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The Bering Sea Ecosystem (1996)
Commission on Geosciences, Environment and Resources (CGER)
 Polar Research Board (PRB)

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6 Causes and Effects in the Bering Sea Ecosystem r 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. The goals of this chapter are to broadly examine the effects of both environmental variation and patterns of human use on the Bering Sea ecosystem, and to develop a scenario for how they may have manifested themselves over the past three decades. The chapter is divided into three sections. The first section attempts to answer the following questions: Is environmental variability a cause of changes in marine mammal, seabird, and fish assemblages in the Bering Sea ecosystem? If so, are the links direct (e.g., a direct population response to physical forcing) or indirect (e.g., bottom-up trophic interactions)? The second section focuses on human effects by addressing the following questions: Is human activity a cause of change in marine mammal, seabird, and fish assemblages in the Bering Sea ecosystem? Has nonextractive resource use, such as waste disposal and pollution, had an effect on the ecosystem! Has extractive resource use i.e., fishing- affected the Bering Sea ecosystem through removals causing short-term changes in density or distribution, or longer-term effects on population abundance, or through indirect impacts on nontarget species (e.g., bycatch)? . The third section presents a conceptual analysis of observed changes in marine mammal, seabird, and fish assemblages in the Bering Sea ecosystem over decadal periods. Some major changes in the Bering Sea ecosystem are known to be the direct result of human extractive use (e.g., the depletion of certain marine mammals and fish stocks of commercial importance), whereas other changes appear to be related to environmental factors. The proposed conceptual model recognizes that both anthropogenic and natural factors have had major influences on the Bering Sea ecosystem the mode! suggests that the current Bering Sea ecosystem is a product of the complex interactions between human use and natural fluctuations caused by natural forcing. The analyses in this section build on the framework provided in Chapter 2. 196

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Causes and Elects in the Bering Sea Ecosystem ENVIRONMENTAL VARIABILITY 197 Much has been said about the regime shift that occurred in the North Pacific starting in the mid or late 1970s (Chapter 3). A relatively warm period was initiated in the northeastern part of the region, oceanic plankton productivity increased in the Gulf of Alaska, salmon production more than doubled throughout Alaska, and several fish populations experienced unusually strong recruitment and grew dramatically. At the same time, Steller sea lions, harbor seals, and some populations of seabirds in the Alaska region declined. Were such changes unique or had they occurred previously in this region or elsewhere in the world? Appropriate data sets to answer this question are scarce. There are suggestions (see Francis and Hare, 1994; Beamish, 1993) that an earlier period of high productivity occurred in the North Pacific about 50 years ago. The apparent abundance of several stocks of sardines in the Pacific showed similar peaks, in the late 1930s and early 1940s and again after the mid-1970s (Kawasaki, 1991). Steele and Henderson (1984) modeled long-term fluctuations in fish stocks and showed that many pelagic fish stocks change rapidly in abundance, with intervening periods of about 50 years. In a later paper, Steele (1991) reviewed changes in several North Atlantic ecosystems, including the North Sea, where there had been dramatic herring declines and demersal fish increases, and concluded that large switches in marine communities can last several decades and that they can occur without human involvement, but can be increased in frequency or amplitude by human actions (see also Chapter 2). While the major cause of such regime shifts appears to be changes in ocean climate of comparable frequency-that is, changes in circulation and mixing, and consequent alteration of surface layer temperature and other conditions-the mechanisms linking environmental and biotic changes are poorly understood. Conventional wisdom is that productivity at lower trophic levels is enhanced (or diminished), for example, by intensification (or diminution) of upwelling, with altered productivity then being felt at higher trophic levels. There may be other significant biotic consequences of climate changes, including: Changes in the community structure as recruitment of different species is differently affected by ambient conditions. Changes in distributions and migration patterns, especially of early life history stages, affecting survival of individuals of species and their availability to predators. Many of the changes in the relative abundances of species in both fish and invertebrate communities observed in the 1960s in the Bering Sea appear to have been associated with the development of large commercial fisheries. This is perhaps most clearly illustrated in the overexploitation of groundfish assemblages (e.g., Pacific Ocean perch, yellowfin sole, and herring in the eastern Bering Sea and Gulf of Alaska). But low-frequency changes in ocean and atmospheric climate have also been documented in the Bering Sea and Gulf of Alaska over this

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198 The Benng Sea Ecosystem period. These environmental changes may have increased the productivity of some species while reducing the productivity of others. Natural Frequencies in the Environment Over relatively short periods, changes in ocean climate will have the most pronounced impacts on the productivity of lower trophic levels. The response of top consumers to such changes may also be evident In the short term (for example .cenhirr1 chink c,~rviv~l) hilt lnne`~r_ term effects maY also be expected. ~ ~ ~ e ~ . · . - ~ 7 A. One important issue in this study is whether the environment is influencing fish population and associated fishery changes. To address this question, it is necessary to first define what is meant by natural fluctuations in the environment and their effects on fish and fisheries in marine ecosystems. Some aspects of Bering Sea variability discussed in Chapter 4 are then revisited in order to see how they may relate to environmental forcing. There are many natural frequencies of variability in the northeast Pacific and Bering Sea. Those most studied by fisheries oceanographers are those that occur at the annual and decadal scales. The sense of annual scale variability is critical for providing the stock assessments needed for annual fisheries management cycles. However, it is the decadal (and longer)-scale variability that seems to provide more insight into marine ecosystem processes, and such scales are rapidly becoming the focus of fisheries oceanography in both the North Pacific and Norm Atlantic. A fundamental question being asked is whether climate can cause rather rapid shifts in the organization of marine ecosystems, and if so, on what time and space scales these effects can be measured. Hollowed and Wooster (1992) postulated that there are two mean states of winter atmospheric circulation in the North Pacific (Figure 6.~. These states are identified by the frequency of intense winter "Aleutian low pressure" events and the resulting ocean temperature and surface wind field responses. In a later paper, Hollowed and Wooster (1994) were able to show a link between years of anomalously low coastal ocean temperatures (caused by intense winter low-pressure events) and simultaneous strong year classes in groundfish species ranging from California to the Bering Sea (Figure 6.2~. Fritz et al. (1993) provided evidence that pollock recruitment is positively correlated with temperature (Figure 6.31. Francis and Hare (1994) have used the methods of tune series analysis (autoregressive integrated moving average, and intervention models) to analyze or describe the spatial and temporal dimensions of the relationship between salmon production and atmosphere/ocean physics. They found very significant and coherent linkages between relatively sudden interdecadal shifts in the North Pacific atmosphere and ocean physics and marine biological responses as evidenced by indices of Alaska salmon production. Figure 3.10 summarizes some of the results of these analyses. The top pane} shows the time series of the North Pacific index (winter atmospheric pressure) during the twentieth century (Trenberth and Hurrell, 19941. The bottom two panels show actual fits of data and intervention analysis models to the index (1900-92) and western Alaska sockeye salmon catch time series (1925-921. Two points stand out from this analysis:

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Causes and Effects in the Bering Sea Ecosystem Type A Type B ..... ~ ~ . . . C,.... c .... . i.. .. I..... ~ I. , ~-.~... 199 20 W! NTE.R 72 it ~ ?~ . , ~ ~ ~ · · ~1 -220 -A - ~ ~ - I ~ - 1 to - LONG 1 TUDE lo A g SO - £ to J ~0 W I NTER 77 L :; ~j . . i i ! tat: · 163 - ~ 10 °: LONG 1 tl-3E Figure 6. 1 Two alternating patterns of atmospheric circulation postulated by Hollowed and Wooster (19921. An example of a winter sea level pressure pattern is illustrated for each circulation type (reproduced from Emery and Hamilton, 19851.

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200 2.CO 1 00 O,SO 0.00 -0.50 -1 .50 The Bering Sea Ecosystem .^ ~ :, :) ` - ~.. - ss sa sl 49 ~7 45 41 ~g 37 ~S ~ o o . . o o o ° O O · O O O' · o 00 · O O O ~ °O ' , o oO 0 0 · O · ~ O O · o ~ o . o o o o · O · o o · · O · ~ o O · O O O · e O e · ~ O ° O ~ O · · O · ° ~ O · O O · ° · O ' · O ° · ° O O ~ O O · O O O e O O · ° ° O O O · O O O ~ · ~ O O e O O O · O ~ · · O · · · ~ ~ (3 ' O O O e e ~ ~ · · O O · O O ° ~ _ _ . High I Low 1 ~i 1 1 1 1 1 l l ALPI 1945-1990 1 1 1 1 1 1 1 t.50 ~·e · · ~ i · ~ ' ~ ~ ~ . ·- O ~ ~ · ~ · · O 00 ' ° ~ ~ · · O ~ · · O O O O · O · · · O O · · O O O O e O · · · · O ~ ~ O ~ O O O ' ~ · - - - - ~ · ~ · · ~ O ° · ~ · · ° · ~ O ~ · · O ~ · · · ~ · · ~ O O · · O O e · · · · 0 e O · 0 0 0 · e ~ O · _ _0 _ - 0 0 0 e · · · e C) _ _ _ .. .. 0~212.2s.o.1 ° · 210.ss.0.1 O · , 1 '.o s.o. 1 · · ~ 1 o.s s.o~l i . l 1 1 1 ' 1 1 1 1 1 1 1 . ~. . 1 1 1 1 1 1 , 1 1 1 1 1 1 ~ 1 1 1 1 1 1 1 1 ~- ~_ ~ _ Figure 6.2 Relationship between sea surface temperature anomalies (upper) and proportion of northeast Pacific groundfish stocks with extreme year classes (lower) (W. Wooster, personal communication) .

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Causes and Elects in the Bering Sea Ecosystem Age 3 Recruits In bililons 1B~ 14F 12 10 4 2 o W W // .A / A A HOW COW/-~ W C / A ·A C TIC AH o 2 W = warm year, C = cold year, A = average year 201 4 ~ E, 10 12 Number of Spawners in billions tat 18 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). Coherence is shown between physical and biological variables primarily at the decadal (regime) scale, and not the annual scale. During the twentieth century, there appear to have been four interdecadal regimes (Figure 3.10) in the North Pacific coupled atmosphere/ocean system: 1900 to 1924, 1925 to 1946, 1947 to 1976, and 1977 to the present. Certainly the well-documented regime shift that occurred during the winter of 1976-77 had a profound impact on many components of North Pacific large marine ecosystems. Venrick and others (1987) show significant shifts in phytoplankton production (integrated chlorophyll-a) just north of Hawaii at about that time (Figure 3.16). Subsequent analysis reveals that the increase may be due to increased production of a deep species of phytoplankton in response to a shift in ocean mixing and a deepening of the mixed layer. Brodeur and Ware (1992) found a significant shift in the zooplankton biomass of the subarctic Pacific (Alaska Gyre), which corresponds to this recent regime shift (Figure 3.17). Later in this section, we point out possible relationships between this climatic regime shift and biological responses in the Bering Sea ecosystem. Baumgartner et al. (1992) looked at natural frequencies in climate-driven fish production of the northeast Pacific over a much longer period. Through the analysis of fish scale deposition rates in anaerobic sediments in the Santa Barbara Basin off southern California, they developed a 1,750-year time series proxy of pelagic fish abundance in the California Current (Figure 6.4,

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202 l 2co 1 '°°°1 BCO c' 00 ;. ~ oo c.` 200 L. 0 y, OOo C05 ~0 0 1S 020 2000 200 co 6 ~b0 1 600 ~ 1 400 q,, 1 200 ~1 000 ;- BOc c, .> 600 400 200 o PACl f IC SARD'N ~ RES~DUALS Il. 76 57 years ~V N~__ J 000 C05 0,0 0 ,5 020 2000 200 ~Co 67 50 Frequency NORTHERN A~C~O' 2ESIOUALS 72 S? rears 99 A ~ ~V~ Period 0 2S ~ Jo t C 25 0 so 0 )5 CyC es/tO ', 40 ,) ~,)e Yea,' ~S, crties/~° y, "c ,:' vec,' ~ ~ t _ _ _ t, . · . The Bering Sea Ecosystem ~... . .. .. .. . 6 ~ 1' Parific Sar`1inP V ~ V! [1- ~ V1 V ~ V V ~! V ~-J ~ i \/ ~ V V ~. ~ I V U ' 1 200 300400 500 600700 eoo 900 ~ooo t100 3200 ,300 ,400 :soo '600 ,700 '900 '900 2^00 Northern Anrhn`A' 0. 0 ~1 200 JoO 4 ~5 ~6 ~700 600 900 1 0~ ~ 1 ~1 200 ~ 3~ 1 4= ~ SCO ~ 6~ ~ 700 ~ 800 ~ 900 2000 YEARS S - - l h,` 'S4~di-t sn. . S-ool`~` 1 1 ~lac`~' so' V W~ ~ ~; J ~.I`l ~` l ~ ,0 20 I ~ J. -_ 11~04 ~ ~ Wh;~t H1, ~1 ' ' ' I tCL, 1~t R;^~, PACIFIC ,: , ' ^. :- V It o o o o - o NORINERH ~o AH CHOVY O 200 400 600 ~oo 1 ~/~ - 1400 t600 1~07~ Y'o' Figure 6.4 Proxy time series of pelagic fish abundance in the California Current (top), power spectra for high-frequency ( 150 year) variability compared with tree ring widths (bottom right) (Baumgartner et al., 1992; T. Baumgartner, personal communication).

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Causes and Elects in the Bering Sea Ecosystem 203 top panels on Pacific sardine and northern anchovy). They partitioned this variability into low frequency (periods greater than 150 years) and high frequency (periods less than 150 years). The low-frequency variability reflects the centennial-scale climate epochs of the last 200 years (e.g., Medieval Warm Period and Little Ice Age), and relates very closely to bristlecone pine tree ring widths' which reflect prevailing atmospheric temperature regimes (Figure 6.4, bottom right panel). The high-frequency power spectra for both sardine and anchovies show peaks at around 60 and 75 years (Figure 6.4, lower left panel). During the twentieth century, pelagic fish populations of the California Current have tended to vary in response to atmospheric forcing similar to Alaska salmon (see Figure 3.14) (Ware and Thompson, 1991). This being the case, the kind of decadal-scale atmospheric forcing in the North Pacific seen during the twentieth century could have persisted for centuries. As discussed in Chapter 3, Royer (1982) has identified possible relationships between regular variations in the production of a number of commercially important North Pacific marine species and the 18.6-year nodal tidal signal. For example, fluctuations in Bering Sea herring abundance since the early 1960s are noticeably similar to the pattern of sea surface temperature (Figure 6.5). Herring spawn inshore and feed in the coastal zone during the summer. Thus, year-class strength is probably related to the seasonal development of the coastal biological community, which is advanced and more prolific in warm years (Cooney, 1981; Springer et al., 1984, 1987). Likewise, the nodal tide signal has been associated with 60 percent of the variance of year class strength of halibut in the Gulf of Alaska (Parker et al., 1994). Of possible greater importance to the biota than the temperature contribution is the alteration of the relative amplitudes of diurnal and semidiurnal tidal components over this 18.6-year period. Organisms that are dependent on one of these tidal components might suffer under these changing conditions. Nodal tide variability might also affect the tidal mixing in the Bering Sea, as suggested by Loder and Garrett (1978) for the Labrador shelf. The point of this discussion is that clunate-driven variability in the large marine ecosystems of the North Pacific is significant, occurs at many different time scales, and affects many components of the ecosystems. It seems clear that climate does cause rather rapid shifts in the organization of these marine ecosystems and that the decadal scale may be more important than the annual scale in its impact. Envirorunental Effects on Marine Mammals and Birds Over relatively short periods, changes in productivity will likely have the most pronounced effects at lower trophic levels, but there may also be longer-term effects on top consumers. Lipps and Mitchell (1976) proposed a model linking the evolution of pelagic marine mammals to changes in the trophic structure of the oceans. They suggested that variations in primary productivity caused by upwelling may explain the invasion of the sea by mammals, and subsequent radiations and extinctions. As large-bodied, long-lived, K-selected species, marine mammals would be expected to be relatively insensitive to many types of environmental fluctuations. As discussed in Chapter 4, most species that occur in the Bering Sea have relatively broad distributions in the North Pacific or Arctic Basin, and are therefore adapted to survival under a range of environmental

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204 The Bering Sea Ecosystem 7 6.5 v: 6 v, 5.5 Eastem Bering Sea HeITing · SST AMP.. Ho-~ Hemng at !~° I,,.\ ~ y .. O 0 b ~Oo 5- r . , 1950 1960 1970 1980 1990 -1500 - - 1000 ° - , u' ct - 500 0 . _ m o Figure 6.5 Bering Sea herring abundance and sea surface temperature (Wespestad, 19911. 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). However, it is clear that pinnipeds living in more southern regions may be affected by relatively severe climatic events such as E1 Nino (Trillmich and Ono, 19911. Climate-caused physical changes in the environment can have significant effects on marine mammal habitats, and thereby on the abundance and distribution of species. In the Bering Sea, sea ice plays an important role in marine mammal ecology, providing important habitat for some species while excluding others (Fay, 1974). Year-to-year variations in the extent of sea-ice cover change species distributions, which may have a variety of biological effects. An example of ice-cover-dependent interspecific interaction is the relatively high rate of walrus predation on seals that occurred in the Bering Sea in 1979. This was a very light ice year, when the overlap in distributions of these species was greater than usual (Lowry and Fay, 1984). Over a longer time frame, changes in sea-ice cover are related to changes in sea level. The two in combination may have a dramatic effect on marine mammal habitat availability (Davies, 1958). These events must have resulted in major shifts in the distributions of pinnipeds that use either land or ice for hauling out. As discussed previously, the recent significant declines in fur seals, sea lions, and harbor seals probably have been environmentally influenced by climate-induced changes in the abundance and availability of food for juveniles during this critical phase of their life histories. More specifically, the problem probably has something to do with the relative availability of appropriately sized pollock and other forage fish (e.g., capelin, sand lance, and herring), and

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Causes and Elects in the Bering Sea Ecosystem 205 would be a decadal (rather than annual)-scale effect. Environmental change, however, may not be the only cause of reductions in food for pinniped species. Bering Sea Fish (Invertebrates) and Fisheries A number of the variations in Bering Sea fish populations described in Chapter 4 appear - '-- -'- ' ~ ~ ~ ways that this kind of forcing mamIesls riser. first, it can realslrlDule marine ilsn populations and their fisheries in space. The second and perhaps more unportant way is by affecting the absolute abundance of marine fish populations primarily through mechanisms controlling recruitment of new individuals. A considerable number of commercially important marine fish stocks of the Bering Sea (and Gulf of Alaska as well) and their fisheries are, at any time, supported by a relatively small number of very strong year classes or cohorts. Looking at a number of figures presented in Chapter 4, one can certainly see evidence of this in the eastern Bering Sea and Gulf of Alaska (see Figures 4.1, 4.2, and 4.3 on king and Tanner crab; Figure 4.5, on walleye pollock; Figure 4.13, on Pacific cod; Figure 4.14, on Atka mackerel; Figure 4.18, on Catfish; Figure 4.21, on Pacific Ocean perch; and Figures 4.24 and 4.25, on herring). Because fisheries tend to develop on stocks that are at high abundance (due primarily to strong recruitment), attempts to attribute cause generally focus on the causes of declines in these stocks rather than their increases even though the high stock levels may be anomalous events. The classic example of such a case in the northeast Pacific is that of pollock in Shelikof Strait (Gulf of Alaska). lo oe related to environmental forcing. In general, there are two _ _ ~ I_ ~ 1~ The _ ~ ·^ ~ - . a , · ^- ~ ~ . Groundfish. A number of groundfish populations of the eastern Bering Sea, Aleutian Islands, and Gulf of Alaska were significantly affected by the well-documented regime shift (general warming) that occurred in the winter of 1976-77 and subsequently manifested itself in the late 1970s and early 1980s. As pointed out by Hollowed and Wooster (1992), several strong year classes of groundfish occurred in 1977 and 1978 (eastern Bering Sea and Gulf of Alaska pollock in 1978, eastern Bering Sea cod in 1977, and Aleutian Islands Atka mackerel in 1977). In particular, these very strong year classes of pollock and cod supported significant expansions of Bering Sea fisheries. The 1978 year class of pollock essentially supported the expansion of the Bering Sea pollock fishery into the Aleutian Basin (Bogoslof and donut hole). In addition, the pollock fishery of the eastern Bering Sea shelf was redistributed to the southeast in the late 1970s in possible response to changes in ocean clunate (see Figures 4.8, 4.9, and 4.10). Although the year classes seem to be more difficult to pinpoint, significant increases in eastern Bering Sea shelf Catfish populations occurred in the late 1970s and early 1980s (yellowfin sole, rock sole, and Alaska plaice; see Figure 4.18). There was also a large increase in arrowtooth flounder in the Gulf of Alaska. At the same tune, the one eastern Bering Sea flatfish species that tends to prefer colder water (Greenland turbot) declined significantly. Juvenile Greenland turbot were pushed to the northwest along the outer Bering Sea shelf as the ocean climate warmed (Figures 4.15 and 4.16).

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206 The Bering Sea Ecosystem 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). As for a number of groundfish populations, the 1977 year class of eastern Bering Sea herring was extremely strong, possibly in response to the concurrent regime shift. It is interesting to note that the three largest year classes of herring (1957, 1958, and 1977) occurred at times of significant warming in the North Pacific (see Figure 6.5), and perhaps also in response to the 18.6-year nodal tidal signal. In the western Gulf of Alaska, a 20-year record of trawl catches of Important indicator species revealed that capelin virtually disappeared in the 1980s (Figure 4.27). Data on seabird diets and from fishery surveys in the Bering Sea also indicate that capelin and eulachon, another smelt, essentially disappeared there during the same time (Decker et al., 1995; Fritz et al., 1993; Figure 4.26). Both species were comparatively abundant in the 1970s, when the physical regime was much different than in the 1980s. Deposition rates of fish scales in Skan Bay on the Bering Sea side of Unalaska Island (eastern Aleutians) declined abruptly in the late 1970s after a period of relatively high abundance (R. Francis, personal communication). None of these short histories runs entirely parallel to that of herring, and all are difficult to evaluate in a decadal-scale context. Salmon. The clear link between decadal-scale variations in Alaskan salmon abundance and regime-scale North Pacific climate fluctuations was discussed earlier in this section. Invertebrates. As discussed in Chapter 4, fluctuations in eastern Bering Sea and Gulf of Alaska crab populations (king and Tanner; Figures 4.1, 4.2, and 4.3) and their fisheries occur in response to infrequent strong year classes or cohorts. The most dramatic of these fluctuations occurred in 1981 in Bristol Bay and 1982 in the Kodiak region of the Gulf of Alaska, when red king crab fisheries in both regions crashed. The Bristol Bay resource supported the largest king crab fishery in the world. Beginning in 1966, this fishery increased annually, to an all-time record catch of 58,938 t in 1980 (Figure 5.7). This increasing trend was mirrored in the biomass estimates from stock assessment surveys performed annually by the National Marine Fisheries Service. However, beginning in 1979, the survey began reporting a decline in overall biomass relative to the prior year's estimate. These preseason forecasts were not readily accepted by the industry. Many people believed that the survey "missed the crab." In 1981, however, the catch dropped from almost 59,000 t to 15,876 t. Although such a decline in catch was predicted, the magnitude of the decline was shocking. In 1982, the catch fell to only 1,361 t, and in 1983 the number of reproducing male and female crabs was believed to be so low that no directed fishery was permitted. Similar collapses were observed in all the other major crab fisheries in the Bering Sea (Figure 5.7). The next most valuable king crab was blue king crab, found most abundantly in the Pribilof Islands region. This fishery experienced similar declines in catch, from a high of 4,991 t in 1980, to 1,998 t in 1982, to only 318 t in 1987. Since then there has been no directed fishery for blue king crab in the Pribilof region. The largest of the Tanner crabs,

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Causes and Elects in the Bering Sea Ecosystem 227 the same time (Figure 6.161. In both regions, inflection points in the trends apparently occurred in the mid-1970s and the mid-late 1980s. Contrasting with both is the apparent stability in numbers of murres and kittiwakes at Cape Peirce, a coastal colony in Bristol Bay (Figure 6.17). Productivity of kittiwakes at Buldir cannot be readily assessed, because it has been determined only since 1988. At Cape Peirce, information on kittiwake productivity is fragmentary from 1970 to 1984 but continuous since, and it shows features similar to the Pribilofs case-productivity is comparatively high now after recovering from a low period during the early to mid-1980s. Kittiwake productivity at offshore colonies in the Gulf of Alaska declined to very low levels in the early 1980s, as water temperature rose (Figure 6.181. However, it did not subsequently recover (as at the Pribilofs and Cape Peirce), but has continued to be generally poor, perhaps because water temperature in the gulf has remained warm (unlike the Bering Sea, which has been cooling). Murres in the gulf have also suffered poor productivity in recent years, and declines have occurred both within and outside of the area oiled by the Exxon Valdez spill and were apparent before the spill (Piatt, 1994). The seabird decline in the gulf occurred at the same time as a shift in pathways of energy flow through food webs there. Capelin was the dominant prey of several species of seabirds during the 1970s but was absent, or nearly so, in diets in the 1980s (Figure 6.19). The shift corresponded with the other indicators of a widespread decline of capelin in the Gulf of Alaska and Bering Sea. Capelin was replaced in seabird diets primarily by pollock and sand lance. It is not known if the absence of capelin by itself has been responsible for the poor productivity of some seabirds or is simply indicative of generally poor prey availability. The declines in the abundance of an important forage species coincided with changes in the physical environment of the gulf (as measured by water temperature), trends that together provide support for the hypothesis that some recent dynamics of seabirds, and perhaps other species at higher trophic levels, may be related to physical changes in the ecosystem. Fish and Invertebrates The main target of the developing international fishery in the late 1950s was yellowfin sole, a bentho-pelagic continental shelf species (see Chapter 4). During a brief period from 1959 to 1961, catches averaged over 400,000 t annually. After 1962, the yellowfin sole population and catches rapidly declined to low levels as a result of heavy exploitation. With the crash of the yellowfin sole population, trawl fisheries began to harvest eastern Bering Sea Pacific halibut, Pacific Ocean perch, and sablefish-all long-lived continental-slope species. With the rapid overexploitation of these species and herring (eastern Bering Sea herring biomass decreased to one-sixth of its former levels during the 1960s, as shown in Figure 4.24), foreign trawl fleets moved into the Aleutian Islands and Gulf of Alaska to target slope rockfish and, in particular, Pacific Ocean perch. The latter species was subsequently "mined out" of these regions during the 1960s (see Figure 4.20 for stock biomass trajectories). During the 1960s, as stocks of Pacific Ocean perch declined, foreign trawl fleets shifted to walleye pollock (Alton 1981). Reported catches of pollock in the eastern Bering Sea increased rapidly from 175,000 t in 1964

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228 100 80 c~ ~ 60 ao .~> 40 ct < 20 100 80 c~ ·> cot au 20 The Bering Sea Ecosystem Kittiwakes ~ 1 of / . ~ 0 O 1970 1975 1980 1985 BLKI-Buld* I. RLKI-Buldir I. BL~KI-Agattu I. 1990 1995 Murres 60 40 ... ~...~ o .o O- ·~eIneBeles 1970 1975 1980 1985 a at / 1 1 TBMU-Buldir I. · o COMU-Agattu I. 1990 1995 Figure 6. 16 Trends in abundance of thick-billed (TBMU) and common (COMU) murres and black-legged (BEKI) and red-legged (RLKI) kittiwakes at Buldir Island (Williams and Byrd, 1992; I.C. Williams, unpublished data).

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Causes and Elects in the Bering Sea Ecosystem 0.5 0~4 ~ ·~ ~ 0.3 c~ ~ ° 0~2 0.1 oi ··~ a ~ I ~ ~ - I ~ · a ~ I at-- a a I 197019751980 1985 1990 100 80 c~ ~ 60 .> 40~ 4 - c~ - 0< 20 O~ 1975 229 _~. · ~n ~e ma ~ ~ COMU ..... o BLKI -- 1 ~ ~ ., I, 1980 1985 p.~0A .....oJ ~ 1990 1995 Figure 6. 17 Trends in productivity and abundance of common (COMU) murres and bIack-legged (BEKI) kittiwakes at Cape Peirce (Hagbloom, 1994)

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230 The Bering Sea Ecosystem 8~5 ~ 8 7 5 ~ En v: v, v' 1 1 Coastal Colonies 7~ 6~5 6 8 5 ~ 8 7 5 7 6~5 ~ -0 8 -0 6 .> e_ c' : -0.4 O SST C' ... o Plroduc~vity -0.2 1950 1960 1970 1980 1990 Offshore Colonies ~ ~ {got - · SST .. 0 .. ~' o Productivity 6- I ~ 1950 1960 1970 1980 1990 ~ 0 - 1 ~0 8 ~0 6 ~ -0.4 sit - 0.2 O 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)

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Causes and Effects in the Bering Sea Ecosystem 50 ~ 40 ~ v 30 o o v z ~90 ~ 10 ~ O ~ tSO ~ ') - 198X-1991 5 40 v o o ~ 30 ~3 20 ~ In 231 1975-1978 L CAPELIN 03 SANDLANCE O O POLLOCK SQUID L 1 1 ~ . 1 1 KIlll~'AKE CONl~lO.N'.\lLiRRE n=328 (n=~: NIARBlEL) SItIRRELET HORNED PlTFFIN' Ttil--l-ED Pt'FF1N (n=l58) (Il=54) (n=44()) 1 1 '1 1 CAPELN E~ SANDLANC:E O PC)LLOCK O SQ()1D 1 _ f~- ICIIlIWAKE CO.\l.\l().N' N1t'RRE (n-~)~) (~1=()~) (Il=73j N1ARBLEl) NIt KRELET H()RN'ED I'l'FFIN' n:FTED PlTFF~' (n= 1 ()O) (n=32'3) Figure 6.19 Diets of seabirds in the Gulf of Alaska, 1975-78 and 1988-91 (Piatt and Anderson, in press).

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232 The Bering Sea Ecosystem 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). Pollock catches in the Gulf of Alaska increased in the early 1980s, to a peak of around 300,000 t in 1984 (due to a more than three-fold increase in biomass between 1974 and 1982), and then fell to near 60,000 t in 1988 as the strong year classes responsible for the surge in the fishery disappeared. Between the late 1970s and mid-l98Os, a number of eastern Bering Sea (EBS)/Aleutian Island (Al) groundfish and pelagic stocks showed rapid increases in biomass (EBS pollock, 2.6 times; EBS cod, 3.1 times; Al Atka mackerel, 4.4 times; EBS herring, 3 times; EBS arrowtooth flounder, 4.7 times; EBS yellowfin sole, 1.6 tomes; EBS rock sole, 1.9 times; and other EBS Catfishes, 3.8 times) (NPFMC, 1993~. The estimated adult biomass of these species combined increased from around 10 million t in the late 1970s to 25 million t in the mid 1980s. Fisheries on these stocks increased accordingly. At the same time, a number of forage fishes, in particular capelin around the Pribilof Islands and Kodiak Island, declined significantly. King crab populations and their associated fisheries, which had declined in the eastern Bering Sea and around Kodiak Island in the early 1960s, grew very rapidly in the eastern Bering Sea in the 1970s and then crashed in both the eastern Bering Sea and Kodiak region in the early 1980s (Figures 4.1, 4.2, and 4.31. Both C. opilio and C. bairdi populations in the eastern Bering Sea declined in the 1970s; C. opilio populations and landings increased significantly in the late 1980s (Figure 4.11. DISCUSSION The Bering Sea ecosystem, like all ecosystems, has undergone natural environmental changes for a very long tune. It has also been significantly affected by human activities for hundreds of years, more significantly in the past 200 years (Chapter 51. Therefore, the changes of the past several decades that have been discussed in some detail in this report cannot be interpreted as perturbations to a "pristine" ecosystem, and they are not unique in many ways. However, until recent decades, good information on many of the major species in the Bering Sea was lacking. Thus the "cascade" changes that began in the 1950s described below crobabIv are only the most recent in a series beginning no later than the nineteenth century. To recapitulate our understanding of the most recent set of changes, from the mid-19SOs to the early 1970s, large populations of both fish and mammals (particularly the large whales) were dramatically reduced. This reduction in biomass likely increased the amount of food (zooplankton and small fish) available to other vertebrate predators. Unfortunately, there is not sufficient information available on a tune series basis for this period to examine the abundances and distributions of such organisms to verify this. However, it seems reasonable to suggest that the dramatic increase in the abundance of pollock, which apparently occurred during the late 1960s in the eastern Bering Sea, may have been in some way linked to overexploitation and reduction of these other populations. In fact, there is little doubt that the eastern Bering Sea fish assemblage switched to a pollock-dominated system in the late 1960s and early 1970s, and that this domination of pollock biomass has persisted since then. What other changes might have been associated with the concentration of resources into pollock biomass? One effect might have been reduced abundance of forage fish species, such ~. . . . . , ~

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Causes and Elects in the Bering Sea Ecosystem 233 as capelin and sand lance, which compete with young pollock for zooplantton. 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. A major climate regime shift occurred in the North Pacific in the late 1970s. Combined with the historical restructuring of parts of the large oceanic ecosystems of the region, this climate shift could have set the stage for the rapid changes in top-level predator populations that seem to have accelerated in the last 15 to 20 years. There is persuasive evidence of very large increases in pisc~vorous fish populations resulting from the synchronous strong vear clns.ce.s that no.~.llrr~.r1 in the 1~tP 1 07na just after the regime shift. 1t 4_~-_1_ /1 ^~\ __ ~, ~_ ~^ ~ _ ~·^- ~4 ~4~ ~4, , ~ A, 1~1~111~ \1~>JJ proposed a very similar hypothesis to explain the recent declines in He Steller sea lion population of the northeast Pacific and eastern Bering Sea. He suggested that there had been a major shift in the trophic structure of the Bering Sea (and probably the Gulf of Alaska) beginning in the mid-1960s, "a decade prior to the initiation of the current fishery management regime." He argued that if there was an actual cause-effect relationship, it lay in the dietary overlap between whales, fish, and some pinnipeds. The significant increase in pollock biomass of the 1960s (eastern Bering Sea) and early 1970s (Gulf of Alaska) (see Figure 4.4) could then have resulted from the release of euphausiid and calanoid copepod prey (preferred prey for juvenile pollock) due to the fishery removal of whales (fin, set, and humpback), Pacific Ocean perch, and herring, and the reduction of juvenile pollock predators due to the significant harvest of northern fur seals. Merrick estimated that "the reduction of fin whales, Pacific herring and Pacific Ocean perch in the Bering Sea and Aleutian Islands could have released 1.36 to 2.81 million mt of zooplankton prey a year...which would be sufficient to feed 4.3 to 8.9 billion age-1 pollock for a year." In addition, he estimated that the reduction in fur seals could remove predation on 2.8 billion age-1 pollock. He concluded that the released zooplankton and reduced predation pressure could have supported pollock abundance at the high levels resulting from recent high recruitments. He also reported that around the time of the 1976-77 climate regime shift' preferred prey (forage fish such as capelin and sand lance) began to disappear in a serial fashion as a number of adult groundfish populations, in particular Pacific cod and a number of Catfish species, grew significantly. In summary, Merrick concluded that the combination of favorable ocean conditions, continued zooplankton release, and low predation in the late 1970s and early 1980s resulted in a series of strong pollock year classes. As a result, adult (but not juvenile) biomass again increased and remained high in the eastern Bering Sea (perhaps due to an effective fisheries management regime using light exploitation rates), and predatory Pacific cod and Catfish populations also increased. The.ce fishes rem the ~ ~..1 ~^ __ _` ~ 1 1 ~ ~ ~ . . ~ . . . ~. pUpUldLlOllb U1 Small Ilsn prey sucn as capelln and sculpms. the reduced biomasses of forage fishes and juvenile pollock abundance were insufficient to meet the dietary needs of Steller sea lions. One piece of this scenario that is not fully understood is why juvenile pollock have been less abundant recently. One possibility is cannibalism, as well as predation by Pacific cod and flatfislles. Another possibility is that although recent environmental conditions have favored pollock recruitment, there have been a few large year classes (especially 1978), rather than a steady supply of juveniles, and the lack of other forage fishes has made the variable supply of juvenile pollock more of a limitation for marine mammals and birds than it normally would have been. Indeed, the year-class strength of pollock has declined recently. What does seem likely

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234 The Bering Sea Ecosystem is that a combination of cascading trophic interactions and a significant cremate 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. Can we expect species to recover from a perturbation (either human-caused or environmental) in a changing ecosystem? For example, consider the large-scale removal of whales and predatory fishes from the Bering Sea. This removal may have resulted in an explosion of one or more prey species (e.g., walleye pollock), as well as an increase in other predators or increased concentration on pollock by predators that feed on these prey, including humans. The resulting reordering of the competitive food web would likely be characterized by an increase or decrease of many of their other components (e.g., crab, sea lions, and seabirds). As a result of this complex reorganization, whales may encounter new competitors for their prey base (e.g., factory trawlers with sophisticated electronics and huge nets) that limit their recovery notwithstanding direct regulatory protection. 2. Time lags and different time scales can produce nonlinear relationships that make it very difficult to explain the changes that occur at any one time. For example, are we still seeing changes in the Bering Sea ecosystem caused by baleen whale removals in the 1960s and 1970s? Was the large pollock year class of 1978 a result of prey release or competitive interactions at lower trophic levels that enabled pollock populations to explode? Or was it a response to warning seawater temperatures after the 1976 regime shift, allowing an increase in water column production and juvenile success? Were both factors partly responsible? Is the current organization of the ecosystem stable or is it likely to shift again, either to its previous condition or to some other condition? 3. At any one time the system may be controlled (limited) by a few key factors (Berryman, 1993). Because these key factors are generally unknown, it is not often possible to know a priori when humans influence one of them, sewing off a series of unexpected, cascading changes throughout the ecosystem. This '~cascade hypothesis t' (described in Chapter 2) might explain some of the rapid and then persistent changes observed in crab, pandalid shrimps, demersal fish, seabird, and marine mammal populations and assemblages. The key question is how this knowledge can be used to manage human actions in order to reduce our impact on the ecosystem. CONCLUSIONS As a general conclusion, the most likely explanation of events in the Bering Sea ecosystem is that a combination of a decadal or regime shift in the physical environment acted in concert with human exploitation of predators (whales, other fish) to cause pollock to dominate the ecosystem and other predatory fish populations to greatly increase in abundance. As a result, some forage fishes that have higher nutritional value than pollock became less available to some marine mammals and birds, leading to their decline. The increase of adult pollock and other predatory fishes in the past 20 years might also be responsible for keeping the forage fishes

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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 materials and birds, especially juveniles. There is no evidence that any human activity other than fishing (including whaling) has had a significant effect on the Bering Sea ecosystem, although better information on various pollutants would be of interest. We emphasize that although this is the most likely explanation or description of events over the past 50 years, it is not one in which there can be a high degree of confidence at present, and it might well be partly but not completely correct. Indeed, until we know more about exploitation rates in the western Bering Sea, we can probably never be very confident of any scenario. Nonetheless, it is worth exploring this scenario in a little more detail. Physical changes have occurred in the Bering Sea ecosystem over the past 50 years. In addition to interannual variations, decadal or regime shifts have occurred; these have included changes in sea surface temperature, the mean extent of ice cover, and atmospheric and oceanic circulation patterns. It is most likely that these changes have altered the distributions and abundances of many members of the ecosystem, especially short-lived species. One of the largest such changes occurred in the winter of 1976-77; since then, the eastern Bering Sea has had warmer surface temperatures and less ice cover on average than in the period before 1976. The brief period of warmer sea temperatures in the 1960s (Figure 6.5) could also have contributed. Human Impacts were also being felt by the Bering Sea ecosystem during the past 50 years-indeed, for the past 200 years at least, and probably much longer (Chapter 5). Intensive hunting of whales continued, along with a significant harvest of fur seals. Heavy exploitation of yellowfin sole caused rapid declines in the population in the early 1960s; other long-lived slope species, such as halibut, Pacific Ocean perch, and sablefish' along with herring in the eastern Bering Sea, also suffered population declines as a result of exploitation. It is extremely plausible that the removal of whales and various fish species that were predators on or competitors with pollock combined with a decadal regime shift to cause a large increase in pollock recruitment. As result, pollock populations increased (as did populations of Pacific cod and some predatory Catfishes, especially arrowtooth flounder), and they came to dominate the ecosystem. It is also plausible but completely untested that increased competition from young pollock for zooplankton or increased predation by growing populations of piscivorous fishes led to the decline of important forage species such as capelin, sand lance, and squid. There is persuasive although not conclusive evidence indicating that some marine mammals-especially Steller sea lions in the eastern Bering Sea-and some seabirds-especially murres and kittiwakes, particularly around the Pribilof Islands-have suffered from food shortages, probably affecting juveniles more severely than adults. Indeed, it seems that some populations have decreased as pollock biomass has increased. It thus appears that the declines were not caused by declines in total abundance of pollock (which have not declined), or even in total abundance of all prey species, although declines in total abundances of capelin, sand lance, squid, and perhaps of juvenile pollock might have contributed to their difficulties. But, especially since the late 1970s, fishing for pollock has been concentrated at some times in some places. This would imply that pollock are effectively removed from some areas at some times, and the local populations would probably take at least days or weeks to be rebuilt

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236 The Benng Sea Ecosystem by in-migration from elsewhere. It is thus possible that food shortages for some mammals and birds- perhaps at crucial tunes and places for juveniles have been exacerbated by this intense pulse fishing. Even if a general increase in pollock has caused a decrease in some other, perhaps more valuable, forage species, it is almost certain that removing pollock from an area could not result in a rebound in the populations of those other species in only a few weeks. As long as overall pollock biomass remains high, it is plausible that the biomass of those other species will remain low. In any event, the above scenario the most plausible that the committee could identify does not lead to the conclusion that pollock have been subjected to ecosystem overfishing. In other words, it is hard to see how the total rate of exploitation of pollock over the past 25 years is directly (or even indirectly) responsible for the decline of mammals and birds. If this is true, it is then impossible to see how reduction of the total rate of exploitation of pollock would be helpful in the short term; it is even possible, although highly speculative, that some mammals and birds would be helped by a temporary increase in the exploitation of pollock. It is more likely that marine mammals and birds have been affected by the distribution in space and time of fishing effort on pollock, and thus that they would be helped by a broader distribution of fishing effort in space and time, especially in areas where they are known to feed. We caution that it is by no means certain that this would be effective enough to reverse or even halt their population declines. It is also hard to predict the effects of protecting other marine mammals. Many whale species have been recovering outside the Bering Sea since whaling ceased or was reduced, and some of those are the ones whose declines might have encouraged pollock increases. It is not clear why recoveries of those species in the Bering Sea have not been observed (Chapter 4), but recoveries are possible. Perhaps if they increase and human exploitation continues, the dominance of pollock in the ecosystem will be reduced, particularly if environmental conditions do not favor their recruitment. Perhaps some of the shorter-lived forage species such as capelin and sand lance would increase quickly enough under those circumstances to provide a significant food source for mammals and birds. At least we can say that it seems extremely unlikely that the production of the Bering Sea ecosystem can sustain current rates of human exploitation of the ecosystem and the populations of all marine mammal and bird species that we believe existed before human exploitation especially modern exploitation began. Over the long term fishing . .. . . . . . . . . O , ~ competes lo some Degree witn at least some top-teve~ predators; therefore, if the goal of management is to have as many top-level predators as possible (not a common management goal), then it seems that ultimately fishing will have to be reduced. We note that "fishing" includes commercial, subsistence, and recreational fishing, all of which can compete among each other as well as with marine mammals, birds, and predatory fishes. Even commercial fishing can be divided into various sectors. Allocating effort among these competing groups remains a major challenge to managers. Finally, we emphasize how difficult it will be for human management to cause a large, complex marine ecosystem to achieve and maintain a desirable balance. If the above scenario is even partly correct, it is clear that many significant factors have influenced the Bering Sea ecosystem over the past 200 years, only some of which have been under human influence (let alone control) and only some of which have been adequately documented. Some of the changes that have occurred might be irreversible over normal human hme frames (say 100 years or less).

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Causes and Elects 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.

Representative terms from entire chapter:

sea ecosystem