Accounting for Fishing Days Without Set, Fishing Concentration and Piracy in the CPUE Standardisation of Yellowfin Tuna in Free Schools for the EU Purse Seine Fleet Operating in the Indian Ocean During the 1991-2017 Period

The time series of EU purse seine fleet catches per unit effort (CPUE) of yellowfin tuna (YFT) from the Indian
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, iv) time spent by centroid cell by boat by day to constrain detectability, v) the Gulland’s index of
fishing effort concentration to measure the extent to which a fleet has concentrated its fishing effort in areas with
higher than average catch rates and, vi) piracy as a presence absence variable. 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 1991-2017. This new standardization approach,
therefore, represents a significant advance over previous efforts, though there are a number of avenues for
future progress.