Effect of shrimp trawling on the commercial ichthyofauna of the Gulf of Nicoya, Costa Rica

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(1)Effect of shrimp trawling on the commercial ichthyofauna of the Gulf of Nicoya, Costa Rica Jorge A. Campos, Bernal Burgos and Carlos Gamboa Escuela de Biología y Centro de Investigación en Ciencias del Mar y Limnología. Universidad de Costa Rica (Received for publicatían February 6 , 1984) Abstraet: Shrimp trawling and industrial fishing has beeo limited to the outer Gulf of Nicoya, Costa Rica, since 1966 to prateet spawning and breeding areas in the inner Gulf. Fish length frequency data gathered in several cruises and data on size at first sexual maturity were used to evaluate this sector as a nursery area and to assess the effects of shrimp trawling on commercial fishes in the auter Gulf. Results show that the moer Gulf serves as a nursery area at least fo! nioe species. Ten ar more of the fish species caught by shrimp boats in the outer Gulf are of a size at capture smaller than that at first sexual maturity. A management option is offered to minimize the stress exerted upon these fish populations.. The Gulf of Nicoya (Fig.. 1). MATERIAL AND METHODS. is the major. tropical estuarine system of the Pacific coast of Costa Rica and according to Blair ( 1 979). Nine stations were sampled on lhe eastern. one of the most important bodies of water in. shore of lhe Gulf of Nicoya (Fig. 1) using a. lhe Pacific coast of Central America. Shrimp trawling as weU as finfish exploitation talce. 30 foot,. place both on artisanal and industrial bases and. 1 .5. towed for. inch stretched mesh shrimp trawl, 10 minutes at each station from. March to November 1982. The fish were kept. harvest mainly penaeid shrimp and over seventy. in cold storage on board and transported to. cornmercial finfish species. The inner Gulf has been elosed to industrial. lhe. laboratory where they were identified,. measured (total length), weighed (wet weight). fishing-lhus exeluding shrimp trawling-for lhe. and dissected for estimation of gonadal ripe. past 1 7 years, since it is considered a nursery. ness (CAM in Table 1).. area for shrimp and fmfish species. This man agement strategy was taken to protect the. to the smallest female found with ripe gonads. faunal assemblages. Shrimp trawling is permited in lhe outer Gulf and personal experience on board shrimp. In Table 1, size at frrst maturity corresponds according to the scale provided by Laevastu (1971). The sizes provided for each species under each gear type and location are the. trawlers indicates lhat many " smaU" finfish species are discarded as by-catch on a regular. midpoints of the modal elasses. This is a very. basis in lhe outer Gulf (Campos, 1981).. each of the gears utilized.. This. study. is based on several research. simple approach to compare the selectivity of Aside. from our data, lhe following data. cmises in the Gulf of Nicoya where length. sources were utilized to build a comparative. frequency data were coUected. Our goal is to. table (Table. investigate whelher the inner Gulf is in fact a nursery area for fish species and to explore. 1):. Lenglh data coUected in 1 98 1 , o n a monthly. possible types of stress that shrimp trawling. basis for lhe commercial finfish fishery of. in lhe outer Gulf may pose for the commercial. lhe Gulf of Nicoya, by lhe Department of. ichlhyofauna.. Fisheries of lhe Ministry of Agriculture ..

(2) REVISTA DE BIOLOGIA TROPICAL. 204. · - 1 0 00'. 1 0· 0 0 _. /l ______ <j 16. 1. � P :::i: �=;� O !"'"-__. ..'. 15. KM. o u te r. N I C O YA. Fig. 1. Location of stations samplcd in thc Gulf of Nicoya, Costa Rica.. These data were eolleeted for the 3ló gillnet (T 3\1), the 6' gillnet (T 6') and longline plus hook and line data (L+C). Length data from a thesis by León (1973) in the Gulf of Nieoya. Part of these data were published in his work listed in the refer· enees (p.L. in Table 1).Thiswork is referred to as P.L. Length data eollected in two eruises-dry and rainy season- in the Gulf of Nieoya and. published by Maurer el al. , in Table 1).. ( 1 980; DEL. The available data were analyzed separately for the inner and outer Gulf to better compare what effeets trawling may have upon the cornmercial ichthyofauna. Since in most cases the 3W gillnet is seleetive for the smaller size speetrum of the speeies under study, eateh data with the 3%' gillnet were eompared with the trawI data, also selective foc small species, in.

(3) 205. CAMPOS et al.: Effect of shrimp trawling on commeIcial ichthyofauna. TABlE I Compararive rabIe showing rhe size offisll and rhe gear urilized by rile anisana/ finfish fishery and by trawlers in rhe jnner and ourer Gulf oiNicoya, Costa Rica lnner Gulf T 3Yl. " 2l. Anisotremus dovii. 27.5. Paralonchurus dumerilli. 32.5 42.5. 760. 27.5 27.5. 16 19 11. Cynoscion phoxocephalus. 32.5. Nebris occidentalis Anisotremus pacifici Sphyraellg ensjs Cylloscioll albus Cynoscion squamipinnis C)I/IOSciOll stolzmanlli. 46. T 6'. L +C. ". 3. 28.5. 39.5. 5. 37.5. 182. 40.4. 55. 42.5. 41. 7.5 11.5 25.5. 87 11. 47.5. 251 328. 52.5. 50.1. 109. 32.5. 458 17. 62.5 29. 436 1. Ophioscjon sciera Polydacly/us panamensis Menticirrhus nasus. 2 7.5 2 7. 5 27.5. 37.5. 34. 6 16. 26.5. Lorimus occfivis lsopisthus remifer Lut¡"onus guttatus eylloscion reticulatus Opllioscion imiceps. 30. 34 .5 45. 57.5. 68. 68. 52.5 45.4 62.5 67.5 .. 5. l8. 2. 3. 90. 47.5 37.5. 5. 2. 36.5. 6 5 112 66. 27.5. 64. 28. 2. 29.5. 23. 27.5 13. 64 51. 24.3. 3. 76 270. 1. 3. 2 1 .5. 42.5. 4 1. 25.5. 15. 17.5. 3 26 1. 12.5. 4 111. 32.5. 81. 8. 1 1. 5. 48. 34.7. 1 2.5. 13 289. 15 42.5. 26.5. 2 7.5. ". 29. 29.5. 28. DEL. 15. 225 75. 1. 27.5. 49. 41. 21. 28.5. 7. 588. Polydact)'lus approximalls. ". 10. 42 1 700. 20. CAM. 37.5. 35. 45 45.5. Micropogonias altipinnis Eucinostomus gracilis. ". 34. 10.5. 8.5. 16. 2. 1. 19.5. 1 69. 5.5 17.5. 8l. 31.5. ". P.l.. ". 4. OUler Gulf T 3Ió. " 3. T 6'. " 1 1. Cynoscion plwxoeephalus" 42.5 Neóris occidentalis" 21 Anisotremus pacifici". 183. 42.6 5 2 .5 39.3. 4. 44.5. Cynoscion a/bus ". 10 175. 67.5. 42.2. 38. 97. 12.5. 207. 57.5. 6 253. Anisotremus dovii". la. Paralonehurus dumerilli". Splt)'raena ensis". Cynoscion squamipinnis" Cynoscion stolzmanni" Mieropogonias alripinnis" Eucinostomus gracilis" OpMoseion sciero Polydoctylus approximans " Polydoctylus panamensis". 42 .5 47.5. 4 7.5 54.7 37.5 27.5. 27.5 25. 13. 28 2l. 32.5. 20 6. lsopisthus remifer". 32.5. 15. Ophioscioll imiceps ". 32.5. 4 2.5 3 2 .5. •. CAM. 27.5. 12. 26.5. 37.5 42.5. 5. 36:5 5 10. 87. 35 52.5 47 .5. 29. 57.5. 6 30. 57.5. 370. 8. 8. 7 7 284 23 8. 10.5 13. 208. 103. 37.5. 24.8. 52.5. 1 71. 34.9. 39.7. 60. 37.5. 2. 4 60. 20 1 1 .5. 24.5 12.5. 14.5. 55. 27.5. 25. 17.5. 3 99. 30. 52. =. 7.5 14. 2. 24. 47 11. 345. JJ. 86 38. 623 985 94. gi!lnet 6 inches stretchcd mesh; l. study in the Gulf (see text);DEL=two cruiscs in Ihe Gulf (see text) and P.l.. 7. 18.. 19. 42.5. T 3lh' = giUnet 3M inches stretched mesh; T 6. 31 1 3.5 18. 32 19. Cynoscion stolzmalllli. 30.5. 17 23. 22. JO. Micropogonias altipinnis. Ophioscioll imiceps. 22 85. ". 35. 24.5. 423. 1 10. 13. 38. Isopistllus remifer. 21. 639. 257. Cynoscioll albus CynosciOll squamipinnis. Larimus acc/ivis. 35 9 23.5. 303. 28 21. 50. 9.5. 33.5. 525. Memici"lIus nasus. 1 0.5. 66. 204. Polydactylus approximans. 349. 24. 23.5. Ophioscion sciera. 2. 2l. 25.5 30.5 29.5. 6.5. 27.5. Cynoscion phoxocepha/us. 24.5. 51. 39. Anisotremus dovii Paralonchurus dumerilli Nebris occidentalis. ". 7.5. 24. Size al First sexual maturilY. Anisotremus pacifici. 20. 1. 11.5. 7.5 14. 20. DEL. 5. 14.8. 37.5. Cynoscions reticulatus". 6. ". 4. Mentici"hus nas/ls" IArimus accliv;s" Lurjanus guUofUS". 25.5. 13. L +C. =. +C = long line plus hook and Une; CAM =a. the study by Pedro León (1973; see tex!).. ycarly.

(4) REVISTA DE BIOLOGIA TROPICAL. 206. arder to evaluate trawling impact to the ichlhy·. ing should continue to be prohibited in the mner Gulf since it seems to serve as a nursery. ofauna under study.. for juveniles of various commercial fish species; olherwise growth overfishing could take place. RESULTS AND DlSCUSSIQN. or be enhanced. Inner Gulf: An analysis of Table 1 shows Outer. that when catche s from all lhree, or at least. Gulf:. The. same. general. pattern. two gears of the spec.ies Hsted are compared.. observed in lhe inner Gulf is present here .. 65%. Nineteen (identified with an asterisk in Table. are captured at smaller sizes with the. 3\\' gillnet than with the 6' gillnet, the longline. 1 ) out of the 20 species listed show smaller. or lhe hook and line. Trawling data (CAM or DEL) show that for 90% of the species listed in Table 1 , the sizes of the specimens cap·. sizes when eaptured by trawl nets lhan when size of lhe organisms captured, when the T 3y,'. tured are smaller than the ones captured with. is compared with CAM or DEL lhe outer Gulf. the other gear.. functions as a nursery area for : CynosciofJ phoxocepha/us, C. squamipinnis, C. reticu/a/us, Para/onchurus dumerilli, Aniso/remus pacifici, Micropogonias altipinnis, Eucinostomus gracilis, Menticirrhus nasus, Larimus acclivis, lsopisthus remifer, and Ophioscion imiceps. Tentatively included are: Sphyraena ensis, Cynoscion a/bus, C. sto/zmanni, and Lutjanus guttatus. When. Fa! SOrne species, the difference in the sizes captured by lhe 3 \\' gillnet and the trawl net is considerable ; Cynoscion phoxocepha/us is captured 35 cm larger with T 3\\' lhan with lhe trawl neto If the data given under T 3\\' which correspond to the smaller specimens are compared with those in columns CAM ar. DEL,. it seems that based on the. size. and number of organisms present, the inner Gulf could be a nursery area for: Cynoscion a/bus, C. squamipinnis, C. reticu/a/us, Para· lonchurus dumerilli, Anisotremus pacifici, Po/ydacty/us approximans and, Isopis/hus remifer. Cynoscion phoxocepha/us, Nebris occidentalis, Micropogonias altipinnis,Ophios don sciera and, Mentici"hus nasus are only. captured with T 3 W . Again , using number and. SFSM was utilized, the following species were. found to use the outer Gulf as a nursery area:. Cynoscion phoxocepha/us, C. a/bus, C. squami· pinnis, C. stolzmanni, Nebris occidentalis, Aniso/remus pacifici, Po/ydacty/us approxi· mans, Larimus acc/ivis, and Isopisthus remiter. The data are not conc1usive for : Ophioscion sciera, Ophioscion imiceps, and MenNcirrhus nasus.. tentatively added to lhis list since the data. The presence of such "small" and irnmature. are not as clear. When biological criteria such. organisms indieates that the outer Gulf is a1so. as the size at tirst sexual maturity (SFSM). utilized as a nursey area. This is a very general. were used to discriminate between mature and. statement since we do not have data regarding. irnmature. specie s. migrating patterns. That is, sorne of the species. were faund to be irnmature at capture : Cynos cion phoxocepha/us, C. a/bus, C. squamipinnis, Para/onchurus dumeril/i, Nebris occiden/alis, Ophioscion sciera, Po/ydacty/us approximans, Menticirrhus nasU$ and, Isopisthus remlfer. Anisotremus pacifici, Larimus acc/ivis and, Ophioscion imiceps are included tentatively. we have discussed may migrate along the inoer. organisms, the following. in this listo. outer Gulf does not imply that they will stay there until they recruit to the parental stock. They could migra te to the irmer Gulf and grow there . However, the fact that juveniles were present in. Based on these results and on preliminary ob",rvations. and lhe outer Gulf for reproduction and feed· ing. Therefore , the presence of juveniles in lhe. by. personnel. of tite. Fisheries. Department, the use of the T 3y,' is considered to be harmful for the spocies under exploita· tion in lhe inner Gulf of Nicoya. The fishermen are asking the Fisheries Department to prohibit its use sinee lhey feel its efficiency has pro· voked a decline in cateh abundance. The trawl net, as mentioned befare , is even more selective for small tish than tite T 3y,'. Therefore, trawl·. the captures indicates that. the outer Gulf is at some point in time, utilized by. non·recruited. juveniles and trawling un·. doubtely imposes a serious fishing pressure on tltem. Species like. Cylloscion a/bus. live almost. exclusively in the inner Gulf (Madrigal, 1983) and move within a relatively reduced area, where most of reproduction and feeding take place. or. Qlher species like. Cynoscion reticulatus. Isopis/hus remifer may show a wider.

(5) CAMPOS el al. : Effect of shrimp trawling on oommercial ichthyofauna. 207. distribution, using both the inner and the outer. toría de Investigación, Universidad de Costa. Gulf in their Jife cycles. Then if a species resides. Rica.. in lhe inner Gulf but its larval or juvenile stages Jive temporarily in \he outer Gulf, any strategy aimed to manage \he resource will be only. RESUMEN. partially effective if applied only to lhe inner Gulf. Therefore, in arder to adequately manage the dynarnics of the species in the Gulf of. tonces, se ha practicado la pesca de arrastre del. Nicoya, it is vital to identify the distribution. camarón en la parte externa del Golfo . Basados. of the fish stocks and for those that move along lhe inner and outer Gulf or those that. en datos sobre frecuencia de taBas de peces cap. reside mainly in the outer Gulf. Detailed biolo. formación sobre tallas a la primera madurez se. gical studies should be directed towards iden. xual, este estudio evalúa en forma preliminar,. turados en varios cruceros en este Golfo y en in. tification of areas and seasons (months) used. la función de la parte interna del Golfo como. for reproduction and juvenile growth, and to. área de crianza para peces y el efecto de la pes. quantify lhe mortality exerted by shrimp trawlers upon cornrnercial juvenile fmfish. la parte externa.. stocks.. la parte interna del Golfo sirve como área de. clase J. La pesca industrial fue prohibida en la parte interna del Golfo de Nicoya en 1966. Desde en. during. A rnanagement option would be to \he. outer. those. Gulf for shrimp. trawling. rnonths when the main peaks. of reproduction of the cornmercial inchtyo. ca de arrastre sobre la ictiofauna comercial en Los resultados muestran que. crianza por lo menos para 9 especies. Diez o más especies. son capturadas por barcos de. arrastre, en la parte externa del Golfo, antes. regarding which. que alcancen la primera madurez sexual. Se. fish species should be protected must be based on their life histories and susceptibility. ofrece una alternativa simple de manejo para. to fishing pressures and other alterations of the environment.. estas poblaciones de peces.. fauna. take. place.. Criteria. man-made. evitar el "stress" que se está imponiendo sobre. As stated above, this work is to be consid. LlTERATURE ClTED. ered as preliminary. There is a need for acurate data regarding size at flfSt sexual maturity and on the selectivity of fishing gear in arder to better assess the effects of capturing what we have caBed "smaB" fish. However, consid ering that it may take months or years befare we have these data at hand \he present research can serve as a preliminary tool to aid in assig ning adequate priorities. AKNOWLEDG EM ENTS We express our gratitude to Eduardo Madri gal, Fisheries Department, Ministerio de Agri cultura y Ganadería, Costa Rica for rus inval uable help; to Dr. David Stevenson for review ing a preliminary version of this paper; to Virginia Monge for proof reading. Part of this work was funded by No. 02071424, Vicerrec-. Blair, K. D. 1979. Coastal Resource Management in the Gulf of Nicoya. WSG-MR 79-4. Campos, J. 1981. Shrimp by-catch pilot study on tile Pacific coast of Costa Rica. M. Sc. Thesis, Graduate School of Oceanography, University of Rhode. Island. Laevastu, T. 1 9 7 1 . Manual de Métodos en Biología Pesqucra. Editorial Acribia, FAO. 1 9 7 1 . León, P. E. 1973. ECOlogía d e l a Ictiofauna del Golfo de Nicoya, Costa Rica, un estuario tropical. Rev. Biol. Trop., 2 1 :5-30. Maurer, D., C. Epifanio, & K. S. Price. 1980. Ecolog ical assessment of finfish and megabenthic inverte brates as indicators of natural and impacted habitats in the Gulf of Nicoya, Costa Rica. Progress Report of tile 1979 International Sea Grant Programo College of Marine Studies. University of Delawan:..

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