|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:
Tony A. Dickens
Ryder E. Erickson
Michael I. Martin
Frank A. Degnan High School
AbstractIn this paper we have written about the many different factors that can be taken into consideration for the downfall of the Steller sea lion population. In reading many documents we believe that predation is an unlikely cause of the decrease in the population. Another factor taken into consideration was parasitism and disease. Though there are some sea lions that die from this, the research on the topic isn't sufficient enough to understand the importance it may play and it must be studied more.
Diseases that cause reproductive failure might be a problem, though blood tests have been taken and most all of the pups were healthy. Changes in the environment could be a contributing factor, but Steller sea lions can tolerate a wide temperature range so this would not be a major contributor. Maybe the temperature is affecting the sea lions food source, but if that were true there would be more than the Steller sea lion suffering. The harvesting of Steller sea lions is not a factor because there has been no commercial harvest of Steller sea lions in America since 1985. Pollution could be a factor, but the only major thing that has happened recently is the Exxon oil spill. A big factor may be all of the contact with humans.
We believe is that fishing has forced sea lions into malnutrition. All of this will be explained further in the pages of this report.
ResearchThere are many different factors that can be taken into consideration as causing the downfall of the Steller sea lion population. Some of them are more likely than others, and it may have been a combination of more than one factor which has caused the population decline.
According to the Recovery Plan prepared by the Steller Sea Lion Recovery Team for the National Marine Fisheries Service, predation is an unlikely cause of the population decline because there is no scientific evidence that predation on the Stellar sea lion by other marine species like killer whales has increased in recent years. Although, according to a study using mathematical computer simulation led by Lance Barrett-Lennard (http://www.zoology.ubc.ca/~consort/research.htm), transient killer whales could play a very significant role in the Steller sea lion yearly mortality rate. Up to eighteen percent of the deaths of the sea lions is due to the predation by killer whales in Alaska. The scientists came to the conclusion that killer whales are not the cause of the population decrease, but are probably now a significant contributing factor (http://www.zoology.ubc.ca/~consort/research.htm).
Parasitism and disease are another factor that can be taken into consideration. The occurrence of disease is difficult to analyze because most of the sea lions studied were healthy when chosen. The parasites that are found on or in Steller sea lions are intestinal cestodes; trematodes in the intestine and bile duct of the liver; nematodes in the stomach, intestine and lungs; acanthocephalans in the intestine; acarian mites in the nasopharynx and lungs; and an anopluran skin louse. Nematodes will make an infection in the sea lions that may cause stomach ulcers.
The stomach ulcers probably only cause a small number of deaths. Research on parasitism isn't sufficient to understand the importance it may play and it must be studied more (National Research Council 1996).
Diseases that cause reproductive failure may be a problem according to the National Research Council (1996). Leptospires are spirochete bacteria that are thought to be causing abortions and adult mortality in California Sea Lions and northern fur seals, but Leptospires don't appear to be a significant problem in the sea lions in the Kodiak area. Chlamydia is another disease, but has not been studied enough and is currently being examined to see what effects it has on the sea lions. A study by Gulland et al. (http://www.zoology.ubc.ca/~consort/research.htm) examined sexually transmitted diseases affecting reproduction. They took blood samples from ten mother/pup pairs at Forrester Island and screened for many different types of antibodies. The results of the tests were negative and all seemed free of any antibodies or diseases that would cause a failure in reproduction.
Environmental changes could also be a problem. El Nino is a small factor because it only mildly affects their population and El Nino's effects are only relevant in the southern regions of the world. Sea ice however could be a problem. Due to some climatic changes, the sea ice could be very thin or too thick for sea lions to surface and stay out of the water. Sea lions are able to tolerate a relatively wide range of environments so that climate change wouldn't affect them directly. Perhaps environmental changes have affected the food source pollock, but that change would not be the only affect on the pollock population (National Research Council 1996).
As of now there is no harvesting of Steller sea lions for commercial use. Back in 1985 when Steller sea lions were harvested (National Research Council 1996), thousands of sea lions were killed and in some places this included up to fifty percent of the pup population (National Marine Fisheries Service 1992). This had a short term effect on the population, and has no effect where the population was harvested commercially.
Pollution is another considerable factor, but the only real hazard that has happened recently would be the Exxon Valdez oil spill. That did account for a low mortality rate in the Steller sea lion population, but that was only for the short while that it took to clean the spill. Some sea lions that were exposed to the oil were found to have traces of hydrocarbon in them. However, this was inconclusive evidence to their population decline. Two other sea lions that were examined were found to have high levels of PCPs and DDT in their blubber, but this was also inconclusive evidence for the decline (National Research Council 1996).
Other contaminants could include human waste materials containing undigested corn, heavy metals or radioisotopes. Scientists are looking into this possibility and researching it further, but so far, nothing is related to the sea lion decline (National Research Council 1996).
According to the University of British Columbia report (http://www.zoology.ubc.ca/~consort/ research.htm), from 1993 to 1997 sea lions were counted daily during their breeding season at two sites at Forrester Island in a study with the Alaska Department of Fish and Game. One of the two sites was repeatedly disturbed by researchers conducting independent studies. The other site remained undisturbed during the whole four years. Scientists suggest that human disturbance may cause females and pups to abandon the rookery earlier in the breeding season than they would otherwise.
Steller sea lions are easily scared off their rookery and haulout sites by humans. Even though the sea lions return to their rookery after being scared, it is possible that human disturbance might be a contributing factor to the population decline. The sites used by Steller sea lions in the Bering Sea and Gulf of Alaska are generally in areas that are remote or are protected from human activities.
According to the University of Idaho report (http://www.uidaho.edu/~crup0675/images/UAsteller.jpg), a research project led by a University of Alaska Southeast instructor suggested that distance restrictions for boaters may not be big enough to prevent disturbances of resting populations of Steller sea lions in Glacier Bay National Park. Beth Mathews, a UAS professor of biology, showed the results of some studies done by the National Park Service that sea lions may need as much as 180 yards between them and an approaching boat or they get scared from their habitat. The studies found that if boats stayed farther from the sea lion rookeries, more sea lions were spotted in their areas.
The factor that may have the largest effect on the Steller sea lions population and the factor that causes the most controversy would be the effect fishing has on the sea lions source of food. Observations have shown that the Steller sea lion's average age of older females has increased in the vicinity of Marmot Island. This means that there are fewer juvenile sea lions surviving. An explanation is that there was 20% per year decrease in the survival of juveniles ages 0-3 which has caused a 5% per year decrease of the total population in the vicinity of Marmot Island since 1975. A limitation on food would effect the juvenile sea lion population. If there was a decrease in food, then the juvenile sea lion would have to increase its search time in order to find the food which would also increase its probability of predation since it would be out at sea longer. It would also have to increase its search effort and if their efforts don't pay off, then it all leads to a decreased body condition. Decreased body condition leads to starvation or slower growth rates. Starvation will cause a decrease in juvenile survival which means an age class shift like that observed at Marmot Island. Slower growth rates lead to delayed maturity and delayed maturity means longer generation times. This too will lead to an age class shift because of an increase in the age of a first birth. So fisheries taking away from the sea lions' source of food could very likely be the cause of the decline (Alaska Sea Grant 1993).
58.3% of the Steller sea lion's diet consists of pollock, 20.6% consists of herring, capelin makes up 7.4%, salmon 5.1%, squids 4.2%, sculpins1.3% and pacific cod, flatfishes, rockfishes, octopus and everything else comprises less than 1%. A natural assumption would be that the fisheries must be taking too much of the pollock, since pollock is their main source of food. According to (National Marine Fisheries Service 1992) in the Gulf of Alaska, walleye pollock biomass declined from 3.7 million metric tons in 1981 to 0.29 million metric tons in 1989 with a slight increase in 1990. These were based on hydroacoustic surveys in the Shelikof Strait. The report at (http://atsea.org/issues/steller.html), as well as most research, shows that the pollock population has been its highest ever during the years the Steller sea lion population had been decreasing. Therefore there is enough pollock in the sea for the sea lions. In fact, the problem seems to be that the pollock are the dominating fish and the amount of nutritious fish is too low. That may be the only reason pollock makes up such a large part of their diet. It's all they can find. Pollock is not a nutritious fish compared to the other fish the sea lions could be eating (http://atsea.org/issues/steller.html). Pollock are also bottom feeders, so the sea lions have to spend more energy for a less nutritious reward. The juveniles might also be having a hard time swimming that deep, which would go along with the decreasing juvenile percentage of the population.
SummationThis problem of not having enough of the right fish and too much of the wrong fish seems like it makes the most sense. This is probably the biggest problem the sea lions face. If the sea lions are going to be saved, then it looks like the nutritious fish, such as herring and capelin, need to grow in ratio compared to the pollock, where the sea lions live. To save the sea lion population in the Gulf of Alaska we would have to limit or stop the fishing of the nutritional fish. This is not likely to happen since this is a livelihood for several cultures and is an entire industry. People can't see an importance of having sea lions. We still feel it may not be just this factor but a combination of some, many, or all the factors mentioned, but it appears that this is the most crucial factor.
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Video: 1998. The Steller Sea Lions in Jeopardy. Co-produced by Marine Advisory Program and the Alaska Department of Fish and Game. Created by Deborah Mercy, Marine Advisory Program.