Zooplankton abundance is also seasonal. There are five dominant subarctic copepods found in the Gulf of Alaska: three species of the genus Neocalanus (plumchrus, cristatus, flemingeri), Eucalanus bungii, and Metridia pacifica. All of the dominant subarctic copepod species are found together or separately in coastal embayments surrounding the Gulf of Alaska. For example, N. plumchrus is abundant in the Strait of Georgia, while N. cristatus and N. flemingeri are apparently absent. Both N. plumchrus and N. flemingeri are abundant in Prince William Sound, where populations apparently build to extremely high levels through an estuarine-type circulation. Other fjords have primarily E. bungii.
The coastal regions of the Gulf of Alaska encompasses a major zoogeographic boundary for fish and crab species. Coastal regions off Southeast Alaska and British Columbia mark the northern boundary of transitional species common to the Oregonian zoogeographic province and the southern boundary of species common to the Aleutian Province (Allen and Smith 1988). The presence of this zoogeographic boundary makes the Gulf of Alaska a desirable location for research on responses of marine organisms to climate variability because the responses of marine stocks are more easily detected at the edges of their distribution.
The dominant species of pelagic fish are pollock, sockeye salmon and herring in order of abundance. Large spawning concentrations of pollock are found in the region near the Shumagin Islands, Shelikof Strait and Prince William Sound. Herring utilize nearshore regions around Kodiak Island, Prince William Sound and southeast Alaska for spawning. Large salmon runs are found in the Gulf of Alaska; one of the most famous runs is the Copper River sockeye run that returns through Prince William sound. Interactions between these three pelagic species is the primary focus of the SEAS research program in Prince William Sound. The SEAS study focuses on the possibility that when copepod abundance is high, predation of salmon and herring by pollock will be reduced.
The Gulf of Alaska also supports a large concentration of benthic fish and crab species. These species reside on the shelf and slope. The abundance of the benthic fish and crab population has been out of phase in recent years, the flatfish population has increased in abundance while the crab population has declined (Albers and Anderson 1985, Blau 1986, Wilderbuer 1994) (Fig. 5). Of notable importance is the increase in the arrowtooth flounder population, which now represents the largest biomass of fish in the Gulf of Alaska. Adult arrowtooth flounder are a major source of predation mortality in the region, consuming small fish as well as euphausiids (Livingston, in review).
The mix of higher trophic level species appears to have changed during the late 1970s, coincident with a major change in ocean conditions.. Abundance of several pinnipeds declined in the region in the 1980s (Merrick et. al. 1987, Hatch and Sanger 1992). One of these, the Steller sea lion, is currently listed as a threatened species under the Endangered Species Act.
There are several notable characteristics of living resources in the Subarctic Pacific. An unusual feature of the subarctic system is the absence of a spring phytoplankton bloom. Explanations for the absence of the bloom include lack of micronutrients such as iron or grazing by microzooplankton (Miller et al. 1991). The dominant copepods of the subarctic Pacific include three species of the genus Neocalanus (plumchrus, cristatus, flemingeri), Eucalanus bungii, and Metridia pacifica. Zooplankton biomass and the catches of epipelagic nekton increased after the mid 1970s (Brodeur and Ware 1992; 1995).
Salmon catches from the North Pacific increased sharply in the late 1970s, especially in Alaska, and exceeded historical levels (Pearcy 1992; Beamish and Bouillon 1993; Francis and Hare 1994). During this recent period of high production, evidence accumulated that several species of salmon, of both North American and Asian stocks, were returning as mature or maturing fish at increasingly smaller sizes (Kaeriyama 1989; Ishida et al. 1993; PICES 1993) (Fig. 6). This suggests density-dependent growth and competition for food in the ocean. Apparently, the carrying capacity of the Subarctic Pacific for salmonids, a major group of epipelagic fishes, was limited, even during this period of exceptionally favorable ocean conditions of the 1980s and 1990s. If the next climate shift is to cooler and less productive conditions, this problem will be exacerbated and have major economic consequences for nations along the Pacific Rim that produce wild and hatchery salmon (PICES 1993).
Besides changes in the productivity per unit area, global warming may also reduce the geographic area that is optimal for salmon growth and survival. Recent observations by Welch et al. (1995; in prep.) suggest that salmon may undertake a reverse, northward migration during the winter. If this is true, and if global warming continues, the area of suitable habitat for salmon could be severely restricted during that season.
Primary productivity on the southeast Bering Sea shelf is spatially variable and highly episodic. Spring blooms are associated with the ice edge and thermal stratification is an important factor in determining whether the production remains in the pelagic or benthic system. Studies indicate that the production on the southeastern Bering Sea shelf is limited by the transport of nutrients onto the shelf from the basin.
Pollock is the dominant species of pelagic fish in the Bering Sea. Large spawning concentrations of pollock are found in the Bogoslof region and the southeast Bering Sea Shelf. Pollock play an integral part in the Bering Sea ecosystem, representing the most abundant forage fish species for marine mammals and sea birds and a major consumer of copepods and euphausiids. Adult pollock may regulate their population size through cannibalism.
The Bering Sea also supports a large concentration of benthic fish and crab species. These species reside on the Bering Sea shelf and mainly consume benthic infauna. The abundance of the benthic fish and crab population has been out of phase in recent years, the flatfish population has increased in abundance while the King Crab population has declined.
Seabirds and marine mammals are abundant in the Bering Sea. Several species of whales migrate to the region to feed. The Pribilof Islands are an important region of pinniped (northern fur seal) and seabird (kittiwakes and murres) breeding in the summer.