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From page 196... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 196 6 Causes and Effects in the Bering Sea Ecosystem Population changes in some Bering Sea marine mammal, seabird, shellfish, and fish species over the last 30 years may indicate changes that could affect the long-term status of the Bering Sea ecosystem as a natural resource. The causes and significance of these changes are the key issues faced by the committee. Read the entire page →
From page 197... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 197 ENVIRONMENTAL VARIABILITY Much has been said about the regime shift that occurred in the North Pacific starting in the mid or late 1970s (Chapter 3) Read the entire page →
From page 198... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 198 period. These environmental changes may have increased the productivity of some species while reducing the productivity of others. Read the entire page →
From page 199... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 199 Figure 6.1 Two alternating patterns of atmospheric circulation postulated by Hollowed and Wooster (1992) Read the entire page →
From page 200... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 200 Figure 6.2 Relationship between sea surface temperature anomalies (upper) and proportion of northeast Pacific groundfish stocks with extreme year classes (lower) Read the entire page →
From page 201... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 201 Figure 6.3 Bering Sea pollock spawner-recruit relationship and relative temperature during the first year of life for each year class. W = warm year, C = cold year, and A = average year (from Fritz et al., 1993a) Read the entire page →
From page 202... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 202 Figure 6.4 Proxy time series of pelagic fish abundance in the California Current (top) , power spectra for high frequency (< 150 year) Read the entire page →
From page 203... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 203 top panels on Pacific sardine and northern anchovy) Read the entire page →
From page 204... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 204 conditions. Conditions that exceed physiological limits and result in the deaths of individuals should occur infrequently, especially in the core of a species' range (e.g., Trites, 1990) Read the entire page →
From page 205... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 205 would be a decadal (rather than annual) -scale effect. Read the entire page →
From page 206... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 206 Forage Fish. Pacific herring biomass in the eastern Bering Sea exhibits patterns similar to those of a number of other groundfish and invertebrate populations -- decadal-scale surges in biomass supported by a small number of very strong year classes (Figures 4.24 and 4.25) Read the entire page →
From page 207... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 207 Chionoecetes bairdi, went from an all-time high in 1976–77 to no fishery by 1986. The smaller C Read the entire page →
From page 208... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 208 a direct pathway of these pollutants to humans (Kinloch et al., 1992; Muir et al., 1992) Read the entire page →
From page 209... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 209 atmospheric fallout but also by oceanographic currents, bioturbation, and greater cation exchange and solubility in seawater, as well as by additional sources derived from nuclear material disposal, particularly in the former Soviet Union. Recent data indicate only low-level contamination of marine sediments (Figure 6.6; Cooper et al., 1995; Grebmeier et al., 1993) Read the entire page →
From page 210... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 210 Figure 6.6 Surface sediment cesium-137 distributions during 1992 and 1993 (Cooper et al., 1995) Read the entire page →
From page 211... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 211 Direct Effects of Extractive Resource Use Chapters 4 and 5 showed that a number of changes in animal populations observed in the Bering Sea are due simply to direct and intentional removals by humans. It is important to recognize these explainable changes, as distinct from the unexplained population declines that are now of major concern. Read the entire page →
From page 212... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 212 the Bering Sea and Gulf of Alaska, Atka mackerel in the Aleutian Islands, herring in the eastern Bering Sea, and king and Tanner crab in the eastern Bering Sea and western Gulf of Alaska. Recalling the discussions in Chapter 2 of ecosystem structure and self-organizing capacities, perhaps the most serious effects of fishing are long-term impacts on ecosystem organization by the rapid removals of large biomasses of long-lived species (Apollonio, 1994) Read the entire page →
From page 213... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 213 where pollock were once abundant (e.g., Shelikof Strait, the Bogoslof Island area, and the donut hole) have been heavily exploited, and pollock stocks are now very low in those areas. Read the entire page →
From page 214... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 214 Probably the most intriguing relationship between trawling and ecosystem patterns is the time-space correlation between intense trawl fishing and the declines in two marine mammal species, the Steller sea lion and the harbor seal. Declines in these two species coincide with the period of tremendous concentration of trawling effort in time and space that accompanied the Americanization of the fishery after establishment of the 200-mile exclusive economic zone (Fritz, 1993a, 1993b) Read the entire page →
From page 215... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 215 Figure 6.7 Size distributions of walleye pollock eaten by five species of marine mammals collected in the Bering Sea, 1975–81 (from Frost and Lowry, 1986) Read the entire page →
From page 216... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 216 reproductive output, and survival of species dependent on small pollock for their nutrition (Anonymous, 1993) Read the entire page →
From page 217... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 217 features, but relatively long-lasting where the trawl causes widespread mortality of the habitat-providing species, as in the case of Sabellaria. If recolonization and recovery rates of the habitat provider are slow relative to the frequency of trawling, alteration of the biogenic habitat could persist indefinitely until trawling operations cease. Read the entire page →
From page 218... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 218 through interference competition, nest predation, or resource competition. In the North Sea, declines in herring gulls, lesser black-backed gulls, and kittiwakes may reflect such indirect effects of subsidizing scavenger food chains by trawler discards. Read the entire page →
From page 219... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 219 populations of blue, sei, fin, and humpback whales were severely reduced by the end of whaling in about 1975. These species fed primarily on zooplankton and small, pelagic, schooling fishes. Read the entire page →
From page 220... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 220 1980s. This was mirrored to some extent by declines in murres and kittiwakes in the Pribilofs and Gulf of Alaska. Read the entire page →
From page 221... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 221 Figure 6.9 Schematic distribution of great whale declines (Committee on the Bering Sea Ecosystem) Read the entire page →
From page 222... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 222 in fisheries have at times been substantial, but they can account for only a relatively small part of the decrease in numbers. Pollution, disease, and predation do not appear to have been important factors. Read the entire page →
From page 223... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 223 kittiwakes apparently have declined during this century. They are now absent from the western Aleutians (where they were abundant nesters and apparently outnumbered black-legged kittiwakes in the late 1800s) Read the entire page →
From page 224... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 224 Figure 6.12 Productivity of black-legged kittiwakes and sea-surface temperature in the eastern Chukchi Sea (Springer, 1991; reproduced with permission of the Minister of Supply and Services, Canada, 1995) Read the entire page →
From page 225... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 225 Figure 6.13 Kittiwake productivity on the Pribilof Islands and sea surface temperature in the southeastern Bering Sea (three-year running mean of kittiwake productivity, and five-year running mean of sea surface temperature) (Anonymous, 1993) Read the entire page →
From page 226... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 226 Figure 6.15 Trends in productivity and abundance of black-legged (BLKI) and red-legged (RLKI) Read the entire page →
From page 227... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 227 the same time (Figure 6.16) Read the entire page →
From page 228... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 228 Figure 6.16 Trends in abundance of thick-billed (TBMU) and common (COMU) Read the entire page →
From page 229... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 229 Figure 6.17 Trends in productivity and abundance of common (COMU) murres and black-legged (BLKI) Read the entire page →
From page 230... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 230 Figure 6.18 Productivity of black-legged kittiwakes in the Gulf of Alaska (three-year running mean of kittiwake productivity and five-year running mean of sea surface temperature) (Hatch et al., 1993; reproduced with permission of the Minister of Supply and Services, Canada, 1995) Read the entire page →
From page 231... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 231 Figure 6.19 Diets of seabirds in the Gulf of Alaska, 1975–78 and 1988–91 (Piatt and Anderson, in press) Read the entire page →
From page 232... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 232 to 1.9 million t in 1972. Much of this shift was due to the more than six-fold increase in eastern Bering Sea pollock biomass between 1964 and 1971 (Bakkala, 1993) Read the entire page →
From page 233... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 233 as capelin and sand lance, which compete with young pollock for zooplankton. Unfortunately, it is not possible to evaluate this hypothesis, even in a general way, because we have few data on trends in the abundance of these fishes and other forage species. Read the entire page →
From page 234... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 234 is that a combination of cascading trophic interactions and a significant climate shift has caused major -- and perhaps irreversible -- restructurings of the oceanic ecosystem of the eastern Bering Sea, Aleutian Islands, and Gulf of Alaska. There are several important questions to consider when analyzing the pattern of declining and changing marine mammal, seabird, and fish populations in the Bering Sea: 1. Read the entire page →
From page 235... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 235 relatively scarce. This food shortage might have been exacerbated by pulse fishing of pollock, which might have removed them from some areas for long enough to cause difficulties to marine mammals and birds, especially juveniles. Read the entire page →
From page 236... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 236 by in-migration from elsewhere. It is thus possible that food shortages for some mammals and birds -- perhaps at crucial times and places for juveniles -- have been exacerbated by this intense pulse fishing. Read the entire page →
From page 237... ... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 237 This difficulty does emphasize the need for an adaptive approach to management and the need for good, long-term data on physical and biological phenomena. Read the entire page →

From page 196...

... CAUSES AND EFFECTS IN THE BERING SEA ECOSYSTEM 196 6 Causes and Effects in the Bering Sea Ecosystem Population changes in some Bering Sea marine mammal, seabird, shellfish, and fish species over the last 30 years may indicate changes that could affect the long-term status of the Bering Sea ecosystem as a natural resource. The causes and significance of these changes are the key issues faced by the committee.