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4 Biology of Higher Trophic Levels The Bering Sea contains many bird, mammal, and commercially important fish and invertebrate species. The Bering Sea ecosystem can be divided into five major regions: the eastern Bering Sea, the Aleutian Islands, the Gulf of Alaska, the Aleutian Basin, and the western Bering Sea. The state of knowledge and the history of exploitation vary for different regions of the Bering Sea. For some species with relatively good histories of exploitation or protection, statistical data are available that provide information on species distributions and magnitude of variability. 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. INVERTEBRATES Life History and Distribution Although there are many benthic and pelagic invertebrates in the Bering Sea, only a few are commercially significant. Because of their commercial importance, the most data are available on the squid, crabs, scallops, and shrimp. Squid (Ber~yteuthis sp. and Gonatus spy Several species of squid inhabit the Bering Sea seasonally and also are widely distributed. The exact nature and size of the resource are poorly understood. Squid populations are thought to be large and mobile and to live at both mid-water and near-surface depths. Spawning for some species may extend from spring to fall; sexual maturity may be reached in two years or less. Fertilization is internal. The number of eggs spawned per individual is low compared to groundfish, and the eggs may be attached to seaweeds or the seafloor. Predators of squid are marine mammals and pelagic fishes. 72

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Biology of Higher Trophic 1,evels King Crab (Paralithodes spp. and L,ithodes sp.) 73 Three species of king crabs are found in the Bering Sea and Aleutian Islands region. The red king crab (P. camtschatica) is the largest, most widespread, and most abundant. 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). In the western Bering Sea, two large populations of red king crab have been identified-the first in the Sea of Okhotsk region along its broad continental shelf and the west Kamchatka shelf (Vinogradov, 1970), and the second in the northwest Ayano-Shantarskii (Rodin and Myasoyedov, 1982)-. In addition, relatively small populations exist along the seacoast, including western Sakhalin, Aniva Bav Tereniva Ptav ~nr1 east K~mrh~tk~ ~nt1 near the cnilthPrn 1{llril" Islands (Rodin, 1989~. _J 7 ~ _ __J 7 ~.,^ ~__~ __ __ v~J~__~_ A_ ^~ __- The blue king crab (P. platypus) has a more limited distribution. Bering Sea populations are found around St. Lawrence Island, St. Matthew Island, and the Pribilof Islands. Life history is similar to that of the red king crab, described below, though the blue king crab may only breed only on a biennial basis, with spawning occurring during the spring (Stevens et al., 1993). When in the juvenile stage, it tends to concentrate in rocly and shell hash substrates (Armstrong et al., 19851. The brown, or golden, king crab by. aequispinaJ is the smallest of the three commercial species, and it inhabits deep water (> 180 m) along the continental slopes of the North Pacific Ocean and the Bering Sea. Little is known of its life history. Whereas adult red king crab feed offshore and migrate inshore for spawning, juveniles are found in the littoral zone and shallower water. In the Bering Sea, adults prefer bottom temperatures of 0 to 5.5 C, suggesting a temperature influence on distribution. Molting and spawning take place in shallow (10 to 50 m) waters in late winter and early spring. Growth of red king crab is discontinuous at time of molts. The U.S. fishery begins to capture them when they are around eight years old. King crab are bottom-foraging omnivores. Principal food sources includes starfish, clams, other mollusks, small crabs, shrimps, other crustaceans, worms, fish, and algae. Predators of king crab include yellowfin sole, Pacific cod, walleye pollock, and Pacific halibut (Wooster, 1992~. Females can carry up to 200,000 eggs for a year in the brood pouch before they hatch. Larvae molt four times in a pelagic phase and then settle to the seafloor. Juveniles move into shallow waters to create dense pods ranging from thousands to hundreds of thousands of individuals. King crah.c reach m~t',ritv at filer or five very Of cap ~nr1 breed annually (Gusey, 1979~. ~_~ _ _ ~_^ ^^, _ ~ _~^ v ~^ ~_ ~ Trawl surveys provide the longest limited information on interannual variation among king and Tanner crab stock units; fishery scientists have conducted independent stock assessments and surveys. Regular systematic trawl surveys for king crab began in the eastern Bering Sea in 1955 and, except for a hiatus from 1962 to 1966, have continued to date. Beginning in 1971, the station pattern of the trawl survey was expanded to include the full range of eastern Bering Sea crabs. Also in 1971, a pot survey for king crab was initiated in the Kodiak region of the Gulf of Alaska. The data from these surveys provide the longest and most consistent time series of survey data available to measure fluctuations in North Pacific crabs. The results of these surveys and comparisons with commercial catch statistics are given In

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74 The Bering Sea Ecosystem Figures 4.1 (eastern Bering Sea; Hayes, 1983; R.S. Otto, personal communication) and 4.2 (Kodiak; Hayes, 1983). Hayes (1983) 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. Similar patterns can be seen for eastern Bering Sea Tanner crab and snow crab. Wooster (1992) chronicled the sporadic fluctuations of king crab over the last three decades. Combining his data (Figure 4.3) with those already shown, patterns in crab dynamics emerge. Red king crab biomass seems to grow quickly, owing to a small number of strong cohorts, and decline quickly as the cohorts either die or are fished out. This happened in Kodiak (central areas in Figure 4.3) in the late 1960s and early 1980s and in the Bering Sea (Figure 4.1) in the late 1950s, late 1970s, and early 1980s. Tanner Crab (Chionoecetes spp.) Tanner crabs are found throughout the Bering Sea region. Two species are relatively large and therefore are of commercial importance. The Tanner crab C. bairdi is the larger of the two species and is usually encountered in the same habitat as red king crab. The snow crab C. opilio is smaller but believed to be the most abundant of the Tanner crabs. It is found in the northern and central Bering Sea on the continental shelf of Russia and Alaska (Slizkin 19891. Eastern Bering Sea Tanner crab (and snow crab) exploitable biomasses peaked in the mid-1970s and again in the late 1980s and early 1990s. The food and feeding habits of Tanner crab are similar to those of king crab; in the larval phase they feed on plankton, in the juvenile phase on benthic diatoms, hydroids, and detritus, and in the adult phase on such benthic organisms as crustaceans, bivalves, brittle stars, echinoids, worsens, fish and gastropods. They are eaten by eelpouts, sculpins, and skates (NPFMC, 1994). Breeding occurs in winter, in waters of 500 to 700 m. Eggs in female abdominal brood pouches can number up to 200,000 and can be carried for up to a year before hatching. Larval release may coincide with spring plankton blooms, and the larvae go through several molts before going to the sea bottom as immature crabs (Lewbel, 1983). Hair Crab (Erimacrus isenbeckii) Korean hair crab, sold commercially by the Japanese, was fished commercially for the first time by the U.S. fleet in 1979. Found primarily near the Pribilof Islands area, this species was used to support an experimental fishery, where crab are shipped live to markets in Japan. Hair crab were first considered as a supplement to ongoing king and Tanner crab operations. Catch quickly increased to about 2.5 million lb by 1980-81 and stabilized at about 1 million lb for the next two years. Beginning in 1983-84, catches have been steadily declining to about 1,000 lb annually beginning in 1988. A similar decline in fishing effort reflects both a decline in the resource as well as poor market conditions.

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Biology of Higher Trophic Levels sor 45 40 35i 30 8 ~ 25 To 20 15 _ 10 - JAPAN _ ~ U.S.S.R. A , ----- \ s 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 Year Swept E ~ \ ~. TRAWL SURVEY ABUNDANCE INDICES OF COMMERCIAL - SIZE BERING SEA CRABS MILLIONS OF CRABS 350 300 250 ~ 200 / if ark 150 1'( 100 >/ 50 f ~ , it, ~ ~ -~` \ ~A A/ ~7~ O I I I T - - ~of %'t- 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 YEAR TANNER >129 x TANNER ~ 134 75 ~ OPILIO ' 109 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).

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76 The Bering Sea Ecosystem ~1 150 100 50 1974 - - ~ ^~11~:79,2 Shots1 1,6 Pot~ft~."17 _, 1978 01 _ ~ ",895 10,~ 1895 ~3 P"'~t~ m Aces e--m ma oats At pa, l~inmmm bed am, 7 - . Reemin _ Sit erab A New Ado:31,202 / \ Sit:1790 / ~P - 'tI~:~ t2 82 ~ `~ ~ iK VS 150 1976 1980 sown: 7~2 _n: 23S2 ,oE. 15Oo: ~ - F 977 ~ 2~ _ 2 200 _ 150 _ 100 _ 50 _ 04 ~ t98 New sheds:73, - 9 for ~Is: ".~" Shiotnolts:441J Shipmate: Po~ft~:1 ~_ f~f~: 1 20 _ _ 145 1982 ~ ~ lo, ens S - malts: Profit: l72l ~ ~ 1 ;~ 145 190 55 100 148 190 Carapace length (mm) 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).

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Biology of Higher Trophic Levels 160 t50 140 _ as ~ 120 _ O' 110 _ ~ 0 90 _ ID .., 80 _ c 70 _ - 60 - 50 zip 40 A- 30 20 10 Ol 1960 77 1 ~ ~ ~ __ _ I! _ Areas~/ I A'~Central Areas ~ ~ I ~ I ~ I I I ' I 'a 64 68 72 76 80 84 YEAR \ Figure 4.3 Alaska king crab landings from central and western areas (central areas include Prince William Sound, Lower Cook Inlet, Kodiak Island, and South Peninsula; western areas include Bristol Bay, Dutch Harbor, Adak, and the eastern Bering Sea) (Wooster, 19921. Shrimp (Family Pandalidae) Eight pandalid shrimp species representing two genera are taken in commercial and subsistence fisheries off Alaska, but only five are of commercial importance. For many years, shrimp fishermen have targeted prunarily pink shrimp (Pandalus borealis), with up to 85 percent of shrimp catch in some years made up of this single species. Other pandalid species, i.e., P. hypsinotus (coonstripe), P. goniurus (bumpy), and P. disbar (sidestripe), are most often taken incidental to the fishery for pink shrimp. Humpy and sidestripe shrimp have at times been dominant in trawl catches from specific areas and have supported small fisheries. Other pandalids, i.e., P. Ulnae (dock), P. montagui tridens, P. jordani (ocean pink), and P. platyceros (spot), are also taken incidentally in trawl fisheries. The former two species are the least common of the pandalids found in commercial catches and dock shrimp are rarely taken in Kodiak and the Bering Sea/Aleutian Island regions. 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. As interest in the species grew, government agencies

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78 The Bering Sea Ecosystem dedicated funds and staff to shrimp research. When the pink shrimp fishery crashed in the mid- 1980s, fiscal priorities likewise changed (in this case to king crab) and nearly all shrimp research ended. As a result, most of the literature concerning the life history of pandalid shrimp consists of studies of pink shrimp (P. borealis). Fishery scientists have been working under the assumption that characteristics of pink shrimp life history closely mirror those of other species within the genus PandaZus, while recognizing that regional and species differences likely occur. Balsiger (1981) presents an excellent review of pandalid research. The life history of pink shrimp can be separated into seven distinct development stages. The female carries from 900 to 4,000 developing eggs in her abdomen. Larger, older females normally carry more eggs than do small females (Fox, 1972; Mistakidis, 1957; Haynes and Wigley, 1969). Female pandalid shrimp carry their fertilized eggs until the eggs hatch; thus, mortality to the female precipitates mortality to her egg clutch. Spawning normally occurs in August through October depending on region, with differences in water temperature the apparent controlling factor (Rasmussen, 1969). The incubation stage may last six months. In the Gulf of Alaska, pink shrimp larvae hatch from February through April or early May, whereas other species hatch from April into mid-July (Buck et al, 1975). The unique water circulation patterns of bays and island regions benefit spawning shrimp by preventing planktonic larvae from drifting into unfavorable areas. Larvae are free-swimming for approximately two and one-half months and then settle to the bottom as postlarvae to assume adult behavior. These juveniles are most abundant in waters deeper than 20 fathoms (120 feet). At the onset of winter, however, they appear to migrate to shallow waters of less than 20 fathoms (Buck et al, 1975). Mortality rates are believed to be high for shrimp but are difficult to quantify. Natural mortality estimates have ranged from 22 percent to 85 percent per year, depending on size and age (Gotshall, 1969; Rinaldo, 1976; Anderson, 1978). Predators include cod, sablefish, sole, flounder, halibut, rockfishes, and salmon. One unique aspect of shrimp life history is individual transformation of sex. Almost all shrimp develop as males, although initial development as females has been reported in a few species. Shrimp reach maturity as adult males two years from hatching. At two and one-half years of age, most breed as males, although at three and one-half years a few have already transformed into females. By four and one-half years of age, all shrimp have transformed into sexually mature females. Six months is the average time required for an individual to change sex. Few females live beyond six years. Little is known of the food and feeding habits of pandalid shrimp, but in general they are predators and scavengers. Shrimp feed principally upon a wide variety of organisms, including worms and larvae (Butler, 1964; Fox, 1972; Dahlstrom, 19701. The most common food item identified in studies on shrimp was crab larvae. Mortality rates are believed to be high for shrimp but are difficult to quantify. Natural mortality estimates have ranged from 22 percent to 85 percent per year, depending on size and age (Anderson, 1978; Gotshall, 1969; Rinaldo, 19761. Virtually any large fish in their vicinity, including cod, sablefish, sole, flounder, halibut, rockfishes, and salmon, is a potential predator. Pandalid shrimp species move off the ocean bottom at night. Each species of shrimp differs in the proportion of individuals that migrate upward and in the upward distance of migration (Barr and McBride, 19671. Different species can be found from the surface to at least 230 fathoms (1,380 feet). In the Kodiak and Cook Inlet areas, large concentrations of pink

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Biology of Higher Trophic Bevels 79 shrimp have been found over sand, silt, or mud substrates. Exploration in more rocky and more difficult trawling areas such as those located in Prince William Sound has been too [united to assess accurately the harvest potential of spot shrimp with trawl gear from these areas. However, it appears that spot shrimp clearly prefer rocky bottoms, usually the bottom of deep canyons or crevices, or pockets along steep, rocly cliffs. As mentioned previously, we understand that Alaska Department of Fish and Game (ADF&G) intends over the short term to conduct a series of pot surveys on spot shrimp. Scallops (Family Pectinidae) Five species of scallops are commonly found in the Bering Sea and Aleutian Islands area. The most common and the primary target of the commercial fishery is the weathervane scallop (Patinopecten caunnus). Taxonomic classification and evolutionary relationships among weathervanes and other commercially important scallop species are described by Wailer (19911. Weathervane scallops are found in intertidal waters to depths of 300 m (Foster, 1991), but abundance seems to be greatest between the depths of 45 and 130 m on beds of mud, clay, and sand (Hennick, 1973). Little is known about the biology of these species, and according to the North Pacific Fishery Management Council (NPFMC, 1993), there have been no routine biological or fishery programs to sample scallops. Scallops develop through a series of growth stages. In the Atlantic, scallop beds are thought to be strongly affected by ocean current patterns (New England Fishery Management Council, 1982), which in turn can affect local recruitment and settling of larvae; the beds tend to be elongated in the current direction and can include different age and/or size groups (Caddy, 1989; Robert and Jamieson, 1986). Weathervane scallops mature by age three, with rapid growth observed through the first 10 years of life (Hennick, 1973). Scallops are long-lived species and may reach an age of 28 years or more. Principal predators of scallops are various invertebrates (such as crabs and starfish) and fish (such as Catfish). The commercial fishery (a primary source of biological information) has experienced a variable history, during which the fishery exists for a number of years followed by a period of no activity. Alaska scallop resources appear to be healthy, which has stimulated renewed fishery activity. 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. Information on the commercial industry that depends on some of these species is covered in Chapter 5. A number of previous histories and compilations (e.g., NPFMC, 1993; Lewbel, 1983; Hood and Calder, 1981) were used as references for this section.

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80 The Bering Sea Ecosystem Most commercial finfish species have experienced some variation in abundance over the years, and in some cases the variability has been extreme. Abundance trends can be measured by a variety of methods. For most species, historical catch statistics and catch per unit effort can be used as measures of relative abundance. Absolute abundance estimates be arrived at only through stock assessment surveys combined with a variety of statistical techniques and modeling. For species such as salmon, run sizes can be determined from a combination of catch and escapement estimates. Most of the information on which these reports and analyses are based is meant to provide advice on the short term (year-to-year) management of commercial fisheries. Thus, those reports and analyses focus on commercially important species and on the distribution and size of the population or stock. This may not be an appropriate scale at which to approach the kinds of ecosystem questions that this study is asked to address. There is information in these reports and stock assessments, however, which provides insight into ecosystem-scale processes that might be operating in the Bering Sea. Once again, most of the information relates to the eastern Bering Sea and Aleutian Islands, with limited information on the Aleutian Basin (pollock only) and sparse information on the western Bering Sea. In addition, many fish species especially pelagic species are not commercially significant and might have significant effects on ecosystem organization. However, little information is available on those species compared with the commercially important ones and we have not discussed them. Details on groundfishi (excluding halibut) can be found in the NPFMC 1994-95 Stock Assessment and Fishery Evaluation reports for the Bering Sea/Aleutian Islands area and the Gulf of Alaska (NPFMC, 19931. In general both NPFMC and the National Marine Fisheries Service (NMFS) are of the opinion that the groundfish stocks of the eastern Bering Sea/Aleutian Islands are in "good" condition. This is certainly the case if the resources of the eastern Bering Sea are compared to those of other regions of the world. However, as should be expected for a dynamic system, all species are unlikely to be at their peaks in abundance at the same time. Some species, such as sablefish, offshore pollock in the Aleutian Basin, and Greenland turbot, are currently at relatively low levels. Others, such as a number of flatfish (yellowfin sole, rock sole, arrowtooth flounder, Fathead 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. Walleye Pollock (Theragra chalcogramma) Walleye pollock is a semidemersal species that inhabits continental slope and shelf waters along the northern rim of the North Pacific, extending from southern Oregon into the southern Chukchi Sea and south along the Asian coast to the southern Sea of lap en (Bakkala, 19931. At present, the walleye pollock stock represents the greatest single-species biomass in the Bering Sea, accounting for approximately 50 percent of the total biomass of all groundfish In the eastern Bering Sea, Aleutian Islands, and BogosIof district (NPFMC, 19931. They probably account for at least that proportion of the biomass in the western Bering Sea as well (Sabolevsky et al., 1989; Wabkabayashi and Bakkala, 19851. ~ Fish normally caught with bottom-fishing gear, usually bottom trawls. Some of these species (e.g., pollock) also spend time higher in the water column.

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Biology of Higher Trophic Levels 81 In the Bering Sea, pollock are found throughout the water column along the eastern and western continental shelves and throughout the deep Aleutian Basin. Massive schools occur on the outer shelf and upper slope from the surface to 500 m. In the eastern Bering Sea, walleye pollock undergo extensive seasonal migrations associated with feeding and reproduction. Overwintering takes place along the outer shelf and upper slope and over deep water where bottom temperatures are relatively high. As temperatures on the shelf rise in spring, part of the walleye pollock population moves to shallower waters (90 to 140 m) to spawn. They first reproduce at the age of three or four years. Spawning occurs primarily in the first quarter of the year in the Aleutian Basin, with major aggregations occurring in the vicinity of BogosIof Island, and from March through October along the eastern Bering Sea outer shelf (Hinckley, 1986), with major spawning concentrations occurring between the Pribilof Islands and Unimak Island. Along the eastern Bering Sea shelf and slope, each female produces approximately 60,000 to 400,000 pelagic eggs. Fecundity is much lower in the basin. Walleye pollock eggs hatch in two to three weeks, depending on temperature, and larvae tend to inhabit surface waters. Larval padlock begin feeding on copepod eggs and nauplii; as they grow, they feed successively on larger prey such as small copepods. Diets of adult pollock consist mainly of copepods, euphausiids, and fish, the majority of which are juvenile pollock. Pollock are a major part of the diets of northern fur seals and other marine mammals, and are also important prey for many seabirds and fish (Frost and Lowry, 198Ib; Pereyra et al., 19761. The population and stock structure of walleye pollock deserves special attention because of its economic and political Importance. Much of the literature on stock structure of pollock in the North Pacific is inconclusive. In terms of genetic differences, Bahama (1993) reported major genetic differences between populations in the Sea of O~otsk and those in the eastern Bering Sea and Gulf of Alaska. Within North American waters, small but detectable genetic differences were found between pollock from the eastern Bering Sea and the Gulf of Alaska. The genetic relationships among pollock from the eastern Bering Sea, Aleutian Islands, and western Bering Sea are less clear. Dawson (1994) provided an excellent overview of these relationships as well as a clear discussion of the concepts of population and stock with particular reference to Bering Sea pollock. He analyzed several independent data sets, using abundance at age from research and commercial samples, size at age, and morphometric analysis, to assess the degree of isolation of populations of juvenile and adult pollock sampled across all four major areas of the Bering Sea. He concluded that there are several phenotypic stocks of pollock. One is on the eastern Bering Sea shelf, with the main spawning area on the southeastern shelf (southeast of the Pribilof Islands) and with a main juvenile rearing ground on the northeastern shelf (northwest of the Pribilof Islands). Another stock is in the Aleutian Basin with its main spawning ground in the southeast basin near Bogosiof Island. This stock probably occupies the same rearing area as the eastern Bering Sea shelf stock. Dawson's analyses led hun to conclude that pollock from the Aleutian Basin and eastern shelf to not move back and forth in large numbers as adults. 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.

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82 The Bering Sea Ecosystem The relationship between pollock in the Aleutian Basin and the eastern and western Bering Sea shelves is of special interest to fishery scientists and managers. Most important for life history and distribution is the fact that only large, older pollock (older than four years) occupy the pelagic waters of the basin, whereas all ages occupy the shelf and Aleutian Island areas. As Bakkala (1993) pointed out, the absence of juveniles in the basin suggests that these pollock originate, or at least spend their early lives, in other areas, presumably one or more of the shelf areas surrounding the deep basin. Figure 4.4 gives estimates of adult biomass of walleye pollock (ages three and up) in the eastern Bering Sea shelf and Gulf of Alaska from 1964 to 1993. Figure 4.5 gives estimates of pollock year class strength. The most recent surges in pollock biomass, in the late 1970s and early 1980s, represent the very strong 1978 year class in the Bering Sea, and a succession of strong year classes from 1972 to 1979 in the Gulf of Alaska. It appears that the strong 1978 year class accounted for the increase in pollock biomass and resultant catch in the area around Bogoslof Island and the Aleutian Basin (donut hole) (Wespestad, 1993) (Figure 4.6). Although the fishery for pollock has remained relatively constant in time and space, if not in biomass removed, in the Gulf of Alaska, the fishery in the Bering Sea has moved considerably in both space and time (Figures 4.6 and 4.7). 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). The strong 1972 and 1973 year classes appear to have supported the fishery in the northwest in the early and mid-1970s (62 percent of the catch between 1973 and 1977 was taken in the northwest). The strong 1978 year class, in contrast, appears to have supported the fishery in the southeast in the late 1970s and early 1980s (75 percent of the 1982-83 catch was taken in the southeast), as well as the fishery in the Bogoslof and Aleutian Basin areas in the 1980s. Figure 4.10 shows how the center of activity of the pelagic pollock winter trawl fishery moved from the slope around the Pribilof Islands in 1979. to the Un~mak/Rn~slof region in 1991 (it. Fritz, personal communication). ~_ a, in, ~ ~ Not only do the centers of activity of pollock fishing move around from year to year in the eastern Bering Sea (including the basin), within a year the distribution of pollock spawning changes considerably from season to season (Hinckley, 1987; Figure 4.11), moving from the basin in winter to the southeast shelf in spring and up the shelf northwest of the Pribilofs in the summer. Bering Sea pollock are quite different in their population dynamics from Gulf of Alaska pollock. This can be understood best by examining the seasonal, annual, and decadal spatial movements of centers of activities of various life history stages (e.g.' spawning and availability of adults to fishing). Pacific Cod (Gadus macrocephalus) Pacific cod also have a broad distribution in the North Pacific, extending along the North American coast from southern California to Norton Sound in the Bering Sea, and along the Asian coast south to the southern tip of the Korean Peninsula and into the Yellow Sea (Bakkala, 1993). This species is common at depths of 80 to 260 m. In the Bering Sea, Pacific cod schools are most abundant on the shelf and upper slope. They migrate seasonally between the

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Biology of Higher Trophic Levels 145 Table 4. 18 Commidee'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 Chemicalpollution 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; notimportant 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 factorin 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 survival Low Low Low Cannot explain the declines Fishery effects on food availability Moderate High High Could have played a major role in all declines Climate effects on food availability Moderate Moderate Moderate Could have played a role in all declines Competition from fish predators Moderate Moderate Moderate Could be a majorfactor in the declines, given observed changes in community structure that were most likely caused by commercial fishing and environmental changes

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146 The Bering Sea Ecosystem Entanglements Fowler (1987) concluded that entanglement of juvenile fur seals in trawl net fragments could account for the decline in the Pribilof population. However, Trite s (1992) examined the evidence supporting the entanglement hypothesis and concluded that while large numbers of fur seals have no doubt died in lost and discarded fishing gear, it is unlikely that entanglement has been the major cause of the decline. 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., 19861. Based on the admittedly limited data, it is unlikely that entanglement in debris is a major factor in the decline of this species (NMFS, 1992~. Entanglement of harbor seals in lost or discarded net fragments has not been observed at Tugidak Island (Pitcher, 1990), the site of the most dramatic decline in numbers, or in the Aleutian Islands (Loughlin et al., 19861. Human Disturbance and Takings Human disturbance of rookeries and haulout sites can have negative effects on pinnipeds (see review by Reijnders et al., 19931. The sites used by pinnipeds in the Bering Sea and Gulf of Alaska are generally in areas that are relatively remote or are protected from human activities. Direct human taking of marine mammals is now limited almost exclusively to subsistence hunting and to incidental kills by commercial fisheries. Based on the results of a simulation model, Trites and Larkin (1992) concluded that direct kills of sea lions in commercial harvests reduced some rookeries and may have stabilized the number of sea lions breeding in the Gulf of Alaska and Aleutian Islands. The commercial harvests, coupled with subsistence harvests and intentional and incidental kills by fisheries, would have had some depressing impact on the population up to about 1980. However, the model indicated that since 1980 over 10,000 sea lions per year were disappearing from the population, and even the highest estimates of fishery kills could not account for these missing animals (Trites and Larkin, 19921. Based on simulation modeling, Pascual and Adkinson (1994) concluded that the observed declines were unlikely to be the result of pup harvests. Merrick et al. (1987) came to the same conclusion. Population declines in northern fur seals of greater magnitude than those observed recently have occurred in the past century at the Pribilof Islands, as a result of commercial overexploitation (Kenyon et al., 19541. The recent decline in pup production at the Pribilof Islands from the mid-1950s to about 1970 can reasonably be explained by the commercial harvesting of adult females between 1956 and 1968 and a series of years of poor juvenile survival (Eberhardt, 1981; Trites and L`arkin, 1989; York and Hartley, 19811. The reason for this poor juvenile survival is not known. The continued decline through the early 1980s at St. Paul Island and to the present at St. George Island cannot be explained by He commercial harvest of juvenile males that occurred until 1985 on St. Paul and until 1972 on St. George. While the data collected by fishery monitoring programs are incomplete, it is unlikely that incidental takes of fur seals have been large enough to be a significant factor in the population decline (NMFS, 1993a).

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Biology of Higher Trophic Levels 147 Subsistence harvests of harbor seals in the Aleutian Islands and southcentral and southeastern Alaska in the late 1960s were estimated to be roughly 5,000 annually (Hoover, 1988a). Approximately 4,000 phocid seals, including an unknown number of harbor seals, were also taken in Bristol Bay and the Yukon-Kuskokwim region (Hickok, 1978, cited in Hoover, 1988). Since 1972, subsistence takes appear to be lower, and probably do not exceed 3,000 per year (Hoover, 1988a; Pitcher, 1984; Wolfe, 1993). It is unclear whether recent subsistence harvests had a local effect in recent declines in seal numbers. Bounty harvests in Alaska varied from about 6,000 to 24,000 seals annually between the 1930s and 1950s. A commercial harvest that developed during the 1960s had peak harvests of between 40,000 and 60,000 seals annually, and by 1967 harvest stabilized at 8,000 to 10,000 annually. Commercial harvesting ended in 1972 (see review in Hoover, 1988a). The influence of these harvests on population size is not known, but their effect on population trend should have ended by the late 1970s. The decline of harbor seals at Tugidak Island began in the mid-1960s and was influenced by commercial harvests, mainly of pups, between 1964 and 1972. However, the population continued to decline in the absence of commercial harvests, and after the effect of the harvests would have passed through the age structure of the population (Pitcher, 1990). The continued decline of harbor seals at Tugidak Island cannot be explained by the harvest history of this population. There are no estimates of the total number of harbor seals killed as a result of fishing operations (Sease, 1992). Recent observer and logbook data suggest that a minimum of several hundred harbor seals were killed as a result of encounters with fishing operations (Sease, 1992). Possible local or regional effects of these takes on harbor seal numbers are unknown. Predation Killer whales and sharks are probably the only significant natural predators of the pinnipeds whose populations have declined. While sharks are known to be major predators of pinn~peds in some other areas (e.g., Ainley et al., 1985; Brodie and Beck, 1983), such interactions appear to be rare in the Gulf of Alaska and Bering Sea. Predation on pinnipeds by killer whales has been documented in the Bering Sea, and there have been some recent changes in killer whale activities (Frost et al., 1992). 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). However, predation could have some effect on the recovery of the currently reduced populations. Direct CInnate Effects on Survival The possible direct effects of weather on pinniped survival are poorly known. Scheffer (1981) reported large numbers of dead northern fur seal pups on the Oregon and Washington coasts after a period of severe storms. LeBoeuf and Condit (1983) documented significant mortality in young northern elephant seals as a result of storm-driven waves. Studies on captive animals have shown that California sea lions cannot rely on physiological mechanisms alone to avoid hyperthermia at air temperatures above 22C (Whittow et al., 1972; Whittow, 1987). A

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148 The Bering Sea Ecosystem number of pinniped species are known to alter behavior to avoid thermal stress which could result in increased mortality, particularly in pups (e.g., Francis and Boness, 1991; Gentry, 1973; Heath, 1989; L~mberger et al., 19861. However, species such as the northern fur seal and Steller sea lion can tolerate a wide range of environments from the cool, wet summers that occur in the Bering Sea to the hot, dr-Y summers of southern California. Several approaches have been used to Study the direct effects of climate on the survival of northern fur seals. Trites (1990) used a physiological mode! to investigate the weather conditions that pups can tolerate during the first week of life on land. The results of this work indicate that healthy, average-sized pups on the Pribilofs can tolerate any combination of wind speed, temperature, and humidity recorded since the mid-1950s. However, the mode! 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 tune on land. However, as Trites (1990) and others have noted, a second critical stage in juvenile survival is the transition from land to the sea. Survival of male northern fur seals from weaning to two years of age is onIv about 40 percent. with most deaths thought to occur in the first winter (Lancler, 1979~. , thermoregulation is energetically more costly for young pinnipeds when they are at sea. York et al. (in press) used cross-correlation analysis to investigate possible relationships among juvenile (i.e., less than two years of age) northern fur seal survival and air ant} sea surface temperatures. Although estimates of early survival on land and at sea were significantly correlated with the environmental inclices, the pattern of correlations did not provide convincing clues about the unclerlying mechanisms. In general, the analysis suggested that environmental conditions hac! a greater influence on survival at sea than on land. These results are consistent with the results of physiological mocleling and observations that young northern fur seals appear to have difficulty in maintaining bocly temperature at low seawater temperatures (Trites, 19901. Pups lose almost 50 percent of their bocly mass cluring the first winter at sea (Scheffer, 19811. Thus, there is some reason to believe that changes in ocean temperature could affect survival ~ e ~ of young p~nmpec as. CInnate events, such as E! Nino, can have markect effects on pinnipec! populations, which are mecliatect through Impacts on the availability of foot} (TrilImich and Ono, 1991~. The 1982-83 E] Nino event produced drastic changes in the marine environment off the west coast of North America, ant! cletailed studies were clone to document the effects on some species of pinnipeds (TrilImich and Ono, 19911. E! Nino had a measurable effect on California sea lion pup production and growth rates in southern California (DeLong et al., 19911. However, the result was only a pause in the long-term pattern of population growth, and this pause was compensated for by a faster rate of increase in pup production after the E] Nino than before (Boveng, 198Sa). A study that compared effects of the 1973, 1983, and 1992 E! Nino events on Steller sea lions at Ano Nuevo Island concluded that there was some indication of limited prey availability in 1992 (Hood et al., 19931. Simulation models indicate that short-term stochastic environmental fluctuations have very likely not been responsible for the recent sea lion decline (Pascual and Adkinson, 19941. O ~ As water conducts heat 25 times more effectiveIv than air ,

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Biology of Higher Trophic Levels 149 Life History Patterns Pinniped reproductive and foraging strategies are constrained by the need to give birth Pinnipeds must also seek haul out at other times of -r~-~-~-~ and temporal separation between seals and their food (Bartholomew, 19701. The extent of this separation is likely tn on land or ice and to obtain food from the sea (Bartholomew, 19701. a solid substrate during the annual molt of their pelage, and they may the year as well. Thus dependence on land can result in a spatial . vary seasonally, and to affect males and females of different ages in different ways. Thus. the effect of changes in food availability is most likely a complex function of many factors. including the age and sex of the individual and the season. Of the three pinniped species known to have declined in abundance, two are members ~ ~ +1-~ ~ ~ l ~ r ~ ~ ~ ~ ~ ~ ~ /_ _ ~1~ ~ 1 _ _ ~ ~ ~ ~ ~ ~ ~ ~ U1 U1C lalilily ~JELIlilUtIC ~llL,rL~lern lur seal and queller sea 1lon), and one IS a member of the family Phocidae (harbor seal). These two groups have very different maternal reproductive strategies: otariids exhibit a "foraging cycle strategy," whereas phocids typically exhibit a "fasting strategy" (see reviews in Bonner, 1984; Costa, 1991). The foraging cycle strategy of otariids is characterized by females having moderate energy stores on arrival at the rookery that are used to support maternal and offspring requirements during a short perinatal fast (Costa et al., 1988). This is followed by regular feeding trips at sea alternating with brief visits to the rookery to nurse young over a period lasting from four months to more than a year. Thus, there is a long period of maternal care during which the foraging success of mothers has direct consequences for the growth and survival of offspring. In contrast, when the larger phocid species give birth, they have large maternal energy stores that are sufficient to support maternal maintenance and the cost of lactation over a 4- to 50-day nursing period (Bower, 1991; Oftedal et al., 1987). This strategy reduces the reliance of females on food availability during the period of maternal care. Although the harbor seal is a phocid, recent studies have shown that females are not large enough to store sufficient reserves to support the energetic cost of lactation, and thus they need to supplement energy stores by beginning to feed after a perinatal fast of about one week (Boness et al., 1994; Bowen et al., 1992). Therefore, as for both northern fur seals and Steller sea lions, the growth and survival of harbor seal pups is likely to be affected by the foraging success of females during the lactation period. Adult male otariids are typically much larger than adult females, and as a result they have greater daily eneraY requirements. and nerhans larder ~ncl/nr diffP.rPnt formed area than f^~ol=o - ~ ~- ~ --- ~ ----I r ----wry ~ ^-~-^~- -a ~`~ ~ `~ll``l~ (French et al., 1989; Stewart and DeLong, 19931. Harbor seal adults of both sexes are similar in size and thus might be expected to have similar foraging ranges. Adult male otariids establish and defend territories on rookeries and do not feed for several weeks during the breeding season. The breeding biology of harbor seals is poorly known, but male seals spend much of their time in the water during the breeding season, and they therefore are not precluded from feeding. Distribution patterns may also influence the response of pinnipeds to food limitation. Northern fur seals that pup and breed in the Bering Sea undertake an annual migration that takes them out of the area, whereas both Steller sea lions and harbor seals are year-round residents of the Bering Sea and Gulf of Alaska. 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.

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150 Fishery Effects on Pinniped Foods The Bering Sea Ecosystem Food The key to understanding the effects of fisheries on pinniped populations most likely lies in understanding the spatial and temporal scales that are critical to the nutrition of individual animals. Normal growth and survival of individuals depends on minimum levels of resources being available at a particular tunes and places, and not on mean abundance of those resources over some large area such as the eastern Bering Sea. Thus, determining that the mean abundance of food at large spatial scales, as is typical in fishery resource assessments, is likely to be at or above the requirements of a pinniped population may be not only uninformative but in fact misleading. Pinnipeds and modern offshore fishing fleets differ greatly in their abilities to determine the distribution and abundance of fishes in the ocean, and to capture them. 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. Analysis of the distribution of fishing effort in the Gulf of Alaska led NMFS to the same conclusion (Fritz, 1993b). The exact nature of these fishery effects cannot be determined from the sparse available data. However, they likely include the direct effects of reduced biomass of prey needed by pinn~peds and indirect effects on community structure that may have resulted in reduced availability of alternative prey (see Chapter 61. A recognition of the importance of considering scale with respect to food requirements of Steller sea lions is reflected in the establishment, in 1992, of fishing exclusion zones around ~rnportant Steller sea lion rookeries. As stated by Fritz (1993b), "NMFS concluded that spatial and temporal concentration of trawl fishing for pollock in the 19SOs could have contributed to the decline in the sea lion population." Thus, year-round exclusion zones of 10 nautical miles were placed around 37 sea lion rookeries, and zones of 20 nautical miles were established around 6 rookeries in the eastern and central Aleutians from January ~ to April 15 during the pollock roe season (Fritz, 19931. It is premature to attempt an evaluation of the effects of these exclusion zones on Steller sea lion population trends in these areas. However, it may be useful to consider the following points. First, it is unfortunate that these exclusion zones are not being designed and used in an experimental way to ensure that maximum knowledge is derived from their use. With the current approach, it will likely be impossible to determine whether changes in sea lion populations are a response to these zones or to changes in other factors. Second, recent work on the distribution of foraging (Merrick and Loughlin, 1993) suggests that these exclusion zones may be too small to effectively separate the local effects of trawlers on sea lion prey from foraging sea lions. ~.. . ~. - ~. . ~. ~

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Biology of Higher Trophic Levels Diets and Food Limitation 151 A reasonable understanding of the composition of diets is necessary to determine how populations satisfy their energy requirements and whether or not they are limited by food availability. It is difficult to accurately describe pinniped diets for a number of reasons, however. Pinnipeds spend much of the year at sea, where it is costly or impossible to observe them. Prey size and species composition of the diets are usually derived from fish otoliths and other hard structures recovered from stomach contents or feces, but such analyses can result in a biased view of prey consumption (Dellinger and Trillmich, 1988; Harvey, 1989; Jobling, 1987; Jobling and Breiby, 1986; Murie and Lavigne, 1985). For wide-ranging species, data collected at or near coastal haulout sites may not adequately represent prey consumed offshore (Bowen et al., 1993). Diets may also differ seasonally and throughout the geographic distribution of the species. Finally, diet composition of pinnipeds may vary in response to both the absolute and the relative availability of the prey (e.g., Bailey and Ainley, 1982; Sinclair et al., 1994). In addition to these inherent limitations, dietary information for eastern Bering Sea and Gulf of Alaska pinnipeds suffers from irregular and inadequate sampling, especially in recent years (see reviews in Lowry et al., 1982, 1989). 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. There is some evidence to suggest that Steller sea lions diet had changed over time, with pollock becoming a more important food in mid 1970 and mid 1980 samples (Calkins and Goodwin, 1988). Calkins and Goodwin also indicate that pups consume somewhat smaller pollock than older animals, and it is likely that the proportion of small pollock in their diet increased in the 1980s (Merrick and Calkins, in press). However, sample sizes in most of these studies are too small to provide much confidence in such conclusions. Furthermore, few data are available through the 1980s to address the question of changes in diet. Considerably more is known about the diet of northern fur seals, but even here there is little recent information. Our knowledge of diets of harbor seals in the Bering Sea is scanty, and the good studies in the Gulf of Alaska are more than 20 years old. Thus, even if food is limiting, it seems clear from the above analysis that we will not find evidence for this by examining trends in the species composition of the diets of any of these species. Data suitable for such an analysis simply do not exist. Other Evidence of Food Limitation The "Is It Food?" workshop (Anonymous, 1993) considered a number of demographic, behavioral, and physiological responses that might be taken as evidence of there being insufficient food to meet the energy requirements of declining pinniped populations in the Bering Sea and Gulf of Alaska. Following that approach, those that the committee considered might be particularly informative with respect to the food limitation hypothesis are listed in Table 4.19. Individually, these responses would be consistent with a number of alternative hypotheses and would not in themselves allow the conclusion that food limitation was responsible for the responses. However, the combinations of these responses, together with information on other potential sources of morbidity or mortality (i.e., pollutants, parasites, and disease), could

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152 The Bering Sea Ecosystem provide more convincing evidence that food was indeed Inniting. For example, reduced fecundity could be a result of food limitation, but it could also result from ingestion of high levels of polychIorinated biphenyls, chemicals that are known to cause reproductive failure (Reijnders, 19861. However, in the absence of certain levels of pollutants or other agents known to cause reproductive pathology, we are more likely to conclude that food limitation was responsible for reduced fecundity. The evidence concerning food limitation for three species of -pinnipeds is discussed below. Northern Fur Seals The northern fur seal is mainly a summer resident of the Bering Sea. After the breeding season all age groups undertake migrations which take them, for the most part, to other areas of the North Pacific (see review in French et al., 19891. In general, rapid growth or fattening occurs during a one-to-three month period before the seals arrive on the Pribilof Islands in July. During the rest of the year, fur seals gradually lose body mass and condition (Trites and Bigg, 19921. Although fur seals do feed in the Bering Sea, there is no evidence to suggest that food is limited in that area. In fact, available data suggest that during the fourth-month lactation period, a time of high energy expenditure, adult female foraging trips were shorter during the 19SOs, which could indicate that food was relatively more abundant, or that food was closer to the Pribilof Islands than it was earlier (Loughlin et al., 19871. Pup body mass at two months of age has also increased over tune, suggesting an increase in food availability for lactating females (Fowler, 19901. The size of juveniles returning to the Pribilof Islands has also increased in recent years (Baker and Fowler, 19921. All of these observations have been interpreted as density-dependent responses to reduced competition for food (Fowler, 19901. Furthermore, since 1984 pup production on St. Paul, where most pups at the Pribilof Islands are born, has been relatively stable (York, 1990), suggesting an Improvement in survival and/or reproductive rates. Recent studies indicate that pups generally leave the Bering Sea within two weeks of weaning (Ragan et al., 1993), such that they are reliant on food resources in the eastern Bering Sea for only a brief period. It has been suggested that low availability of food in the Aleutians could compromise pups as they leave the Bering Sea (Trites, 19921. At present, there are no data to support this speculation, which stems from the temporal correspondence between the rapid development of commercial fisheries in the early 1970s and the "unexplained" continued decline of the fur seal population. 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~. However, it is not possible to directly relate these changes to the survival of fur seal juveniles.

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Biology of Higher Trophic Levels 153 Table 4.19 Demographic, behavioral, and physiological responses that could indicate food limitation in pinnipeds Factor Expected to Change Observed Change Northern Fur Seal Steller Sea Lion Harbor Seal Lower birth mass - Unknown No change Unknown Reduced growth rate Increased Reduced Unknown Reduced body condition Increased Reduced Unknown Reduced milk production/intake Unknown Unknown Unknown Increased foraging trip duration Decreased No change Unknown Reduced weaning mass Increased No change Unknown Reduced juvenile survival Reduced Inferred Unknown Increased age at first birth Unknown Maybe Unknown Decreased birth rate Unknown Maybe Unknown Reduced adult survival Inferred Inferred Unknown Source: Committee on the Bering Sea Ecosystem. Steller Sea Lions Two lines of evidence have been used to support the hypothesis that food limitation may be responsible for the population decline of Steller sea lions. Calkins and Goodwin (1988) compared size-at-age data for sea lions in the Kodiak area in 1985-86 with similar data for animals collected during 1975-78 (Calkins and Pitcher, 1982). They found that sea lions ~ to 10 years old were shorter and leaner and weighed less in the 1980s than those examined during the 1970s. With the declining trend in population size over this period, a density-dependent increase in size at age should have occurred if the overall amount of food availahie for .cea lions . . _ had remained constant. The decreased growth rates suggest that fond availahilitv most have declined more than population size. Although this conclusion appears to be warranted. it is id +~ ~r~l~ 41~ :^ :~ to ~ 1: ~_1 1_~_ _ 1_ Illl~ULLdIlL LU rUm~mDer Inal 11 IS Dasea on 1lmllea Gala from only one area where sea lions have declined. Similar comparisons have not been made in southeast Alaska where the sea lion population has increased. The second line of evidence indicating nutritional stress was a reduction in hemoglobin levels in seal lions examined during the 1980s compared to those sampled in the 1970s (Catkins and Goodwin, 1988). Hemoglobin level~is a useful indicator of anemia, dehydration, or iron deficiency (Bossart and Dierauf, 1990). 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) and for Steller sea lion pups two to three weeks of age (Castellin~ et al.,

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154 The Bering Sea Ecosystem 1993). Although the coronal range of erythrocyte indices has not been established for Steller sea lions, the suggestion that sea lions in the 1980s sample were anemic is equivocal. The hypothesis that reduced survival of juveniles may be the cause of the population decline (York, 1994) is attractive because recent studies have shown that weaned pups have limited foraging abilities compared to older sea lions (Merrick and Loughlin, 1993). Using satellite-linked, time-depth recorders, Merrick and Loughlin (1993) studied the foraging ranges and diving behavior of 19 adult female sea lions and 5 pups. They found that during the summer adult females with pups generally foraged within 20 km of land, made brief trips of less than two day's duration, and dived to depths of < 30 m. In winter, some females traveled over 300 km offshore for up to several months and often dived to > 250 m. Pups, on the other hand, were able to cover more than 300 km in a trip but their dives were generally brief (less than 1 minute) and shallow (less than 20 m). These preliminary results suggest that pups may rely heavily on prey near the surface, and that small changes in prey abundance in near-surface waters may have large effects on pup foraging success and thus on pup survival. There is also evidence that, like other pinnipeds, sea lion pups consume smaller prey than those taken by older sea lions (Merrick and Calkins, in press; Lowry et al., 1989). This may further constrain the array of prey available to pups compared to older animals. Based on studies on other pinniped species (e.g., Costa et al., 1989; Trillmich and Ono, 1991), one would expect changes in the availability of food to be reflected in the foraging behavior of lactating female sea lions and the growth and behavior of their suckling offspring. However, when Davis et al. (1993); Merrick et al. (1995) compared data for a declining population at Chirikof Island to data for a stable population at Lowrie Island they found no significant difference in female foraging trip duration, pup birth mass, or pup growth rates between the two areas. This suggests that mothers are not food-limited during the early part of lactation. Given the long period of maternal care in this species, it is possible that the effect of food limitation on lactating females and pups may become evident only later in lactation. Thus, further study is needed to determine both interannual variation and how these patterns may change during the entire lactation period. As noted earlier, pregnancy rates in Steller sea lions are lower than for other otariids for which data are available. 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). Harbor Seals There are no data that can be used to examine the hypothesis that food limitation has caused or contributed to the decline in the number of harbor seals in the Bering Sea or Gulf of Alaska (Table 4.19). A reasonably comprehensive study was conducted in the late 1970s to describe the biology of this species (Pitcher and Calkins, 1979). However, virtually no biological research was conducted on the biology of Alaskan harbor seals between then and the time of the Exxon Valdez oil spill in 1989. Recent studies initiated in response to the oil spill

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135 genera populalion decline ma provide some insight into bclors Euclid Baylor seals in Case areas.