A historical index of abundance for the blue shark ( Prionace glauca) in the western North Atlantic

Abstract

The blue shark has been subject to bycatch fishing mortality in the western North Atlantic for almost a half-century. Nevertheless, stock status is ambiguous and improved input data are needed for stock assessments. It is particularly important to obtain reliable indices of abundance because of the uncertainty in estimates of bycatch. An index of relative abundance (catch-per-unit effort, CPUE) is developed for western North Atlantic blue sharks, starting from the mid-1950s, when industrial pelagic longline tuna fisheries began. Longline catch and effort records from recent observer programs (1980–1990s) were linked with longline survey records from both historical archives and recent cruises (1950–1990s). Generalized linear models (GLM) were used to remove the effects of diverse fishing target practices, geographical and seasonal variability that affect blue shark catch rates. The analysis revealed a decline in blue shark CPUE of approximately 30% in the western North Atlantic from 1957 to 2000. The magnitude of this CPUE decline was less than other recently published estimates and seems reasonable in light of the high productivity of the blue shark revealed by life-history studies and preliminary stock assessments.