Selectivity of the longline system used by the Mexican fleet in the exclusive economic zone

Abstract

The longline is the most common fishing gear used to catch pelagic fish species. It consists of a principal line or mother line from which secondary lines with hooks hang. This fishing gear has demonstrated its efficiency to catch tuna, shark and billfish. The relative specific selectivity, according to some authors, seems to be influenced by factors like fishing strategy, horizontal and vertical species distribution, and the kind and size of bait. In this paper, a practica1 method to estimate hook operating depth is proposed, taking into consideration the catenary line formed by the mother line in each section. This estimate was validated with data obtained by means of a CTD meter placed at the same depth as the hooks during the longline operation. Moreover, an analysis of the species composition of the catch at each hook depth was performed. The results indicate that there is a difference of more than 30 m between operating depths in each section. Likewise, significant differences (at level α = 0.05) among catches at each operating level were observed.