CPUE standardization for skipjack tuna (Katsuwonus pelamis) of the EU purse-seine fishery on floating objects (FOB) in the Indian Ocean

Abundance indices for Katsuwonus pelamis (SKJ) in the Indian Ocean were derived from the European purse seiner CPUE series (2010-2021) for fishing operations made on floating objects (FOBs). GAMM and GLMM approach were used to standardize the SKJ catch per floating object set. The GLMM approach has been applied to compare the outputs when using an alternative modelling approach and both approaches have been compared to nominal annual CPUE time series. To account for the effort creep, additional explanatory variables have been included in the models. FOB sets have been classified to non-followed FOBs (i.e., randomly encounter FOBs for which the purse seiner has no previous information) and followed- FOBs (dFADs for which the purse seiner is likely to have previous information and therefore the dFAD was not randomly encounter). Densities of instrumented buoys at the -month scale and vessel capacity have also been included as explanatory variables. The time of the set relative to local sunrise has been estimated by comparing logbook catch-effort data with VMS vessel trajectory data and this variable has been integrated in the analysis to account for changes in fish aggregations around the FOBs over the course of the day.