Standardized CPUE series for blue and shortfin mako sharks caught by the Portuguese pelagic longline fishery in the Indian Ocean, between 1999 and 2012

Portuguese longliners targeting swordfish and operating in the Indian Ocean regularly capture elasmobranch fishes as bycatch. Of those, the blue shark (Prionace glauca) and the shortfin mako (Isurus oxyrinchus) constitute the two main shark species captured. A recent effort by IPMA (Portuguese Sea and Atmospheric Institute) has been made to recover historical catch data on elasmobranchs captured since the late 1990’s to the present date in that fishery. Nominal CPUEs for these two major sharks were calculated as kg/1000 hooks and standardized with Generalized Linear Models (GLM). Several different modeling techniques were tested and compared, chosen depending on the specific proportion of zeros in the catch data for each species. The models tested included the delta method, tweedie, gamma and lognormal models. Model validation was carried out with residual analysis, and relative indexes of abundance for the two species were calculated. The results presented in this paper update a previous analysis on the trends of elasmobranch catch rates available from the Portuguese longline fishery operating in the Indian Ocean.