MAXIMUM YIELD ESTIMATES FOR THE PACIFIC THREAD HERRING, 

 OPISTHONEMA SPR, FISHERY IN COSTA RICA 



David K. Stevenson' and Francisco Carranza' 



ABSTRACT 



Linear and exponential surplus production models were applied to annual 1969-79 catch per unit effort 

 and effort data obtained from the Costa Rican Pacific thread herring fishery. Effort was estimated as 

 the number of calendar days at sea and standardized to account for presumed increases in fishing 

 power and "real" fishing time (time spent searching for fish) after 1973. Independent regression 

 analyses were performed using standardized and unstandardized effort estimates. In addition, values 

 of the independent variable were estimated both as simple annual effort and as a moving 2-year 

 average of effort. All regressions were statistically significant, but the best fit was obtained with 

 standardized average effort and the exponential model. The "best" estimates of maximum equilibrium 

 yield and effort (Ys = 6,430 t;^ = 1,036 days or 1,096 standard days) indicated that the resource was 

 overexploited in 1974-75, 6 years after fishing began. In 1976-77 catch remained at approximately 

 Yg levels while effort remained above f^. Catch and observed effort declined dramatically during 

 1978-79 although standardized effort remained slightly above 1,100 standard days. Future manage- 

 ment of the resource may require limited catch and/or effort. The validity of the model is discussed 

 primarily in terms of the nonindependence of the A' and Y variables. 



Thread herring (genus Opisthonema) are school- 

 ing pelagic clupeids which inhabit tropical and 

 subtropical coastal waters of the western Atlantic 

 and eastern Pacific Oceans. According to Berry 

 and Barrett (1963), three Pacific species {Opis- 

 thonema libertate, O. bulleri, and O. medirastre) 

 are found in continental waters between northern 

 Mexico and Peru while a fourth species (O. herlan- 

 gai) is limited to the Galapagos Islands. These 

 authors differentiated individual species on the 

 basis of the number of gill rakers, a meristic 

 character which was found to be positively related 

 to standard length. The most abundant Atlantic 

 species (O. oglinum) is distributed from southern 

 Brazil to the Gulf of Maine. 



Thread herring school in nearshore waters 

 (Klima 1971; Magnusson 1971) and are 

 planktivorous feeders. Fuss et al. (1969) observed 

 copepods and larval pelecypods, gastropods, and 

 barnacles in the stomachs of Atlantic thread 

 herring. Peterson (1956) reported mostly phyto- 

 plankton from Pacific thread herring stomachs 

 collected in Costa Rica. The presence of fine sedi- 



ments in the stomachs of Atlantic thread herring 

 indicated that these fish may spend some time 

 feeding on the bottom. 



Atlantic thread herring spawn during April- 

 August in the eastern Gulf of Mexico. Fuss et al. 

 (1969) reported a maximum mean gonad index in 

 June, and Houde (1976) collected eggs when sur- 

 face seawater temperature ranged from 22.5° to 

 30° C. Most eggs and larvae were collected within 

 50 km of the coast. Based on the histological 

 examination of reproductive tissue, Paez Barrera 

 reported peak spawTiing for O. libertate in Mexico 

 during June and July at water temperatures of 

 25°-29° C. Peterson (1956) collected small indi- 

 viduals ( <60 mm SL, standard length) in the Gulf 

 of Nicoya, Costa Rica, during all months of the 

 years 1952-54. His observations indicated that 

 spawning may have been continuous. Further- 

 more, nearly all collections of adults included 

 some individuals which were sexually mature. 

 Continuous recruitment on the Pacific coast of 

 Costa Rica was also implied by the presence of a 

 predominant size group at 18-20 cm SL in most 



'Department of Zoology, University of Maine, Orono, ME 

 04469 or Maine Department of Marine Resources, West Booth- 

 bay Harbor, ME 04575. 



^Departamento de Estudios Bioldgicos de Fauna Marina y 

 Continental, Ministerio de Agricultura y Ganaderfa, San Jose, 

 Costa Rica. 



Manuscript accepted May 1981. 



FISHERY BULLETIN; VOL. 79, NO. 4. 1981. 



sPaez Barrera, F. 1976. Desarrollo gonadal, madurez, de- 

 sove y fecundida'd de sardina crinuda, Opisthonema libertate 

 (Guntheri de la zona de Mazatla'n, ba.sados en el ana'lisis his- 

 toldgico de la gdnada. Mem. del Simp, sobre Recursos Masivos 

 de Mexico, Ensenada, B.C., 28-30 de Septiembre de 1976, p. 

 207-255. 



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