Population dynamics of the blue shark, Prionace glauca, in the North Atlantic Ocean

Large numbers of blue sharks are caught as bycatch, and have even become the target species in pelagic longline fisheries in the North Atlantic Ocean. The status of the stock is ambiguous due to the limitations of the fishery-dependent and -independent data. In chapter 2, fishery-independent demographic and risk analyses are developed for the North Atlantic blue shark. An age-structured matrix population model in which the life-history parameters are stochastic was build. A risk analysis to evaluate the future prospects of the population under different harvest rates is presented and implications for management discussed.;Chapter 3 develops a historic index of abundance for the blue shark in the western North Atlantic. Pelagic longline catch records from several fishery observer programs that operated since the late 1970s are linked to observations from historical fishery-independent longline surveys undertaken in the late 1950s, 1960s and 1970s. Generalized linear model (GLM) techniques are used to derive standardize catch rates (catch-per-unit-effort, CPUE). The index of abundance revealed a decline in blue shark CPUE of approximately 30% in the western North Atlantic from 1957 through 2000. The magnitude of this decline is less than other recently published estimates and seems reasonable in light of the high productivity of blue shark revealed by the demographic analyses in Chapter 2.;Chapter 4 analyzed the available tagging data in a spatio-temporal analysis to derive estimates of movement and fishing mortality rates for the blue shark in the North Atlantic Ocean. The analyses are based upon the historical blue shark tag-recovery data archived by the U.S.-NMFS Cooperative Shark Tagging Program (1965-2004). Fishing mortality rates (F) were highly heterogeneous across the North Atlantic Ocean. Estimates of F for the western North Atlantic were historically lower than 0.10 yr-1, well below a critical reference point, Fc (=0.20 yr -1), derived from the risk analysis in Chapter 2. Such low fishing mortality rates are consistent with the low declines in catch rate estimated for the western North Atlantic in Chapter 3. In contrast, F estimates over the most recent decade (1990s) in the eastern side of the Atlantic were found to be rapidly approaching Fc.