Standardized catch rates for the blue shark (Prionace glauca) caught by the Spanish surface longline fleet in the Indian ocean during the 2001-2013 period

Abstract

Standardized catch per unit of effort (CPUE) were obtained for the blue shark stock (Prionace glauca) of the Indian Ocean using General Lineal Models (GLM) from a total of 1838 trips of the Spanish surface longline fleet fishing swordfish during the 2001-2013 period. A base-case and two sensibility runs were conducted. The main factors considered into the base-case were year, area, quarter, gear and ratio between swordfish and blue shark catches. The significant base-case model explained the 82% of the CPUE variability of the blue shark. A major part of this variability was explained by the proxy of the targeting criteria defined as the ratio between the two more prevalent species caught during the trip, the swordfish and the blue shark. Other factors were also significant but less important. The standardized CPUE trend obtained in the base-case suggests a stable trend over time of the Indian Ocean blue shark stock. The sensitivity analyses showed similar trend as the base-case but in one of this runs the general trend obtained was slightly more optimistic when the ratio factor is excluded from the model.