Continental slope and deep-sea fisheries: implications for a fragile ecosystem

Exploited deepwater (>500 m) species generally exhibit clear “K-selected” life-history characteristics markedly different from most shelf species: extreme longevity, late age of maturity, slow growth, and low fecundity. Many also aggregate on restricted topographic features such as seamounts, and as a consequence are notably unproductive, highly vulnerable to overfishing, and have potentially little resilience to overexploitation. Since 1964, deepwater fisheries have contributed 800 000–1 000 000 t annually to global marine fish landings. Underlying this apparent overall stability is the “boom and bust” cycle that has characterized many individual fisheries. The accumulated biomass of previously unfished stocks is typically fished down, often within 5–10 years, to the point of commercial extinction or very low levels. Most deepwater stocks are today overfished or even depleted. Depletion of species from deep-sea environments that dominate mid to upper trophic levels may have long-term ecological implications, but the risks of reduced stock size and age structure to population viability, the potential for species replacement, and the impacts on prey and predator populations are not generally known. However, trawl fisheries have been shown to have potentially severe impacts on the benthic fauna of seamounts, where these fish aggregate. This fauna, dominated by suspension feeders, such as corals, is typically restricted to the seamount environment and is characterized by high levels of endemism, which suggests limited reproductive dispersal. The ability of the benthic community to recover, following its removal by trawling, is not known.