CPUE standardization of Skipjack tuna in Sri Lanka

Large meshed drift gillnets are widely used in the tuna fishery in Sri Lanka and the key target species for this gear is Skipjack tuna. The fishery conducted by this gear is characterized by the inboard engine fishing vessels, relatively longer fishing trips, use of supplementary fishing gear with gillnets, and harbour-based landings of multispecies catches. The present study was undertaken to standardize the catch per unit effort (CPUE) of skipjack tuna in the tuna drift gillnet fishery in Sri Lanka. Ten years of port sampling data (2013- 2022) were used for the CPUE standardization. A delta-lognormal model comprising a Gaussian-based Generalized Linear Model (GLM) for positive catch rates and a Bernoulli-based GLM for binary data of skipjack tuna was used for the CPUE standardization. The explanatory variables considered for the study include year, month, vessel category, gear used, number of net panels used, trip duration, gear setting time, and fishing area. All variables except the “fishing area” in Gaussian-based GLM were significant at 0.01 level. The abundance index of skipjack tuna is largely influenced by the “vessel category”, “gear” and “year” variability. A remarkable variation in the annual abundance index was observed during the studied period. A similar standardized CPUE series obtained for an extended period could perhaps be beneficial in the future when stock assessments of skipjack tuna in the Indian Ocean are conducted