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The Bering Sea Ecosystem (1996) / Chapter Skim
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4 Biology of Higher Trophic Levels
Pages 72-155

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From page 72...
... Generally, the most information is known about the eastern Bering Sea, Aleutians, and Gulf of Alaska, followed by the Aleutian Basin and then the western Bering Sea. This imbalance in data and knowledge on the five regions and the species themselves makes the study of the Bering Sea ecosystem as a whole very difficult.
From page 73...
... They are generally found on the continental shelf in the eastern Bering Sea, Aleutian Islands, and Gulf of Alaska to northern British Columbia, generally at depths of 180 m or less (Hayes, 1983)
From page 74...
... concluded that interannual variations in abundance of the exploitable portion of king crab stocks have ranged over an order of magnitude in both the eastern Bering Sea and Kodiak. These fluctuations result from the occurrence of particularly successful cohorts that can be observed to grow and progress through successive sample years, indicating that the sources of these variations operates early in the crab's life history.
From page 75...
... BIOLOGY OF HIGHER TROPHIC LEVELS 75 Figure 4.1 Abundance estimates for large male king crabs, Paralithodes camtschatica, from National Marine Fisheries Service trawl surveys in the eastern Bering Sea (solid lines are swept estimates, dotted line is an index based on Japanese catch per unit effort (CPUE) , and dashed line is an index based on USSR CPUE)
From page 76...
... BIOLOGY OF HIGHER TROPHIC LEVELS 76 Figure 4.2 Kodiak male red king crab size distribution, for each year from 1974 to 1982 (as determined by research pot sampling; skipmolt crab -- the black area -- masks new shell crab) (Hayes, 1983)
From page 77...
... Spot and coonstripe shrimp have supported small pot fisheries in Prince William Sound and in the eastern Gulf of Alaska. The extent of pandalid research on Alaskan species has closely followed the rise and fall of the commercial shrimp fishery.
From page 78...
... . Fishery scientists have been working under the assumption that characteristics of pink shrimp life history closely mirror those of other species within the genus Pandalus, while recognizing that regional and species differences likely occur.
From page 79...
... FINFISH Life History, Distribution, and Variability Of the approximately 450 fish, shellfish, and crustacean species in the eastern Bering Sea, only about 25 are commercially important. The discussions that follow provide background data on the known life habits and biological attributes of significant species in the Bering Sea, including their spatial and temporal variability.
From page 80...
... Others, such as a number of flatfish (yellowfin sole, rock sole, arrowtooth flounder, flathead sole, and Alaska plaice) on the eastern Bering Sea shelf and Atka mackerel in the Aleutian Islands are currently at relatively high levels of abundance.
From page 81...
... A third stock was in the Aleutian Islands. Less information was available for western Bering Sea pollock, although Dawson speculated that a fourth stock might be in the southwestern Bering Sea and that pollock in the northwestern Bering Sea in Russian waters might be part of the eastern Bering Sea shelf stock.
From page 82...
... . Figures 4.8 and 4.9 give the distribution of catch and relative contributions of year classes in the 1970s and early 1980s to the pollock fisheries of the eastern Bering Sea shelf and slope, northwest and southeast of the Pribilof Islands (Francis and Bailey, 1983)
From page 83...
... BIOLOGY OF HIGHER TROPHIC LEVELS 83 Figure 4.4 Estimates of adult (ages three and up) biomass of walleye pollock in the eastern Bering Sea and Gulf of Alaska, 1964–93 (NPFMC, 1993; Bakkala, 1993)
From page 84...
... BIOLOGY OF HIGHER TROPHIC LEVELS 84 Figure 4.5 Pollock year class strength in the eastern Bering Sea and Gulf of Alaska (showing a succession of strong year classes 1972–79 in the Gulf of Alaska) (Wespestad, 1993)
From page 85...
... . Figure 4.7 Eastern Bering Sea pollock catch quarterly distribution (Wespestad, 1993)
From page 86...
... of pollock in the eastern Bering Sea northwest and southeast of the Pribilofs, 1973–82 (Francis and Bailey, 1983)
From page 87...
... BIOLOGY OF HIGHER TROPHIC LEVELS 87 Figure 4.10 Pelagic trawl fishery locations (Fritz, in press)
From page 88...
... BIOLOGY OF HIGHER TROPHIC LEVELS 88 Figure 4.11 Center of activity for pollock fishing (Hinckley, 1987)
From page 89...
... . Pacific cod are used as food by northern fur seals, halibut, beluga, and sperm whales.
From page 90...
... BIOLOGY OF HIGHER TROPHIC LEVELS 90 Figure 4.12 Pacific cod catch (based on unpublished data from the Alaska Fisheries Science Center [AFSC] , NMFS, [National Oceanic and Atmospheric Administration]
From page 91...
... BIOLOGY OF HIGHER TROPHIC LEVELS 91 Figure 4.13 Trends in biomass and recruitment of Pacific cod in the eastern Bering Sea (based on unpublished data from the [AFSC] , NMFS, [National Oceanic and Atmospheric Administration]
From page 92...
... BIOLOGY OF HIGHER TROPHIC LEVELS 92 Figure 4.14 Biomass and recruitment of Atka mackerel (top and middle graphs: NPFMC, 1993; bottom graph: based on unpublished data from [AFSC] , NMFS, [National Oceanic and Atmospheric Administration]
From page 93...
... Because similar stock assessments are not available for Atka mackerel in the Gulf of Alaska, Figure 4.14 also gives catch trajectories for the Aleutian Islands and Gulf of Alaska. Like cod in the eastern Bering Sea, Atka mackerel biomass surged in the Aleutian Islands in the late 1970s and early 1980s as a result of one or two strong year classes (in particular, for the year 1977)
From page 94...
... After hatching, the larvae are pelagic and found at 30 to 130 m depth, until they reach a length of about 80 mm, when they transform and become demersal. The overall life history of Greenland turbot is unique among flatfish in that they spend the first three or four years of life on the continental shelf and move to the continental slope where the older juveniles and adults reside (Bakkala, 1993)
From page 95...
... BIOLOGY OF HIGHER TROPHIC LEVELS 95 Figure 4.16 Distribution and density of Greenland turbot on the continental shelf of the eastern Bering Sea, as shown by northwest and AFCC survey data, 1979–86 (Bakkala, 1993)
From page 96...
... BIOLOGY OF HIGHER TROPHIC LEVELS 96 Figure 4.15 Biomass estimates of Greenland turbot and arrowtooth flounder (top graph: based on unpublished data NMFS, NOAA) ; recruitment estimates for Greenland turbot (bottom graph: NPFMC, 1993)
From page 97...
... BIOLOGY OF HIGHER TROPHIC LEVELS 97 Figure 4.17 Pacific halibut indices of abundance (IPHC, 1994)
From page 98...
... , extends south along the Asian coast to the Sea of Japan. The two species may overlap considerably in the eastern Bering Sea and Aleutian Islands (Bakkala, 1993)
From page 99...
... BIOLOGY OF HIGHER TROPHIC LEVELS 99 Figure 4.18 Catch and biomass estimates of flatfish of commercial importance in the eastern Bering Sea (NPFMC, 1993)
From page 100...
... . They are more abundant in the Aleutian Islands region and particularly in the Gulf of Alaska than in the eastern Bering Sea.
From page 101...
... . Figure 4.20 gives biomass trajectories for the Pacific Ocean perch complex,2 and Figure 4.21 gives recruitment time series for the eastern Bering Sea, Aleutian Islands, and Gulf of Alaska.
From page 102...
... BIOLOGY OF HIGHER TROPHIC LEVELS 102 Figure 4.20 Biomass trajectories of age six and older Pacific Ocean perch (NPFMC, 1993)
From page 103...
... BIOLOGY OF HIGHER TROPHIC LEVELS 103 Figure 4.21 Estimated recruitment of two- and three-year-old Pacific Ocean perch for the three regions (NPFMC, 1993)
From page 104...
... In the eastern Bering Sea, they generally winter northwest of the Pribilof Islands, migrating to the Alaska coast in the spring to spawn from Bristol Bay to the Yukon River (Figure 4.22)
From page 105...
... . Figure 4.23 Migration routes to the eastern Bering Sea winter grounds from coastal spawning sites as inferred from fishery and research catches (Wespestad, 1991)
From page 106...
... . Figure 4.25 Estimated abundance of Pacific herring in the eastern Bering Sea at age one, 1958–84, as estimated by cohort and catch at age analysis (estimates are deviations from the geometric mean year class)
From page 107...
... . The long-term dynamics of Pacific salmon production in the eastern Bering Sea (and Alaska region in general)
From page 108...
... BIOLOGY OF HIGHER TROPHIC LEVELS 108 Figure 4.26 Generalized distribution of smelts in the eastern Bering Sea based on NMFS groundfish trawl surveys and fisheries observer data (Os and Is represent outer and inner shelf areas, respectively, over which fisheries observer data are pooled for analysis of eulachon abundance trends) (upper figure: Fritz et al., 1993; lower figure: Hinckley, 1987)
From page 109...
... , and trends in CPUE (catch per unit effort) of eulachon from fisheries observer data on outer and inner shelf areas of the eastern Bering Sea (hours represent total duration of sampled trawls in region for each year)
From page 110...
... Eastern Bering Sea salmon production is dominated by sockeye, whereas western Bering Sea production is dominated by pink salmon. Summaries of recent trends in fish populations and biomass are given in Table 4.1, Table 4.2, and Figure 4.29.
From page 111...
... BIOLOGY OF HIGHER TROPHIC LEVELS 111 Figure 4.28 Estimated salmon production (catch and escapement) for eastern and western Bering Sea, 1952–91 (based on unpublished data, from Alaska Fisheries Science Center, NMFS [National Oceanic and Atmospheric Administration]
From page 112...
... (4) 167,000,000 197,000,000 147,000,000 EBS/Al- EBS/Al High, GOA- Stable, GOA High Stable/ Decreasing King Crab (4)
From page 113...
... (AI) 13,900 2,200 2,800 1,745 2,200 Average Declining Pacific 47,100 1,850 1,910 1,906 1,850 Low Stable Ocean Perch (EBS)
From page 114...
... BIOLOGY OF HIGHER TROPHIC LEVELS 114 Figure 4.29 Trends in fish populations and biomass (based on data from NMFS, ADF&G, and the Committee on the Bering Sea Ecosystem)
From page 115...
... BIOLOGY OF HIGHER TROPHIC LEVELS 115 Table 4.3 Numbers of seabirds and seabird colonies in Alaska. Species Number of Sites Catalog Total Estimated Total Northern fulmar 30 1,451,980 2,000,000 Sooty shearwater 0 - 21,000,000 Short-tailed shearwater 0 - 20–80,000,000 Fork-tailed storm petrel 60 1,148,500 5,000,000 Leach's Storm petrel 38 1,709,600 4,000,000 Double-crested cormorant 82 4,701 7,000 Brandt's cormorant 1 11 100 Pelagic cormorant 285 40,888 90,000 Red-faced cormorant 179 51,613 130,000 Pomarine jaeger - - ?
From page 117...
... Table 4.5 Trends in the abundance of geese nesting and wintering in the eastern Bering Sea Species Nesting Wintering Status Greater white-fronted goose + Increasing Emperor goose + + Decreasing Brant + Decreasing Cackling Canada goose + Increasing Aleutian Canada goose + Increasing Source: Springer (1993)
From page 118...
... demonstrate the impacts of commercial fisheries on seabird food availability, citing the experience of various species declines offshore California, Salvador, and Norway. In the Norwegian case, for example, Atlantic puffins, common guillemots, and razorbills suffered drastic breeding reductions due to intensive herring fishing from 1958 to 1969.
From page 119...
... . Whereas there are three species of cormorants in the Bering Sea region, the red-faced cormorant is found mostly in the Aleutian Islands and Alaska Peninsula.
From page 120...
... 1977–1991 0 Nysewander and Dipple, 1992 Puale Bay 1976–1991 0 Nysewander and Dipple, 1992 Prince William Sound 1973–1989 0 Irons et al., 1987; unpubl. data Middleton I
From page 121...
... . Black-legged kittiwakes on or near the coast of the eastern Bering Sea -- Bluff (Norton Sound)
From page 122...
... These birds are found in the Bering Sea, northern Pacific, and North Atlantic. The greatest concentrations of murres in the Bering Sea region are on the Pribilof Islands, St.
From page 123...
... In contrast, murres have been gradually increasing nearby at Cape Lisburne, at least since the mid-1970s. Introduced Mammals Introduced mammals have devastated seabird populations throughout the Aleutian Islands and on many islands in the Gulf of Alaska (Bailey, 1990; Bailey and Kaiser, 1993; Murie, 1959)
From page 124...
... . Japanese high-seas and land-based salmon fisheries operated in the western North Pacific south of the Aleutian Islands and in the western Bering Sea since 1952.
From page 125...
... further argues that murre populations were in decline in the Gulf of Alaska well before the oil spill and the recent declines must be viewed in a broader context. Hunting Four species or subspecies of geese nest along the coast of the Bering Sea, particularly on the YukonKuskokwim delta: greater white-fronted geese, emperor geese, cackling Canada geese, and black brant.
From page 126...
... . Several species of the North Pacific fauna (e.g., harbor seal, Steller sea lion, sea otter, beluga whales, and Dall's porpoise)
From page 127...
... bEndangered and threatened refer to Endangered Species Act listings. OSP means within the optimum sustainable population range as defined by the Marine Mammal Protection Act.
From page 128...
... Pinnipeds are generally more abundant than cetaceans with the population size of several species estimated to number over 100,000. Steller sea lions are classified as threatened under the Endangered Species Act, and northern fur seals on the Pribilof Islands are listed as depleted under provisions of the Marine Mammal Protection Act.
From page 129...
... . Table 4.10 Relative importance of major prey types in the diet of cetaceans in the Bering Sea Species Pelagic and Demersal Octopus Copepods Euphausiids Nekto- Epifaunal Marine Semidemersal Fishes and benthonic Invertebrates Mammals Fishes Squids Invertebrates Right -- - -- - -- - Major Minor -- - -- - -- whale Bowhead -- - -- - -- - Major Major Minor -- - -- whale Blue Minor -- - Minor Minor Major -- - -- - -- whale Fin Major -- - Minor Major Major -- - -- - -- whale Sei Minor -- - Minor Major Minor -- - -- - -- whale Minke Major -- - Minor Minor Major -- - -- - -- whale Gray Minor -- - Minor -- - -- - Major Major -- whale Sperm Minor Major -- - -- - -- - -- - Minor -- whale Beaked Minor Major Major -- - -- - Minor -- - -- whale Killer Major Minor Major -- - -- - -- - -- - Major whale Beluga Major Minor Minor -- - -- - Minor -- - -- whale Harbor Major -- - Minor -- - -- - Minor -- - -- porpoise Dall's Minor Major Major -- - Minor Minor -- - -- porpoise Source: Based on Frost and Lowry (1981a)
From page 130...
... Similar information is presented for some of the large whales, because the large reduction in the abundance of these species could have contributed to changes in the ecosystem that are of concern today. Steller Sea Lion Steller sea lions inhabit waters of the North Pacific rim from California through the Aleutian Islands to Japan (Loughlin et al., 1984)
From page 131...
... Source: Committee on the Bering Sea Ecosystem. Table 4.12 Counts of adult and juvenile Steller sea lions at rookery and haulout sites in seven subareas of Alaska during June–July, 1975–92 Gulf of Alaska Aleutian Islands Year Southeast Alaska Eastern Central Western Eastern Central Western 1975 19,769 1976 7,053 24,678 8,311 19,743 1977 19,195 1979 6,376 36,632 14,011 1982 6,898 1985 19,002 6,275 7,505 23,042 1989 8,471 7,241 8,552 3,908 3,032 7,752 1990 7,629 5,444 7,050 3,915 3,801 7,988 2,327 1991 7,715 4,596 6,273 3,734 4,231 7,499 2,411 1992 7,558 3,738 5,721 3,720 4,839 6,399 2,869 Change 1975/79–1992 +19% -47% -77% -55% -76% -83% -80% Source: Sease et al.
From page 132...
... In Russia, the situation is similar to that in the eastern Bering Sea and Gulf of Alaska, with the present abundance of sea lions much reduced from historical levels. According to Perlov (1991)
From page 133...
... These data showed marked seasonal and geographic variation in the species consumed. In the eastern Bering Sea, pollock, squid, and capelin accounted for about 70 percent of the energy intake by fur seals (Perez and Bigg, 1986)
From page 134...
... BIOLOGY OF HIGHER TROPHIC LEVELS 134 Table 4.13 Geographic and temporal distribution of samples used to determine the composition of northern fur seal dietsa Area n Year Major Prey Source Bering Sea 373 <1900 Pollock, squid Lucas, 1899 1749 1958–74 Pollock, squid, capelin Perez and Bigg, 1986 73 1981, 1982, 1985 Pollock, squid Sinclair et al., 1993 Aleutians 309 1958–74 Sand lance, capelin, mackerel Perez and Bigg, 1986 North Pacific 562 1958–74 Herring, anchovy, squid Perez and Bigg, 1986 Gulf of Alaska 38 <1900 Rockfishes Alexander, 1892 36 <1900 Squids, rockfishes Lucas, 1899 12 1930s May, 1937 104 1950–51 Herring Wilke and Kenyon, 1952 1163 1958–74 Sand lance, capelin, herring Perez and Bigg, 1986 a Major prey are listed only for samples of 15 or greater.
From page 135...
... . Table 4.14 Estimates of population abundance and pup production for northern fur seals on the Pribilof Islands Year(s)
From page 136...
... . In recent years, the numbers of harbor seals and California sea lions (Zalophus californianus)
From page 137...
... data 7 1954 Wilke, 1957 11 1959–62 Kenyon, 1965 17 1968–73 Shrimp, mysids, octopus Burns and Gol'tsev, 1984 Gulf of Alaska 3 1972 Lowry et al., 1979 67 1945–46 Eulachon Imler and Sarber, 1947 269 1973–78 Pollock, octopus, capelin Pitcher, 1980 99 1945–46 Pollock, herring, flatfishes Imler and Sarber, 1945 a Major prey are listed only for samples of 15 or greater.
From page 138...
... BIOLOGY OF HIGHER TROPHIC LEVELS 138 Table 4.16 Counts of harbor seals in Bristol Bay Year North Side of Alaska Peninsulaa Nunivak Bayb 1975 17,245 2,918 1976 23,805 -- 1977 12,584 -- 1979 -- - 2,000 1981 -- - 3,100 1983 -- - 2,500 1985 11,728 -- 1990 10,105 470 1991 10,192 400 aSum of maximum counts at each of four haulouts made in June. bMaximum count made in August-September.
From page 139...
... Between 1960 and 1967, an additional 4,000 were killed in the northern Gulf of Alaska, which virtually eliminated the species from that region as well. During the same intervals, about 5,000 sei whales were killed in the northern Gulf of Alaska, and about 1,500 blue and 3,300 humpback whales were killed in the northern Gulf of Alaska and Aleutian islands.
From page 140...
... North Pacific <20,000 21,113 >100 Source: NMFS (1991b)
From page 141...
... . By such criteria, the observed declines in the numbers of Steller sea lions, northern fur seals, and harbor seals in the Bering Sea and Gulf of Alaska have also been ''drastic." In describing these declines in this way, there is the clear implication that in light of existing knowledge, the nature of these declines is cause for concern.
From page 142...
... This comparative approach could be extended to ask whether species in the Bering Sea and Gulf of Alaska other than fur seals, sea lions, and harbor seals have shown similar changes in abundance. Although population trends in ice-breeding seals of the northern Bering Sea and cetaceans in the Bering Sea and Gulf of Alaska are poorly known, it is clear that not all species are declining or failing to recover from overexploitation.
From page 143...
... examined the information available on population biology and feeding of Bering Sea marine mammals, they concluded that three species overlapped significantly in the foods they prey on and commercial fisheries. The species were northern fur seals, Steller sea lions, and harbor seals.
From page 144...
... in the numbers of fur seals, sea lions and harbor seals. These are discussed in Alverson (1992)
From page 145...
... BIOLOGY OF HIGHER TROPHIC LEVELS 145 Table 4.18 Committee's assessment of potential causes of recent declines in abundance of fur seals, sea lions, and harbor seals in the eastern Bering Sea and Gulf of Alaska Likelihood of Involvement in Declines since 1980 Cause Fur Seal Sea Lion Harbor Seal Comment Disease Low Low Low Few data, but no evidence of increased effects that could explain declines Chemical pollution Low Low Low Few data; levels low relative to known effects in other populations Entanglement Moderate Low Low Could only have been a contributing factor in the fur seal declines Harassment Low Low Low Local effects only; not important for the geographic scale over which the declines have occurred Commercial harvest Low Low Low Could not explain observed declines beyond the mid 1970s Subsistence harvest Low Low Low Not a factor in the widespread declines Incidental take Moderate Moderate Low Contributed to the declines, but not a major factor; cannot explain continued declines Predation Low Low Low Few data; could not explain the declines, but may affect recoveries Direct climate effects on Low Low Low Cannot explain the declines survival Fishery effects on food Moderate High High Could have played a major role availability in all declines Climate effects on food Moderate Moderate Moderate Could have played a role in all availability declines Competition from fish predators Moderate Moderate Moderate Could be a major factor in the declines, given observed changes in community structure that were most likely caused by commercial fishing and environmental changes
From page 146...
... Steller sea lions also become entangled in packing bands and net fragments, but available data indicate that entangled adults are rarely seen, and entanglement rates of pups and juveniles appear to be even lower than those observed for adults (Loughlin et al., 1986)
From page 147...
... . Although there are no estimates of total predation mortality on Bering Sea/Gulf of Alaska pinnipeds, it is unlikely that increased predation could account for the declines, particularly the widespread declines in Steller sea lions (NMFS, 1992)
From page 148...
... However, the model suggested that pups of low birth weight could have reduced survival during periods of cold, wet, and windy weather. Overall, these results suggest that direct effects of weather are unlikely to cause increased pup mortality in northern fur seals, or in the larger, newborn Steller sea lions and harbor seals, during their time on land.
From page 149...
... Thus, the population dynamics of northern fur seals will be influenced by events that occur both in the Bering Sea and elsewhere in the North Pacific. Steller sea lions and harbor seals, on the other hand, are more likely to be affected by regional conditions.
From page 150...
... To date, there has been inadequate attention to the importance of scales in the analysis of nutritional requirements of pinnipeds in the Bering Sea and Gulf of Alaska. After reviewing the available information on the temporal and spatial pattern of declines in Steller sea lions and harbor seals, and the pattern of fishing in areas that are thought to be important feeding locations for these species, one can hardly avoid the conclusion that the removal of fish biomass from these areas has in some way contributed to the declines, at least on localized bases.
From page 151...
... . Available data on the diets of Steller sea lions, northern fur seals, and harbor seals are summarized in Tables 4.9, 4.11, and 4.13.
From page 152...
... It is also possible that large-scale changes in ocean climate have reduced winter food availability in areas of the North Pacific outside of the eastern Bering Sea, through effects on either prey population abundance or distribution. Large-scale changes in the environment are known to have occurred in the North Pacific (see Chapters 3 and 4)
From page 153...
... . However, the mean levels measured in Steller sea lions in both periods are well within normal ranges reported for several other otariid species (Bossart and Dierauf, 1990)
From page 154...
... It is not known whether low food availability is a contributing factor to these relatively low rates. It may be significant, however, that adult Steller sea lions consume more fish of the size range removed by commercial fisheries than do other Bering Sea pinnipeds (Lowry and Frost, 1985; Lowry et al., 1989)
From page 155...
... BIOLOGY OF HIGHER TROPHIC LEVELS 155 and the general population decline may provide some insight into factors affecting harbor seals in these areas.


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