|This paper was written as part of the 1999 Alaska Ocean Sciences Bowl high school competition. The conclusions in this report are solely those of the student authors.|
in part by each of the following:
Juneau-Douglas High School
IntroductionSteller Sea Lions (Eumetopias jubatus) are the largest members of the otariid family. These pinnepeds inhabit the north Pacific Ocean, ranging from 318 kg females to 1 ton bulls.
During the 1950's, scientists began studying the population dynamics of Steller sea lions. Two areas of particular interest were the Bering Sea and the Gulf of Alaska (See Figure 1). Since the study began, the population has decreased 80% in these two areas (See Table 1). This report examines possible contributors to the decline, including food web interactions, human impact, climate influence, and the changes in population dynamics. Also included are recommendations to further understand the causes of population decline and promote recovery of the species.
DeclineSince the late 1950's, scientists have been studying the decline of Steller sea lions (SSLs) in the Gulf of Alaska and the Bering Sea. The first counts made in 1956-1960 suggested that at least 140,000 SSLs existed in the Gulf of Alaska (GOA) and the Aleutian Islands (See Table 2) (Merrick et al., 1987). This high count decreased 81% by 1990, to 27,860 (See Table 2) (Merrick et al., 1987), and has decreased steadily by approximately 5% each year through 1996 (Pennoya et al., 1998). Prior to this decrease in population, Alaska was home to approximately. 75% of the world's SSL population, with 38% in the GOA and 37% in the Aleutian Islands (NMFS, 1992a; Braham et al., 1980; Merrick et al., 1987). A major decline was first detected in the eastern Aleutian Islands in the mid-70s (Braham et al., 1980). By the late 1970s and the early 1980s, the decline had spread eastward to Kodiak Island and westward to the central and western Aleutian islands (Merrick et al., 1987; Byrd, 1989). The greatest decline was observed in the eastern Aleutian Islands and western GOA. In the eastern Aleutian Islands, the population dropped from 52,530 in 1960, to 4,875 in 1990, a 91% decrease in population (See Table 2). In the western GOA, the population dropped from 24,320 in 1956 to 5,331 in 1990, a 78% drop (See Table 2). Population surveys of non-pups conducted at Walrus Island have shown an 88%-90% drop. SSL non-pup inhabitants have dropped from 4,000-5,000 in 1960, to 600 in 1982 (Kenyon, 1962; Loughlin et al., 1984), and less than 500 in 1988. Other non-pup population surveys conducted in the western Aleutian Islands illustrate a smaller drop in population. Data collected from 1977-1980 and 1988 illustrate a 65% drop, from 27,228 to 9,517 (Byrd and Nysewander, 1988). Reports following 1988 have illustrated a wide range of decline in this area.
Studies done between 1984-1989 showed that from the Kenai Peninsula to Kiska Island, the adult and juvenile population declined 63%, from 67,617 to 24,953 (Loughlin et al., 1990).
Between 1989-1990, pup counts at Kiska Island decreased by 25%, however, pup counts elsewhere appeared to increase. At Bogoslof and Seguam Islands, the pup count increased by 29%. The count of adults and juveniles increased at 12 of the 25 rookeries. These apparent increases may be due to the difference in the number of the sites surveyed during the different survey dates. In 1989, 87 sites were surveyed (Merrick et al., 1991), however, in 1990, 152 sites were surveyed (Loughlin et al., 1990). The changes in population between 1989-1990 may be within range of natural fluctuations.
In an attempt to clarify the collected population survey data, scientists focused on a group of sites which have been included in every major survey. The data collected from the trend sites between 1989-1990 illustrated a steady decline, from 23,064 to 22,754, in contrast to the increase shown by the studies of 87 sites in 1989, and 152 sites in 1990. A comparison of the trend sites between the late 1950s and 1990 showed an overall decline of 78%, from 105,289 to 22,754 (See Table 1) (Merrick et al., 1992).
Counts of SSL pups provide reliable index of population status and productivity (Lowry & Loughlin,1990). The population percentage of older SSLs is increasing. Twenty percent of the population on rookeries used to be of juvenile SSL, now it is down to five percent. The pup population at Atkin Island, in the west GOA, dropped 77% between 1979 and 1986, from 4,538 to 1,046 (Lowry and Loughlin, 1990). Similar declines have been discovered at Seguam Island, in the central Aleutian Islands. The pup population dropped 79% between 1979 and 1989, from 2,475 to 529 (Lowry and Loughlin, 1990). Marmot Island, an incredibly successful breeding ground in 1978 and 1979, has seen it's pup population drop 63% between 1978 and 1989, from 6,000 in 1978 and 1979, to 3000 in 1987 and 1988, and 2,200 in 1989 (Lowry and Loughlin, 1990). This drastic decline in pup population is an undeniable predictor of future population decline.
Human Impacts-directThere are four main types of direct human impact on SSL populations; commercial harvesting, subsistence harvesting, incidental trawl catches, intentional agitated killing due to SSL interference with fishing. Currently, there is no commercial harvesting of SSLs. However, Since the late 1950's, scientists have been studying the decline of Steller sea lions (SSLs) in the Gulf of Alaska and the Bering Sea. The first counts made in 1956-1960 suggested that at least 140,000 SSLs existed in the Gulf of Alaska (GOA) and the Aleutian Islands (See Table 2) (Merrick et al., 1987). This high count decreased 81% by 1990, to 27,860 (See Table 2) (Merrick et al., 1987), and has decreased steadily by approximately 5% each year through 1996 (Pennoya et al., 1998). Prior to this decrease in population, Alaska was home to approximately. 75% of the world's SSL population, with 38% in the GOA and 37% in the Aleutian Islands (NMFS, 1992a; Braham et al., 1980; Merrick et al., 1987). A major decline was first detected in the eastern Aleutian Islands in the mid-70s (Braham et al., 1980). By the late 1970s and the early 1980s, the decline had spread eastward to Kodiak Island and westward to the central and western Aleutian islands (Merrick et al., 1987; Byrd, 1989). The greatest decline was observed in the eastern Aleutian Islands and western GOA. In the eastern Aleutian Islands, the population dropped from 52,530 in 1960, to 4,875 in 1990, a 91% decrease in population (See Table 2). In the western GOA, the population dropped from 24,320 in 1956 to 5,331 in 1990, a 78% drop (See Table 2). Population surveys of non-pups conducted at Walrus Island have shown an 88%-90% drop. SSL non-pup inhabitants have dropped from 4,000-5,000 in 1960, to 600 in 1982 (Kenyon, 1962; Loughlin et al., 1984), and less than 500 in 1988. Other non-pup population surveys conducted in the western Aleutian Islands illustrate a smaller drop in population. Data collected from 1977-1980 and 1988 illustrate a 65% drop, from 27,228 to 9,517 (Byrd and Nysewander, 1988). Reports following 1988 have illustrated a wide range of decline in this area.
Studies done between 1984-1989 showed that from the Kenai Peninsula to Kiska Island, the adult and juvenile population declined 63%, from 67,617 to 24,953 (Loughlin et al., 1990).
Between 1989-1990, pup counts at Kiska Island decreased by 25%, however, pup counts elsewhere appeared to increase. At Bogoslof and Seguam Islands, the pup count increased by 29%. The count of adults and juveniles increased at 12 of the 25 rookeries. These apparent from 1959-1972, commercial harvesting took place in the eastern Aleutian Islands and the GOA (Merrick et al., 1987). Harvesting began in 1959 with the experimental harvesting of 616 adult males (Thorstein and Lensink, 1962). From 1963-1972, a total of 45,178 pups were harvested in the eastern Aleutian Islands and the GOA (Merrick et al., 1987). The largest harvest between 1963-1972 occurred at Sugarloaf Island, where 16,763 pups were taken, and Marmot Island, where 14,180 pups were taken. The largest harvests between 1970-1972 took place at Ugamak Island (3,773 pups) and Akutan Island (6,036 pups). Pup harvests reached 50% of total pups at the rookeries at time (Merrick et al., 1987).
The Marine Mammals Protection Act (MMPA) allow subsistence hunting to continue throughout Alaska, even in times of depletion, and has been documented in Prince William Sand, lower Cook Inlet, Kodiak Island, Alaska Peninsula, Pribilof Islands, and the Aleutian Island (Haynes and Mishler, 1991). The impact of subsistence hunting appears minor, with a total of 207 taken between 1980-1990.
From 1956-1984, SSL catches by fisheries escalated steadily (See Figure 2)(Trites and Larkin, 1992), peaking in the 70's and early 80's. A sharp decline occurred, beginning in 1984, and dropping rapidly, most likely due to changes in fishing techniques and the area and times fished (Stellar Sea Lion Recovery Team, 1992). With an estimated 20,000 SSL total incidental catch by trawl fisheries from 1966-1988, Perez and Loughlin believe that incidental trawl catches were a contributing factor to the SSL decline during the 1970's (Stellar Sea Lion Recovery Team, 1992).
Deliberate shooting of SSLs interfering with fishery operations have been acknowledged. An estimated 305 SSLs were killed during the 1978 Copper Riber Delta Salmon gillnet fishery (Matkin and Fay, 1980). SSLs have also been killed in the eastern Aleutian Islands, to be used as crab bait. These killings may have been part of the declining populations at haulouts and rookeries or encouraged migration (Loughlin and Nelson, 1986; Merrick et al., 1987).
Human Impacts-IndirectCommercial fisheries appear to be the main source of human impact upon SSLs by creating competition for food. Competition for food between commercial fisheries and SSLs include the removal of spawning biomass of prey to the extend of reduction in reproductive capabilities of fish, and competing for the same size of fish, which, amongst pollock can alter the rate of cannibalism by altering size selection (NMFS, 1998).
Pollock fisheries have been the main focus in the study of SSL decline, with 1.2 billion tons of pollock removed from the Bering Sea each year (Stellar Sea Lions in Jeopardy, 1998).
Diving depths of SSLs occur within the range of trawling depths for pollock which range from 30 m to 500 m (NMFS, 1998).
Biologists believe that 70% of pollock catch comes from critical SSL habitat. With fishing efforts increased, and 10 month fishing seasons decreased to 3 months of very intense fishing (Speiss, 1998), pollock populations are wiped out, with less time to repopulate.
From the 1970s to the 1980s, pollock consumption's by SSLs increased from 22% of their diet, to nearly half. However, total average volume of food decreased from 1317 ml to 745 ml, suggesting that pollock is one of the only available foods left for SSLs.
ClimateBy affecting the available food, it is possible that changes in climate have contributed to the decline of SSL population. Food limitation may be caused by climatically-driven ecosystem shifts, favoring increased production of pelagic and demersal predatory fish. These predatory fish include adult pollock, cod, salmon, and various flatfishes, which prey on capelin, juvenile pollock, and herring (Francis et al., 1998). This increase of predators, in combination with exploding pollock populations (Michele Drummond, personal communication) could help explain the elimination of capelin from SSL diet.
Food Web InteractionsThe quality of the SSL diet is not determined only by the specific species consumed, but also by the variety of species consumed (Merrick et al., 1997). Data from Merrick's studies (1997) and the NMFS (1995) indicate that walleye pollock and atka mackeral are the dominant prey in the SSL diet. Atka mackeral distribution ranges from the Kamchatka Peninsula through the Aleutian Islands and the GOA. The main area of abundance appears to be in the Aleutian Islands, from the Seguam Islands to Buldir Island (NMFS, 1998). Distribution of atka mackeral stocks are debated. Studies done by Levada (1979) and Lee (1985) suggest that separate stocks exist in the Bering Sea and the GOA. However, genetic studies indicate that the stocks of atka mackerel in the Bering Sea and the GOA are mixed (Lowe et al., 1998). Larger atka mackerel in the GOA, and a sensitivity to local fishing suggests that the GOA population is recruited from the Bering Sea population (Lowe and Fritz, 1997). During spawning season, atka mackerel schools aggregate near the bottom of the ocean to lay their eggs (Musienko, 1970). This spawning season peaks in August in the Aleutian Islands (McDermott and Lowe, 1997). Pollock are distributed through the North Pacific in temperate and subarctic water (Wolotira et al., 1993). The largest spawning ground in the east Bering Sea is located in the region north of Unimak Pass. In the GOA, the largest spawning concentrations are located in Shelikof Strait and the Shumigin Islands (Kendall et al., 1996). Smaller pollock (40 cm) from the east Bering Sea tend to remain on the shelf in the summer and move to the slope in the winter (Springer, 1992). Ninety percent of pollock smaller than 25 cm may be found on the shelf, west and northwest of St. Matthew Island, with only 3% in the east Bering Sea, south of the Pribilof Islands (Springer, 1992). It is hypothesized that a decrease in whale population led to an increase of krill, resulting in an explosion of walleye pollock (Michele Drummond, personal communication).
From 1975-1986, the variety of SSL diet changed drastically. In 1975-1978, SSL diet was dominated by walleye pollock (22.8% of stomach contents), capelin (43% of stomach contents), and pacific salmon (27.9% of stomach contents). By 1985-1986, the dominant species in SSL diet were walleye pollock (42.2% of stomach contents), flatfish (25.2% of stomach contents), and octopus (26% of stomach contents). The frequency of the consumption of walleye pollock among SSL near Kodiak increased 19.2%, and accounted for 19.4% more of the food found in their stomachs, at 42.2% (See Table 4)(Calkins and Pitcher, 1981; Calkins and Goodwin, 1988). Following the dietary chance, nearly half of SSL diet consisted of walleye pollock, capelin consumption ceased entirely in the Kodiak region, and pacific salmon accounted for only .6% of stomach contents, down from 27.9%. The total amount of food found in SSL stomachs decreased from 1317 ml in 1975 to 745 ml in 1986, a 44% decrease (Calkins and Pitcher, 1981; Calkins and Goodwin, 1988).
The value of prey type should by quantified on the basis of the net gain in calories and nutrients (NMFS, 1998). The reduction, not only in the amount of food consumed, but the type of food consumed, suggests a lack of quality in SSL diet. The caloric content of salmon and the caloric content of capelin differs greatly from the caloric content of walleye pollock, therefore, the lack of salmon and capelin in SSL diet could be drastically depriving SSL of their caloric needs.
Caloric content of salmon ranges from 130 calories, with 22 grams of protein and 4 grams of fat, per 3 oz portion, in pink salmon, to 200 calories, with 22 grams of protein and 11.5 grams of fat, per 3 oz portion, in king (Chinook) salmon. The caloric content of capelin per 3 oz portion consists of approximately 190 calories, with approximately 14 grams of fat and 12 grams of protein. The caloric content of walleye pollock per 3 oz portion lacks significantly in comparison, containing 90 calories, with 19 grams of protein and 1 gram of fat (Alaska Seafood Marketing Institute, 1998). The incredible shift in the variety of food consumed, and the lack of caloric value, suggests a substantial reduction of nutrition.
Nutritional stress appears to be behind the growth reduction in the SSLs in the GOA. In 1958, the average length of female SSLs over 9 years of age was 2,444 mm. By the 1970s, the average length for adult females was 2,320 mm, and by the 1980s, the average length was 2,312 mm (See Figure 3)(E. Becker et al., 1997).
The foraging patterns of SSLs change in accordance to the season. Summer foraging trips are much shorter than the trips taken in the winter (Merrick and Loughlin, 1997). It is likely that this is due to the pupping which occurs in the summer months. Trip duration in the summer months average 18-25 hours, covering 17 km, with dives occupying 4.7 hours per day. During the winter months, trip length averaged 204 hours, covering 133 km, with dives occupying 5.3 hours per day (Merrick and Loughlin, 1997). Foraging trips made during the winter by the young-of-the-year were in between the summer and winter foraging trips made by adult females, lasting 15 hours, covering 30 km, with dives occupying approximately 1.9 hours per day (Merrick and Loughlin, 1997).
SSL dives are generally shallow, with maximum depths reaching 250 m, and winter young-of-the-year dives reaching a maximum depth of 72 m (Merrick and Loughlin, 1997). The most common depth for females during the summer months was 10-20 m, 4-10 m for winter young-of-the-year, and 4-10 m for females during the winter months (See Figure 4)(Merrick and Loughlin, 1997).
SSLs generally feed on near shore prey in estuaries and marine waters in 180 m deep or less (Fiscus and Baines, 1966). In the 60's, observations concluded that SSLs left the shore in groups of thousands, and split into smaller feeding groups about 8-24 km out (Fiscus and Baines, 1966). Groups in the 1970's typically consisted of 2-12 SSLs (gentry, 1970), however, observations reported by Connell et al (1983), stated that feeding groups generally consisted of groups of 2-4.
Changes in the size of SSLs, in combination with the reduced availability of volume and variety of food suggests that SSLs are having to work harder to obtain food, and consuming unusual prey. Large marine animals are occasionally eaten. Some of these animals include harbor seals, spotted seals, bearded seals, ringed seals, fur seal, and sea otters (Gentry and Johnson, 1981; Pitcher and Fay, 1982; D. Calkin, unpublished data). The consumption of octopus near Kodiak has increased, from accounting for .2% of the volume of food found in their stomachs, to 26% of the volume of food found in their stomachs (See Table 4) (Calkins and Pitcher, 1981; Calkins and Goodwin, 1988).
Population Dynamics: (Age group, Reproduction, Disease and Mortality)
From 1978-1994, it was observed that the population percentage of older SSLs were increasing, while pups were drastically decreasing. In 1978, pups accounted for 20% of SSL populations at rookeries. By 1994, pups accounted for only 5% of the total rookery population (Stellar Sea Lions in Jeopardy, 1998).
Examining female to male ratios, it was determined that they have remained balanced in side natural boundaries, slightly favoring males (Stellar Sea Lion Recovery Team, 1992; Calkins et al., 1982).
SSL migration habits suggest that they generally return to the same rookeries, haulouts, and stopover sites of their birth (Kenyon and Rice, 1961; Fiscus and Baines, 1966; Fiscus et al., 1976; Bonnell et al., 1983). However, shifts in distribution suggest a northward migration from rookeries in British Columbia to Southeast Alaska (Bigg, 1988).
Reproduction rates appear to have slowed since the 1970s in SSLs, perhaps due to lack of nutrition and reduction in growth rate. In SSLs, sexual maturity relates directly to size. With SSL females maturing at slower rates, it appears as though reproduction would be increasingly delayed in females from the 1980s. Females collected in the 1970s were larger than females of the same age collected in the 1980s, suggesting that SSLs in the 1970s would experience more successful repopulations (Calkins et al., 1998).
Fetal mortality rates appears to have increased since the 1970s. The components of reproduction, pregnancy rates and birth rates seem to have declined since the 1970s. The pregnancy rates of sexually mature females in the GOA declined 7% from 67% in 1975-1978 to 60% in 1985 (Pitcher and Calkins, 1981; Calkins and Goodwin, 1988). Data provided from the 1970s and 1980s suggests a 97% pregnancy rate after the mating season, which declined to 67% in the 1970s and 55% in the 1980s. These reductions imply an increasing occurrence of fetal mortality. Fetal mortality may be a common strategy of SSL reproduction (Pitcher et al., in review), perhaps in conjunction with reduction in available nutrition.
Research conducted on Haptoglobin (HP) levels in SSLs throughout Alaska strongly implies that the Bering Sea and GOA populations are experiencing significant physical trauma in comparison to SSLs in Southeast Alaska. HPs are blood proteins which increase considerably in response to infection, inflammation, tumor, or trauma (Nguyen, 1989). The levels of HP in SSLs from the Bering Sea and the GOA, where population is declining, were considerably higher than HP levels found the SSLs from Southeast Alaska, where population status remains stable.
HP levels in Southeast Alaska SSLs averaged 143.1 in adults, 97.8 in juveniles, 87.2 in pups, and 35.4 in newborns. Pups sampled in the Aleutian Islands had an average HP level of 253.6. Adults SSLs examined in the GOA had an average HP level of 250.7, 263 in pups, and 68.6 in newborns (Adams et al., 1996). The higher HP levels in newborns in comparison to newborns in Southeast Alaska suggest physical trauma beginning from birth, possibly due to lack of nutrition in the mother. The higher levels of HP found in pups from the Aleutian Islands in comparison to adults from that same region illustrated the stress occurring in the pup populations and their declining numbers.
There appear to be three main diseases which are afflicting SSLs: Sam Miguel Sea Lion Virus (SMSLV), Leptospria, and Chlamydiosis. SMSLV was detected in 49% of adults and 21% of fetuses in the GOA, however, only 4% of the SMSLV titers discovered in SSLs were known to cause reproductive problems (Goodwin and Calkins, 1985). Chlamydial antibodies were detected in 62% of adults, and were absent in fetuses. In the Bering Sea, Leptospria, known to contribute to fetal, pup, and adult mortality (Gilmartin et al., 1976; Morton, 1981; Britt and Howard, 1983) was detected in 2 of 6 SSLs (Fay et al., 1978).
Socio-economic Effects of Fisheries in Alaska
Altering the fishing industry would be the most obvious attempt at aiding the SSL population, by reducing competition for food. However, this takes only the scientific needs into consideration. Altering fishing industry would affect not only commercial survival, but coastal community survival.
Unalaska, the United States' biggest fishing port for the past nine years, has passed a resolution to initiate a review of Alaska's Bering Sea pollock allocation system. The Unalaska resolution is the fourth calling for an examination of this system. "Shore-based pollock processing is the heart and soul of our local economy. We depend on it for our thousands of private sector jobs and most of our local tax revenues." Changes in fish takes may cause "hundreds of jobs to be lost and Unalaska's strong economy to falter." Unalaska Mayor Frank Kelty, speaking on the devastating effects changes in fishing allocations could wreak on small-town economies.
The Unalaska resolution stated that Alaska's pollock system should comply with the Magnuson-Steven Fishery Management Act. This act requires that fisheries should maximize the benefit to local communities and take into account factors which could adversely affect communities.
Another economic boost for small Alaskan communities was instigated by the Community Development Quota program already sends more than twenty million dollars a year into Alaska's poorest fishing villages.
In September of 1997, a federal agency allocated resources used by Alaskans. In Seattle, the North Pacific Fisheries Management Council met to make decision on the nation's most valuable fisheries, it is estimated that the annual final market value of Alaskan pollock is nearly one billion dollars. (ASPA, 1997)
The loss of SSLs in Alaska would have a negative effect on coastal communities as well, by disrupting traditional harvesting by subsistence hunters. However, with only 100 SSLs being harvested on average annually, subsistance appears to have little impact. For the economy and sustenance of coastal communities is far more dependent upon availability of fish than the availability of SSLs.
The drastic decline of the SSL population in the Bering Sea and the GOA is clear. These areas have lost 80% of total population since the 1950's.
After analyzing numerous possible components, we have determined that a reduction in the quantity, quality, and variety of available food may be contributing to failing immune systems and reduced growth of SSLs in the Bering Sea and the GOA. We recommend conducting further research in the reduced quantity of capelin and salmon found in SSL stomachs, and how this may be affecting their overall health.
Pollock fisheries have been widely disputed as a contributing factor, by creating competition for food. We suggest researching modified techniques in trawling procedures in order to reduce this competition. Altering trawling intensity, time of year, and trawling depths would be three important components to consider.
The future of the SSL population is unsure. Population declines have been experienced by other pinnepeds, who have generally been able to survive. An undetermined simple cause of the drastic reduction of SSLs suggests a complex system of contributing factors. It is possible that a link of the system will fail, and the species will further decline, or that a link will strengthen, and the species will once again flourish.
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