STEVENSON and CARRANZA: MAXIMUM YIELD ESTIMATES FOR OPISTHONEMA SPP. 



were available from the beginning of the fishery as 

 well as from the period characterized by diminish- 

 ing yields. 



Management Implications 



This assessment of the Costa Rican thread her- 

 ring resource indicated that a maximum equilib- 

 rium yield of approximately 6,430 t was surpassed 

 in 1974-75, leading to stock depletion and reduced 

 catches in subsequent years. Declining catches 

 during 1976-79 were certainly due only in part to 

 reduced effort. Effort increased dramatically after 

 1973 and remained in excess offs (1,036 d) during 

 1975-77. "Real" effort may have remained slightly 

 above optimum even in 1978 and 1979 when catch 

 declined to about 5,000 t. 



Management thus far has been self imposed by 

 the industry in response to rapidly declining 

 CPUE. Economic returns have presumably suf- 

 fered at least as much as biomass yields. To protect 

 the resource, Costa Rican Government regulation 

 would only be necessary if effort again approached 

 1,000 d at sea or if catch exceeded 6,500 t. Regula- 

 tion of effort should concentrate on "real" effort, 

 not merely the number of days vessels spend at 

 sea. It would be important, therefore, to maintain 

 effort at some level below 1,000 d if fishing power 

 were suddenly increased, say, by the use of aircraft 

 to spot schools offish. Since real effort is not easily 

 measured, a catch quota might be a more practical 

 management strategy. Given the small number of 

 vessels in the fishery, allocation of vessel quotas 

 would be feasible. A reduction in the number of 

 vessels, however, would more effectively maintain 

 acceptable economic returns for the industry. 

 Maximum economic returns would presumably be 

 achieved at some effort level below /"s. 



Once an exploited population is depleted, 

 biomass can be restored to a level which supports 

 maximum equilibrium yield by reducing the har- 

 vest of adults, thus reducing adult mortality and 

 stimulating increased recruitment. Catch should 

 be maintained at reduced levels for a period of time 

 equal to the delay between spawning and recruit- 

 ment. As a general policy, fishing at below Ys also 

 protects to some extent against additional stock 

 depletion due to poor recruitment. For the Costa 

 Rican thread herring population, some increase in 

 stock size (and therefore CPUE) following over- 

 fishing in 1974-75 should have been noticeable 

 after a year or two of reduced catches such as 

 occurred in 1978-79. The observed increase in 



CPUE since 1976 (Table 2, Figure 2) was small and 

 may not have been "real" since standardized 

 CPUE stabilized after 1976 (Table 6). However, 

 catch and effort data available for the first 5 mo of 

 1980 ( Table 9 ) showed a continued low CPUE when 

 compared with the same period in 1979. 



Table 9. — observed catch, effort, and CPUE data compiled 

 from sales receipts for the fi rst .5 mo of 1979 and 1980 for thread 

 herring captured on the Pacific coast of Costa Kica. Source: 

 Oficina df IVscm. Minisierio de .Auricultura y (ianaderia. 

 Puntarenas. Costa Rica. 



Period 



Catch (t) 



Effort (d at sea) CPUE (t/d at sea) 



Jan-May 1979 

 Jan.-May 1980 



2,270 

 1,860 



330 

 340 



6.88 

 547 



Unit stock identification is an important area 

 requiring further research. Although thread her- 

 ring on the Pacific coast of Central America may 

 not be highly migratory, it seems likely that there 

 is some mixing across national boundaries. The 

 rapid decline in population size in Costa Rican 

 waters — as inferred from reduced CPUE 

 estimates — would not have been possible, how- 

 ever, if there had been a significant influx offish 

 from other areas along the Central American 

 coastline. The Panamanian thread herring fishery 

 is only active in the Gulf of Panama, although 

 thread herring were detected acoustically in the 

 Gulf of Chiriqui (see Figure 1) and off Guatemala, 

 El Salvador, and Nicaragua during a 1970 survey 

 (Magnusson 1971). If the same stock is presently 

 being exploited in Costa Rican and Panamanian 

 waters, stock assessment and resource manage- 

 ment should be a joint activity of both countries. 

 The possibility also exists that more than one 

 stock is present in Costa Rican waters, either geo- 

 graphically or taxonomically. Clearly, the assump- 

 tion that the three species which currently make 

 up the catch can be treated as a single unit stock 

 needs to be confirmed with additional ecological 

 and life history information. In the meantime, 

 efforts should be made to determine the species 

 composition of landings, and to compile catch and 

 effort data by species. 



SUMMARY 



1. Catch and effort data were compiled for three 

 species of Pacific thread herring harvested in 

 Costa Rican waters during 1968-79. Effort was es- 

 timated as the number of calendar days at sea. 

 Data were available from the beginning of the 



701 



