Case study: trends in blue shark abundance in the western north Atlantic as determined by a fishery-independent survey

Abstract

The blue shark (Prionace glauca) is the most abundant large, pelagic shark inhabiting upper oceanic waters. Because of its widespread distribution and relatively high fecundity, the blue shark has been depicted by some as being more resistant to the impacts of fishing pressure than other shark species. To test this hypothesis, we investigated historical trends in the abundance of blue sharks in the western North Atlantic during a period in which commercial and recreational catches of pelagic sharks were substantial. We used catch and effort data from the R. V. Geronimo, a fishery-independent longliner that operated consistently in the summer months from 1977 to 1994 in US continental shelf waters south of Massachusetts, Rhode Island, and New York. The catches included juveniles and adults of both sexes, but very few adult females. When catch per unit of effort was analyzed using a generalized linear model, male blue sharks showed an approximately 80% decline between the mid-1980s and the early 1990s. A significant change in female catch rates could not be demonstrated primarily because of the lower numbers of females in the catch. These results suggest that a dramatic decline occurred in the abundance of male blue sharks inhabiting a portion of the western North Atlantic. The broader significance of this finding is not known, but it challenges the common view that the relatively prolific nature of these sharks makes them immune to the effects of overfishing.