The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems

The impact of fishing on chondrichthyan stocks around the world is currentl y the focus of considerable international concern. Most chondrichthyan popu lations are of low productivity relative to teleost fishes, a consequence o f their different life-history strategies. This is reflected in the poor re cord of sustainability of target shark fisheries. Most sharks and some bato ids are predators at, or near, the top of marine food webs. The effects of fishing are examined at the single-species level and through trophic intera ctions. We summarize the status of chondrichthyan fisheries from around the world. Some 50% of the estimated global catch of chondrichthyans is taken as by-catch, does not appear in official fishery statistics, and is almost totally unmanaged. When taken as by-catch, they are often subjected to high fishing mortality directed at teleost target species. Consequently, some s kates, sawfish, and deep-water dogfish have been virtually extirpated From large regions. Some chondrichthyans are more resilient to fishing and we ex amine predictions on the vulnerability of different species based on their life-history and population parameters. At the species level, fishing may a lter size structure and population parameters in response to changes in spe cies abundance. We review the evidence for such density-dependent change. F ishing can affect trophic interactions and we examine cases of apparent spe cies replacement and shifts in community composition. Sharks and rays learn to associate trawlers with food and feeding on discards may increase their populations. Using ECOSIM, we make some predictions about the long-term re sponse of ecosystems to fishing on sharks. Three different environments are analysed: a tropical shelf ecosystem in Venezuela, a Hawaiian coral reef e cosystem, and a North Pacific oceanic ecosystem. (C) 2000 International Cou ncil for the Exploration of the Sea.