore, 20 min trawlings were carri ed out to source mated females for successive experiments.
Mated females spawned from 10:00 PM until 03:00 AM, with a peak at 12:00 PM.In fact, 50% of the MF spawned during midnight, from 11:00 PM to 02:00 AM (Fig. 2).Due to spawning time, special care during this interval was given to optimize the nauplii production.-------------------------- A total of 2.2 x 106 nauplii were produced from females naturally mated (N=14) during four days of effective commercial fishing.Mean fertility rate of MF varied from 75 ± 5 to 87 ± 7 %.Mean egg production ranged from 122 ± 26 to 244 ± 40 x 103 eggs per spawn.Mean hatch rate varied from 61 ± 11 to 90 ± 5 %, with a mean yield ranging from 83 000 to 189 000 healthy nauplii per spawn (T able 1).Ashmore (1988), Wyban and Sweeney (1991), Bray & Lawrence (1992) and Robertson et al. (1993), under controlled conditions on the mainland, reported lower values: their mean fertility rate for L. vannamei varied from 40 to 70% and mean hatching rates ranged from 32 to 66. Differences are probably caused by age, eyestalk ablation, nutritional status, genetic performance and enviromental conditions (Bray & Lawrence 1992).Offshore spawning under the condition of ibis satisfies nauplii production needs for a small-scale hatchery.However, interannual variations of MF abundance must be monitored regularly to estimate how many MF could be caught using this approach.
Mature female transport is a difficult task for larviculture operations.High mortality can occur due to stress caused by trawling and confinement (Beltrán-Félix 19 87).Low density confinement trials (91 shrimps per m2) during transport on a shrimp trawler produced highest survival in 24 hr.Survival at 48 hr of transport at this density yield no further mortality (T ukey's HSD tests p <.05).Since the nearest ports are 24 hr from the main spawning ground, low and medium density confinements are adequate.
This study shows the great potential of spawning mated shrimp females on-board of shrimp trawlers during commercial operations.Nauplii production is a reliable and alternative source on-board.The possibility of recovering wild reproductive outputs in places where fertilized products are continuously discarded during shrimping season in tropical waters represents a valuable resource for the shrimp larviculture.
Secretaría de Pesca (SEPESCA-México) authorized Permiso de Pesca de Fomento No. 1998 granted to the frrst author.Oswaldo Morales, Alonso Medina, and crew of FV Suriano XV provided technical assistance during cruise.Anonymous reviewers improved the English version.

Fig. 1 .
Fig. 1.Shrimp catch per trawling time ánd the survival of shrimps cau g ht in each trawlin g time.Values are means :!: standard deviation (SD).

Fig. 2 .
Fig. 2. Incidence time of spawnings of mated shrirnp females caught in the Gulf of Tahuantepec.Val ues are means ± SD.
Mean statistics ( ± SD) for spawns of white shrimp Litopenaeus vannamei on-board of cornrnercial shrimp trawler in