FISHERY BULLETIN: VOL. 79, NO. 4 



fishery and from a period of diminishing and de- 

 clining yields. 



2. Effort data were standardized to account for a 

 presumed 25% increase in capture efficiency be- 

 ginning in 1974 when larger, more powerful ves- 

 sels with larger nets first entered the fishery and 

 for a presumed 5% annual increment in the pro- 

 portion of time spent searching for fish as stock 

 size declined beginning in 1975. 



3. Estimates of maximum equilibrium yield ( Ys) 

 were obtained by fitting linear and exponential 

 forms of the surplus production model to plots of 

 catch per unit effort (CPUE) vs. effort. Eight yield 

 analyses were performed using observed and 

 standardized effort to calculate CPUE and a mov- 

 ing 2-yr average of observed and standardized ef- 

 fort as well as simple annual observed and stan- 

 dardized effort as the independent variable. 



4. All regression analyses were statistically sig- 

 nificant at the 99% confidence level. The standard- 

 ization and averaging of effort data, and the use of 

 the exponential model improved the degree of 

 statistical fit. The predicted yield curves fit ob- 

 served catch data fairly well. 



5. Ys estimates ranged from 6,290 to 7,890 t, 

 observed /s ranged from 888 to 1,067 d and stan- 

 dardized fs ranged from 1,041 to 1,117 standard 

 days. The best fit to the data was obtained with the 

 exponential model applied to standardized CPUE 

 vs. average standardized effort data. This analysis 

 produced a Ys estimate of 6,430 1 and afs estimate 

 of 1,096 standard days. The best estimate of un- 

 standardized effort was 1,036 d. 



6. The "best" Ys estimate was exceeded by 760 t 

 in 1974 and by 1,160 1 in 1975. Overfishing proba- 

 bly contributed to catches of 5,000 t and lower in 

 1978-79. Observed effort abruptly exceeded fs in 

 1975 by 20%, remained above 1,000 d in 1976 and 

 1977, and then declined dramatically in 1978-79. 

 Standardized effort remained slightly above stan- 

 dardized/;, even in 1978 and 1979. 



7. Attempts to reduce the dependence of the 

 CPUE and effort variables in the regression 

 analyses by averaging effort were of doubtful 

 value since there was a trend toward increasing 

 effort as the fishery developed and since only 2 yr 

 were used to average effort. Thus, significant au- 

 tocorrelations between f^. and f could not be 

 avoided. Obvious time trends in the deviations 

 between predicted and observed CPUE provided 

 evidence for the nonindependence of the variables 

 in the regression analyses. This problem, however. 



was not believed to have seriously altered the re- 

 sults of the assessment. 



8. The industry should be encouraged to either 

 maintain effort below 1,000 d at sea or annual 

 catch below 6,500 t as was done in 1978 and 1979. 

 If regulations are necessary, a catch quota may 

 prove to be more feasible since it could fairly easily 

 be allocated to individual vessels and since effort 

 limitations must consider "real" changes in fish- 

 ing effort which are difficult to quantify. On the 

 other hand, a reduction in the number of vessels 

 would improve economic returns for the fishing 

 vessels which remain in the fishery. 



9. Unit stock identification is an important area 

 requiring further research. The inferred rapid re- 

 duction in stock abundance in Costa Rican waters 

 suggests that there was little net immigration of 

 thread herring from neighboring countries, but 

 the degree to which the stock (or stocks) exploited 

 by the Costa Rican fleet may also be exploited 

 elsewhere (Panama, in particular) needs to be 

 clarified. 



ACKNOWLEDGMENTS 



The authors wish to thank the vessel captains 

 and the owners and managers of the three Costa 

 Rican canning companies for their cooperation; 

 the biologists, assistant biologists, and statisti- 

 cians of the Ministry of Agriculture Fisheries 

 Office in Puntarenas, Costa Rica, for compiling 

 catch and effort data; and to Eduardo Bravo, Mil- 

 ton Lopez, and Eduardo Lopez of the Ministry of 

 Agriculture Fisheries Resources and Wildlife Di- 

 vision, San Jose, Costa Rica, for their encourage- 

 ment and support. Thanks are also extended to 

 Adan Chacon of the Costa Rican Ministry of Ag- 

 riculture and David Sampson of the Maine De- 

 partment of Marine Resources for their assistance 

 with mathematical and statistical procedures and 

 to four anonymous reviewers for their constructive 

 criticism. 



LITERATURE CITED 



Berry, F. h., and i. Barrett. 



1963. Gillraker analysis and speciation in the thread 

 herring genus Opisthonema . [In Engl, and Span.] 

 Inter-Am. Trop. Tuna Comm., Bull. 7:111-190. 

 BEVERTON, R. J. H. 



1963. Maturation, growth and mortality of clupeid and 

 engraulid stocks in relation to fishing. Rapp. 

 P.-V. Reun. Cons. Perm. Int. Explor Mer 154:44-67. 



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