Pelagic shark bycatch in the tuna- and swordfish-directed longline fishery off southern Africa

摘要

The capture of pelagic sharks as bycatch of the South African pelagic longline fleet targeting tuna Thunnus spp. and swordfish Xiphias gladius was investigated during the period 1998-2005. In all, 26 species were caught, of which six are listed by the International Union for Conservation of Nature (IUCN) as Vulnerable and one (scalloped hammerhead Sphyrna lewini) as Endangered. Blue shark Prionace glauca and short-finned mako Isurus oxyrinchus were the most commonly caught species (69.2% and 17.2% respectively). Generalised linear models explained 70.4% of blue shark and 22.2% of short-finned mako bycatch patterns and showed that vessel name was the most important explanatory variable. Other significant explanatory variables included month, year, area, bathymetry, bait type, moon phase and time of set. South African-flagged, swordfish-directed vessels caught more sharks (11.7 blue sharks and 1.4 short-finned mako sharks per 1 000 hooks) than Asian-flagged, tuna-directed vessels (1.8 blue sharks and 0.9 short-finned mako per 1 000 hooks). The catch per unit effort of blue sharks and short-finned mako sharks started to decrease from 2001 and 2000 respectively. This was accompanied by a decrease in average length for both species over the period 2002-2007. Three techniques for extrapolating total shark mortality were investigated. Simple extrapolation yielded an estimate of 73 500 sharks per year; if vessel flag was taken into account, the estimate decreased to 39 200 sharks per year and to 43 100 sharks if vessel flag and area (5 grid cells) were taken into account.