•158- 



recruit. Given a 3-1 b size-at-entry, increasing effort would not 

 substantially increase yield- per- recruit in each of the cases presented. 



III.B.4. Results of Spawner/Recruitment Analysis 



Large catches of 1 year-old yellowfin were taken from the 

 CYRA in 1978 and 1979. This raises questions as to whether these 

 relatively greater catches of young fish were due to increased 

 recruitment, increased vulnerability of small fish resulting from average 

 recruitment, a shift of effort to areas where small fish are more abundant, 

 a shift in the population structure away from large fish, or some 

 combination of these. If the first case is true, large catches could be 

 expected in subsequent years when the fish from the large recruitment 

 become available as medium- and large-sized fish. However, in the second 

 and third cases, the opposite would be true due to a scarcity of medium- 

 and large-sized fish after the small ones had been heavily exploited. In 

 1979 the fish recruited in 1978 appear to have contributed heavily to the 

 fishery as 2 year-olds, suggesting above-average recruitment in 1978. In 

 general, the data suggest that there has been an increased dependency of 

 the fishery upon 1 year-old fish in recent years. 



III.B.5. Results of Other Analyses/Simulations 



Cohort analysis was used to estimate the numbers and 

 weights of fish of various cohorts at the time of recruitment and at 

 various intervals thereafter. 



During 1968 to 1971 the total CYRA yellowfin biomass 

 averaged about 319,000 mt and was comprised of a large proportion of older 

 fish; this resulted from above-average 1966 and 1967 recruitments which 

 were lightly exploited as juveniles (Figure 8). Below-average recruitment, 

 together with increasing rates of exploitation during 1969 to 1972, led to 

 a decline in the biomass of both young and old fish during 1972 and 1973. 

 The lower biomass of older fish persisted through 1974, while the abundance 

 of smaller fish began to increase due to an extremely large 1974 year- 

 class. The 1974 year-class accounted for most of the increase in biomass 

 of large fish late in 1975, during 1976, and into early 1977. The largest 

 catch of yellowfin in the history of the eastern Pacific fishery was made 

 during 1976. However, poor recruitment in 1976 and 1977, coupled with 

 heavy exploitation of young fish since 1973, has resulted in the biomass of 

 both young and old fish decreasing to the lowest level yet observed in the 

 fishery. Although recruitment in 1978 was high (exceeded only by that of 

 1974), the 1978 year-class is not expected to contribute significantly to 

 the fishery in 1980 and 1981 because so many individuals were captured as 1 

 and 2 year-olds. Preliminary estimates of the 1979 recruitment appear to 

 be slightly lower than average. 



