A Comparison of Cohort Analysis and Catch per Unit Effort for Dover Sole and English Sole

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Abstract
Indices of year‐class strength, stock size, and surplus production of female Dover sole (Microstomus pacificus) and female English sole (Parophrys vetulus) were calculated by two methods: Catch per unit effort (CPUE) and cohort analysis. For each species we derived two CPUE indices and two cohort analysis indices based on different age‐groups. For Dover sole, both CPUE indices show strong year classes in the early and mid‐1940ˈs, declining recruitment in the late 1940ˈs and early 1950ˈs and an increase in strength up to 1962; however, the year by year correlation of the two CPUE indices with each other was not significant. A similar correlation between cohort analysis estimates was significant, however. Both sets of cohort analysis estimates show a gradual decline in female stock size during the 1950ˈs, and a leveling out at a lower level in the 1960ˈs. Catch per unit effort indicates that stock size has increased in the 1970ˈs. These trends in stock size seem to lag behind parallel trends in recruitment. Surplus production plots based on both CPUE and cohort analysis indicate that slightly higher yields are possible with increased effort. For English sole, the two sets of CPUE indices are highly correlated with each other. They both show extraordinary recruitment success for the 1961 year class, but no long‐term upward or downward trends in recruitment. Female stock size increased when the 1961 year class was recruited, and has since decreased. The discrepancy between the two indices of Dover sole CPUE may be due to unusable effort data. The difficulties seem to be due to fluctuating market demand and the assignment of effort by species in a multispecies fishery. We feel, therefore, that changes in year‐class strength of Dover sole are better determined by cohort analysis than by CPUE. For English sole, however, CPUE seem to be as reliable as cohort analysis, apparently because the greater desirability, shallower distribution, and behavior of English sole make the effort figure more valid.

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