Inferences on Ecosystem Impacts of Bycatch Removals

Shark species typically inhabit the upper levels of the food chain, regardless of whether they are demersal sharks inhabiting primarily benthic environment or pelagic, inhabiting primarily surface and mid-water oceanic environments. Evidence of impacts may be seen in demographic parameters, in abundances at mid-trophic levels, reduced commercial catch of some species, reduced cohort strength of the shark species and, under appropriate ocean circumstances, trophic cascades. Top-down effects such as these are well-known in many environments around the world that support commercial or even artisanal fisheries. The effects are described for several fisheries including tuna fisheries in the north Atlantic Ocean where pelagic blue sharks are the by catch and in the pollock fishery in the eastern Bering Sea where benthic sleeper sharks are the by catch. In the blue shark case both the fins and the tissue have marker value, In the sleeper case there is no market value associated with captured sharks. Changes in estimated rates of natural mortality are demonstrated in the presence of by catch. It is also shown that ecosystem level impacts are best seen when the size structure of the shark population is known.