Proportions of Atlantic sailfish (Istiophorus platypterus) and spearfish (Tetrapturus pfluegeri) caught in tuna longline fisheries

Abstract

The reliability of estimations of catches of pelagic longline fisheries is especially worrying in the case of non-target species such as the sailfish (<em>Istiophorus platypterus</em>) and spearfish (<em>Tetrapturus pfluegeri</em>), because they are often recorded aggregated in onboard maps. For this reason, generalized linear models were used to identify a number of important explanatory variables and their effect on sailfish and spearfish proportions. The response variable (number of sailfish in the aggregate capture) is assumed as binomial using the logit link function. The explanatory variables used were foram <em>year</em>, <em>qrt</em> (quarter), <em>fleet</em>, <em>de</em> (distance from equator) and <em>d</em> (distance from areas above sea level). The selected models showed that the equatorial and coastal regions presented higher densities or catchabilities of sailfish when compared to spearfish, as in the case for fleets of China Taipei and the United States. The lowest mean proportion of sailfish [sailfish/(sailfish + spearfish)] of the third quarter were driven mostly by the low estimations calculated for Japan and Brazil.