Standardisation of yellowfin tuna CPUE for the EU purse seine fleet operating in the Indian Ocean.

The EU purse seine fleet catches of yellowfin tuna (Thunnus albacares) from the Indian Ocean were standardized using the framework described in Katara et al. (2016, 2017) with a Delta-lognormal generalised linear mixed model developed specifically for the standardisation of tropical tuna catch per unit effort (CPUE) time series. The CPUE time series were treated by fishing mode: free school (FSC) sets and sets associated with floating objects (FOBs).
CPUE for FSC was defined as the catch per hour of large yellowfin tuna (> 10 kg) since FSC sets are mainly dominated by adult fishes. For FOBs sets, CPUE was defined as the catch per positive set of small yellowfin tuna (< 10 kg) – a positive set defined as a set with small yellowfin catches > 0 - since mainly juveniles are caught under FOBs. The time series considered were 1986-2017 and 2010-2017 for FSC and FOB, respectively. The two time-series are of different length due to the availability of covariates that likely affect them. In both cases, the least absolute shrinkage and selection operator method was applied for model selection.
The resulting time series of standardised CPUE for FSC and FOBs do not show significant trends. Environmental variables were shown to affect catchability.