1970s the physical regime appears to have shifted as well, resulting in higher sea-surface temperatures and less ice cover than before, conditions that seem to favor pollock recruitment.
As a result, the ecosystem appears to have become more dominated by pollock than it was before. In recent years other predatory fishes—mostly flat-fishes—have increased as well. These predators might be responsible for the decline of species normally favored by marine mammals and birds, such as capelin (Mallotus villosus), Pacific sand lance (Ammodytes hexapterus), and squid (Berryteuthis sp. and Gonatus sp.) As a result of these changes, juvenile marine mammals and birds have been deprived of their preferred foods. Thus, fishing (including whaling) appears to have contributed to a significant change in the structure and functioning of this large marine ecosystem, although the total biomass removed by the current pollock fishery does not seem to be a major contributor to the problem.
The complexity of marine ecosystems and the number of potential factors involved make it difficult to have great confidence in our understanding of the precise mechanisms that relate fishing to the populations of top predators. The NRC report (NRC 1996a) pointed out that, although there has been heavy fishing pressure in the North Sea, in the upwelling areas off South Africa and Namibia, and off Peru, there have not been clear effects on the populations of pinnipeds.
Nonetheless, it seems likely that continued removal of large portions of various trophic levels from marine ecosystems will affect ecosystem structure and functioning. One issue that needs resolution is the effects on marine ecosystems of populations of marine mammals as they recover from very heavy exploitation. Baleen whales eat zooplankton and thus compete for food with many commercially important fish species; toothed whales eat fish and squid and thus compete directly with humans for food. The recovery of whale populations is one of many examples where different policy goals (i.e., protection of marine mammals, catching fish for food) have the potential to conflict.
The Barents Sea, off the extreme northwestern coast of Russia and the extreme northern part of Norway, contains heavily exploited populations of cod (Gadus morhua), capelin (Mallotus villosus), and herring (Clupea harengus) as well as marine mammals (whales and seals). Collapses of fish populations, crises in the fisheries, and a destabilization of the ecosystem occurred during the 1980s, a consequence of overfishing in the Norwegian and Barents seas.
The mature stock of Atlanto-Scandian herring is fished primarily in the Norwegian Sea, but its young use coastal regions of the Barents Sea as a nursery,