Comparison of effects of fishing with effects of natural events and non-fishing anthropogenic impacts on benthic habitats

The effects of demersal fisheries include destruction of habitats and shifts in species composition or age structure of populations and numbers of benthic invertebrates and fishes. Natural events and other anthropogenic impacts may lead to similar effects. A striking phenomenon found in the marine realm is the possibility of regime shifts. Periods with high numbers or biomasses suddenly alternate with periods with lower numbers. Both in the North Sea and in the North Pacific Ocean, sudden, large-scale changes or regime shifts were observed in 1977-1978 and 1988-1989, hinting at possible large-scale causes of these phenomena. In this article, both observations and possible causes are described. Human-induced pollution may also lead to declines in populations. Tri-butyl-tin (TBT) leading to imposex in whelks Buccinum undatum is an example. However, despite high TBT loadings in the Dutch coastal zone, fisheries are still seen as a major cause of the whelk's local extinction. It is argued that the trends that we observe in the marine ecosystem are the result of a very complex interplay between natural and human-induced causes, the final result being an integrated summation of the effects of manageable and nonmanageable factors. Comparing the direct effects of fisheries, sand extraction, and oil or gas extraction in the Dutch part of the North Sea, it can be concluded that, presently, the impact of fisheries on the benthic fauna is a thousand times higher than that of sand extraction and a hundred thousand times higher than that of the oil and gas exploration. Since eutrophication and pollution cannot be expressed per unit area, a direct comparison with effect of fisheries is impossible. However, in general, pollution effects are local, whereas fisheries effects cover the whole North Sea. In conclusion, both natural events and fisheries are the major driving forces of changes observed in the marine ecosystem in the temperate zone.