Results of a fishery-independent survey for pelagic sharks in the western North Atlantic, 1977–1994

Abstract

Catch and effort data from a fishery-independent longline survey in the western North Atlantic were used to investigate the biology, distribution and abundance of pelagic sharks. Data were restricted to the period from 1977 to 1994, and for the months of June–August. Catches were dominated by the blue shark ( Prionace glauca), with smaller numbers of shortfin mako ( Isurus oxyrinchus), porbeagle ( Lamna nasus), thresher ( Alopias spp.) and silky ( Carcharhinus falciformis) sharks. Detailed analysis was restricted to P. glauca due to small sample sizes for the other species. Male blue sharks were caught more often than females. The catches included juveniles and adults of both sexes but very few adult females. Annual sex ratios varied, but normally favored males. Monthly sex ratios were even in June, but favored males in July and August. Due to differences in occurrence between males and females, distribution and abundance data were analyzed separately for each sex. Male catch per unit effort (CPUE) was highest between 15 and 20 °C, while female CPUE peaked at approximately 15 °C and decreased with increasing water temperatures. CPUE for males and females were standardized using a generalized linear model with years, months and day-night as factors. Males showed an approximately 80% decline between the mid-1980s and the early 1990s, while a significant change in female catch rates could not be demonstrated. The results of the analyses are discussed in relation to fishery-dependent studies in the western North Atlantic, along with previous work on the biology and distribution of pelagic sharks.