|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
IntroductionThere has been a drastic decline in Steller sea lions (SSL) population, throughout the past three decades, in the Western Gulf of Alaska (WGOA), Eastern Bering Sea and Aleutian Islands (See Figure 1). The SSL has been listed as an endangered species. It is the purpose of our report to list possible causes of the decline, factors which contribute to the problem, and suggestions for further research. Many relevant options have been explored. Included in this report are food web interactions, eating habits, affect of commercial fishing on available food supply, climate changes, pollution, habitat, foraging patterns and human interactions. It is this report's intention to clarify the actual problem and suggest a line of action.
Population DynamicsA study of trend sites, between the late 1950's and 1990 show an overall decline of 78%, from an astonishing 105,289 SSL to 22,754 SSL (NOAA Final Recovery Plan, 1992). In a single year, data collected between 1989 and 1990 illustrates a steady decline in SSL from 23,064 to 22,754. Based on these results, one might suggest further studies in specific trend sites.
However, studies of trend sites slightly contradict collected population survey data, which tell of the count of adult and juvenile SSL increasing at 12 of 25 rookeries. These apparent increases may be due to the difference in the amount of sites surveyed during different survey dates. During 1989, 87 sites were surveyed ( NOAA Final Recovery Plan, 1992 ), but during 1990, 152 sites were surveyed ( Merrick et.al, 1987 ). The differences in population between 1989 and 1990 are believed to be due to normal fluctuations, and the results are not extensive enough to determine the answer. For this reason a study of trend sites, as opposed to collected population survey data, is recommended.
Observations of the pup population in correlation with fetal mortality is another option that should be researched in greater depth. The pup population gives scientists a crude indication of the rate of repopulation and the trend for future generations.
The pup population at Atkin Island, in the WGOA, dropped 77% between 1979 and 1986, from 4,538 to 1,046 pups (Lowry and Laughlin, 1990). Similar results were discovered at Seguam Island, in the central Aleutian Islands. The pup population decreased 79% between 1979 and 1989, from 2,475 to 529 (Lowry and Laughlin, 1990). This is another example of why site specific work is so valuable for gaining accurate statistics. Marmot Island, a breeding ground of much success in 1978 and 1979, has watched a 63% decline in pup population between 1978 and 1989, from 6,000 to 2,200 pups (Lowry and Laughlin, 1990). Currently, this decline shows no sign of slowing, and with the SSL in the WGOA and the Aleutian Islands listed as endangered,further research is essential.
However, some population data showns signs of increase. The White Sisters Islands have recently begun to be used as rookeries, increasing from 3 pups in 1979, to 95 pups in 1991. Pup production at Forrester Island, the largest SSL rookery in the world from 1989-1991, has increased from 2,187 in 1979, to 3,261 in 1991. The largest rate of increase occurred at Hazy Islands, from 30 pups in 1979 to 808 pups in 1991. (ADFG, unpublished data).
Habitat/EmigrationNatural factors that deplete the SSL habitat are overpopulation, disease, non-human predators, and climatic change. Climatic change appears to be the only factor contributing in any changes of habitat in the WGOA and Bering Sea. The principal factor responsible for the unfavorable trends in marine birds and pinnipeds is availability of food sources which may be a result of recent ecosystem shifts, driven by the climate. These changes favor the increased production of pelagic and demersal predatory fish (i.e. adult pollock, cod, salmon, and various flatfishes) which occurs at the expense of forage species (i.e., capelin, sand lance, herring, and myctophids) (Francis et.al, 1998). These are the very fish upon which SSLs have historically depended . One of the selecting factors for areas of congregation by SSLs is the amount of available food.
Human related factors involved in habitat change include marine debris from fishing operations and pollution. These factors are fairly insignificant when compared to other events contributing to changes in habitat.
Rookeries are breeding grounds. The emigration patterns of SSL program them to return to the rookery in which they were born to give birth themselves. The SSL are fierce and protective mammals. Competition for a birthing spot can be tough, but this is not the current problem. The 80% population decline has decreased the amount of competition for birthing spots within the rookeries. This does not seem to have a negative impact on the SSL population. Life on the rookery lasts each year during the birthing months of May through October. In June and July especially, the SSL are less seen out at sea. Another component of the SSL habitats are haulouts, the gathering and socializing spots, consisting of rock shelves, ledges, and slopes made from boulder or cobble. When considering the affect of decline of SSL populations on avaliable habitat, there is no shortage of haulouts (Calkins and Pitcher, 1982). Haulouts are determined by season, exposure, amount of food sources, and tradition of use. Non breeding adult/sub adult males go to adjacent haulouts (Hoover, 1998).
ClimateA major climatic regime shift occurred during the mid-1970's. At the same time, pollock fisheries increased dramatically ( www.1, 1998), and SSL population declined. A decrease in capelin is assumed, due to analysis of SSL stomach content in scientific studies (NMFS Final Recovery Plan, 1992). It could be suggested that a regime shift is contributing to the declining SSL population. A regime shift occurs when many components of the ecosystem change at one time. This change affects the interactions between the system, which results in a total change, affecting the life in the ocean. Climate variability is linked to ecosystem change (Francis et. al 1998).
The most important effect occurring within SSL habitat as a result of the climatic regime shift may be the changes in mixed layer temperature which could directly influence metabolic rates of marine mammals, including SSL. Overall, the temperature of the ocean rose as part of the previously mentioned regime shift (Francis et. al 1998).
Rapid changes, as a result of the regime shift are reflected in ocean surface currents and circulation. This is due to newly varied distribution of atmospheric pressure. Oceanic ecosystems respond to variations in physical conditions. Herein lies the linkage between atmosphere/ocean biological responses. Ecosystem response to specified climate variation is difficult to predict due to the complexity of the atmosphere-ocean-biosphere system (Francis et. al, 1998). The primary conditions resulting from climate change are changes in surface wind stress which affects horizontal and vertical flow, and mixing in the surface layer of the ocean, and the depth of the surface layer. Air-sea heat exchange will also be changed, as will the location and character of oceanic fronts and other mesoscale features (Francis et. al, 1998).
Climate change leads to variablity in primary production, the timing of blooms, mix of phytoplankton species, and their concentration and larger scale distributions. These fluctuations are reflected in the secondary production (zooplankton) level and in transfer efficiencies between levels. Before and after a major shift in climate, the upper trophic levels are likely to be supported by substrates that differ in abundance, concentration, and location of key zooplankton and the associated small nektonic species. Upper trophic level species will be selectively favored by changed conditions before and after a climate regime shift, so one might expect different total production and species mix in the two states. In the case of sustained large changes in abundance of certain key species, there should be changes, possibly compensatory, in other areas.
Growth and survival of upper trophic levels is affected by climate change primarily through direct effects on the metabolic requirements of juveniles, the availability of adequate food to satisfy those needs, the local distribution and abundance of predators and competitors, and the availability of suitable habitat for protection from predators. If everything changed correspondingly, the system would be expected to follow climate change, especially at the lower trophic levels where replacement time is short. Systematically, the regime shift has the ability to change all areas of life that are linked with the ocean ecosystem, and this is bound to result in some type of change for the SSL that live within it (Francis et. al, 1998).
Food Web Interactions
The nutritional content of SSL's prey can be measured by the variety it receives and the caloric value. A positive correlation was discovered between diet diversity and the amount of decline in prey in an area: as diet diversity decreased, population decreased (Richard L. Merrick et. al 1996). Due to limited resources, the SSL is unable to obtain as much caloric value as it once did. This makes it very difficult for the SSL to obtain the amount of food it needs to survive.
SSL are opportunistic feeders. They eat herring, cod, squid, rock fish, pollock, capelin and a variety of bottom feeders. Pollock, a fish with little nutritional value, is the main food source for SSL. Capelin was once the main food source for SSL, providing them with more caloric value than pollock. There are 17.3 grams of fat per 99.05 grams of capelin. This is approximately a 17.5% fat content, where as with pollock, they are only receiving a 0.6% fat content fish. Pacific Cod has decreased in the amount found in SSL stomachs, suggesting a decline in capelin stocks may reduce resources of other fish in northern seas (http://www.fish.is/products/capelin.htm).
According to Calkins and Pitcher (1981), and Calkins and Goodwin (1988), the SSL in Kodiak used to consume 43% volume in Capelin in 1975-78. In 1985-86, they were completely eliminated from the SSL diet (NMFS Recovery Plan 1992). In 1975-78 pollock was 22.8% of their consumption volume. During 1985-86 pollock had jumped to a high of 42.2% of their total intake.(Calkins and Pitcher 1975-78 and Calkins and Goodwin 1988) (see figures). Capelin was once the most heavily consumed prey by SSL. The fact that these highly nutritional fish are no longer ingested by the SSL, may indicate that there is something lacking in their diet.
A reduction of food sources is a leading hypothesis behind the decline of SSL in the WGOA, Aleutian Islands and Bering Sea. The SSL population decreased throughout the western portion of the SSL region in Alaska. The decline appeared to have started in the eastern Aleutian Islands and spread throughout the range (with the exception of southeast Alaska). However, in southeast Alaska, White Sisters Islands, rookeries pup counts have been increasing. In 1979, there were three pups, where as in 1991, there ere 95 pups. Pup production at Forrester Island , the largest SSL rookery in the world from 1891-1991, has gone up from 2,187 in 1979, to 3,261 in 1991. The most impressive increase has occurred at Hazy Islands, from 30 pups in 1979, to 808 pups in 1991 (NOAA Final Recovery Plan, 1992). In contrast, walleye pollock biomass in the Eastern Bering Sea rose from less than five million metric tons in the 1960s to a peak of over 13 million tons in the early 1970s and has since gone down to about 8 million tons in 1985 (Merrick et. al., 1986).
The growth of the SSL, as measured by standard length (see figure 2), axillary girth, and mass was significantly reduced in the 1980s, supporting the nutritional stress hypothesis . During the 1980s, the female SSL in the GOA were reported to be shorter, thinner, and had lower masses than samples of animals from the 1970s (Becker, 1996). Food limitations, resulting from an absolute reduction of food, a reduction in per capita food availability because of an increase in consumer population, or a reduction in quality of food, can result in reduced body size of marine mammals, (pinnepeds). If food supplies are low, or the nutritional content of the food consumed is low, lactating females have a hard time finding food to nurse their pups (Oxman,1998). Studies done by NOAA have shown that pups stick around longer with their mothers on the rookeries (NMFS, Steller sea lions in jeopardy, 1998). Rookeries are found on beaches of relatively remote islands. Access by humans and other animals is made difficult due to the exposure of wind and waves. Here, the mother gives birth and nurses the pup for approximately a year. When the mother is not getting proper nutrition, her milk will not be as nutritious for her pup (Oxman, 1998). Pups have been found up to the age of 37 months, still suckling off their mothers (Calkins, 1982). If there is another pup, this could be fatal for the smaller one. The mother will not be able to provide for both of her pups, if she is not receiving her nutrition. This is a major problem because baby sea lions are biologically predisposed to providing for themselves within a year. If a mother is unable to teach her pup to hunt properly, the mother is forced to continue feeding the pup for a longer period of time than the biological norm. This problem relates directly to the foraging habits of baby pups, and why a reduction in fish doesn't allow them to learn proper hunting skills within one year. (NMFS, Steller Sea Lions In Jeopardy, 1998).
Foraging HabitsWhen fewer fish are available to the female SSL, they are unable to obtain as much food on each dive. This means diving more frequently, and less time spent with the pup on the rookery. A greater amount of energy is expended for a fish of lesser nutrition. In order to receive proper nutrition, the SSL has to dive more frequently, because walleye pollock, for example, is less nutritious than capelin. Until juvenile SSL perfect their foraging skills, they are unable to catch fish which are found at deeper depths. (Steller Sea Lions In Jeopardy, 1998)
In a study conducted by Una G. Swain and Donald G. Calkins (1993), in order to find the mean dive depth and duration of a juvenile SSL, four juvenile SSL were tranquilized with Telazol and fixed with a time-depth recorder. After each sea lion was rendered unconscious, the satellite monitored time-depth recorders were fixed with a fast setting epoxy resin to the mid-dorsal region of the sea lion's back. The test subjects consisted of one two year-old male, one yearling male, one-two year old female, and data taken in 1993 from a yearling female in Southeast Alaska. All four were observed suckling, although the oldest male only suckled once during the three months of observation. Every dive made by these sea lions in the three month period of study was recorded for both depth and duration. These dives were broken into six time duration (see figure 3) and six dive depths (see figure 4).
Human InteractionsAlaska Natives are authorized by MMPA to harvest and use Steller sea lions. As long as this use is for subsistence purposes and is not wasted, it may continue even if the species is listed as depleted. For centuries Alaska Natives harvested and used sea lions for subsistence purposes. Currently, sea lions are not commercially harvested. From 1959 to 1972, they were commercially harvested in the eastern Aleutian Islands and Gulf of Alaska. From 1963 and 1972, a total of 45,178 pups of both sexes were harvested in the eastern Aleutian Islands and Gulf of Alaska. Pup harvests, which sometimes reached 50% of the total pup production from a rookery, could have depressed recruitment in the short term. This may explain why there was a decline at some sites during the mid-1970s. Unfortunately, it does not explain why there was a decline in sites where harvesting was not present. In some places, the decline did not occur until approximately 20 years after harvesting (NMHS final recovery plan 1992).
Data collected from 1975 to 1985 in the GOA showed that Steller sea lions may have been entangled in fishing gear. However, no records of entanglement have been recorded. Only one juvenile sea lion out of 3,847 has been recorded as entangled in debris. According to this research, entanglement does not seem to be a main contributor to the depletion of sea lions (NMFS, final recovery plan 1992).
In addition to subsistence hunting and net entanglement, pollution may play a very minor role in reduced population. In California, sea lions have experienced reproductive failure when associated with a pollutant, organochlorine (NMFS, final recovery plan 1992). Impaired immune systems have also been related to exposure to the toxin. Relatively high concentrations of organochlorine compounds have been found in Alaska's SSL population. These levels have not been directly related to any changes in health so far. In addition to the presence of organochlorine SSLs were directly exposed to oil in Prince William Sound after the Exxon Valdez oil spill. So far no significant long term effects have been confirmed. At the present time, it does not appear that pollutants are expected to be a leading cause in the decline of the SSL population.
Socio-Economic EffectsHumans, as well as relying on the pollock industry as a food source, rely on the fishery for income. The Puget Sound at-sea processing fleet alone provides jobs for over 10,000 people, and is worth approximately $750 million per year (http://www.fakr.noaa.govpmtc/gopllck.htm, 1998). As far as its affect on Alaska's economy, the fishery is required by law to allocate 35% of the total harvest to onshore processors (Clark, Maureen, Associated Press, 1998), and 7.5% to rural native villages through the CDQ (community development quota) program. This program, since 1992 has brought approximately $12 million in wages to the 56 ANCSA (Alaska Native Claims Settlement Act) villages (http://www.fakr.noaa.govpmtc/gopllck.htm, 1998). These numbers indicate the fishery's importance to many people's livelihoods, and the U.S. economy.
ConclusionsDue to the complexity of the variables surrounding the declining population of the SSL throughout the Aleutian chain, Gulf of Alaska, and Bering Sea, we recognize that conclusions and recommendations must necessarily be conservative in order to appropriately reflect this complexity.
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SourcesOxman, Don. (Personal Interview, Dec, 1998) Phd. Candidate. University of Alaska Southeast 11120 Glacier Highway Juneau, AK 99801