Accounting for fishing days without a fishing set in the CPUE standardization of yellowfin tuna in free schools for the EU purse seine fleet operating in the eastern Atlantic Ocean during the 1993-2018 period

Abstract

The time series of EU purse seine fleet catches per unit effort (CPUE) of yellowfin tuna (YFT) from the Atlantic Ocean were standardized using an extension of the Delta-lognormal GLMM to three components. The aim was to depict the trend in abundance for adult YFT observed in free schools (FSC). The originality of this work relied on the inclusion of i) null sets, considered as presence of YFT FSC, ii) fishing days without set, considered as absence of FSC, iii) EU fishing agreement in the exclusive economic zones driving EU purse seine fleet presence in these areas, and iv) time spent by centroid cell by boat by day to constrain detectability. Standardized CPUE for FSC was thus defined as the product of the number of set (positive and null) by spatio-temporal strata, the proportion of sets with large YFT (>10 kg) and the catch per large YFT set. To detect strata without sets, all activities recorded in captain logbooks were used for the period 1993-2018. This new standardization approach, therefore, represents a significant advance over previous efforts, though there are a number of avenues for future progress.Versión del edito