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Multi-species interactions
Feeding habits of fish, and other marine animals in Icelandic waters, have been investigated during the last decades. The feeding habits of marine fish are highly variable, in Icelandic waters as well as in other marine ecosystems. Generally, however, prey size increases with increasing size of the predatory fish. Pelagic fish, e.g. capelin, herring and blue whiting, prey mostly on zooplanktonic animals throughout their entire life, but to a limited extent on other fish. Demersal fish, on the other hand, start by preying on pelagic or demeral crustaceans during their first years of life, switching to other prey later on. Many demeral fish, e.g. cod, Greenland halibut and whiting, switch over to other fish like capelin and sandeels during this "second" phase. Other fish, e.g. haddock and long rough dab, switch over to benthic animals such as polychaetes and ophiuroids. Still other fish, like redfish, stick to the crustaceans but prefer larger animals as prey. Notwithstanding such main patterns, feeding habits of marine fish are subject to distinct variations with respect to seasons, oceanic areas and various environmental factors. The trophic webs of marine environments are, therefore, highly complex.

Fisheries in Icelandic waters had limited effect in terms of multi-species interactions during more than half of the last century, excluding the herring fishery. The fishery was directed towards fish species, mainly cod, which were not preyed upon by other fish. This situation changed drastically in the early 1960s with a large scale capelin fishery, and in the late 1980s with a fishery for deep-water shrimp. Those fisheries constituted a harvesting of a new trophic level, i.e., the level of prey of demersal fish, with basically unknown consequences for the stocks in questions.

In view of such profound changes, species interactions became an increasing object of research in following decades. Clear interactions have become apparent, e.g., between the size of the capelin stock and the growth of cod. The size of the capelin stock was shown to be an important factor for the yield of cod. It has also been shown that the cod stock has a large predatory effect on the recruitment of deep-water shrimp. Thus, survival and yield of deep-water shrimp is dependent upon the size of the cod stock, although the latter is not significantly affected in terms of growth. Such relationships reflect only a fraction of the interactions among marine animals, but demonstrate nevertheless that species interactions are relevant in fisheries management.
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