Addition and subtraction of species

Direct take in fisheries has had an immense impact on open-ocean food webs, virtually clearing many parts of the ocean of large predators and marine mammals. Of about 200 fisheries tracked by the FAO, fully one-third are severely depleted, and many are on the verge of collapsing, or have already collapsed, commercially. Many fisheries which were thought to be successfully managed for "sustained yield" prove to have experienced a scries of sequential collapses of different target species close enough together in time to create the illusion of sustained yield for several years; once the last target species is exhausted, the fishery disintegrates. Exploitation is not limited to adult fishes or mammals. Humans take everything from top carnivores to copepods. The methods employed in pelagic fisheries are particularly non-selective and thus ecologically destructive. Large numbers of sharks, marine mammals, and other non-target species are part of an incidental catch that may equal or exceed the commercial take.

Another important human impact comes from the transport and introduction of many marine species into novel environments via tanker bilge water, etc. (Carlton 1989; Carlton and Geller 1993; see section on productivity).It is often assumed that open-ocean taxa are more cosmopolitan in distribution and that open-ocean systems should be less prone to the introduction of novel organisms, but this has yet to be tested. Introduction of new organisms into an environment may result in the exposure of endemic taxa to novel diseases or predators (Carlton 1989; Carlton and Geller 1993), potentially altering the food web.

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