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Fish diet a factor in harbor seal declines

Four new harbor seals arrived at the Alaska SeaLife Center late last June and now are making their public debut. The seals -- Atuun ("song"), Qilak ("cloud"), Susitna ("sleeping lady"), and Miki ("small one") -- are more than pretty faces. They are research animals vital to understanding the dilemma of the seal decline off Alaska's shores.

The number of harbor seals has declined in several areas of the Gulf of Alaska and Prince William Sound since the mid 1970s. At Tugidak Island near Kodiak, numbers have declined by nearly 85 percent from approximately 9,300 seals to some 1,000 by 1990. Areas such as Prince William Sound have experienced roughly a 63 percent decline since 1983. Alternatively, since the mid-1990s, the Kodiak area is showing signs of recovery. The reasons for declines and increases are unknown, but scientists at the Alaska SeaLife Center are working hard to find out.

A change in prey availability is proposed as a leading hypothesis in explaining the declines. This prey abundance change is linked to ecological warming in Alaskan waters that were first noted in winter of 1976-77 but may well have occurred periodically in the past. The shift seems to indicate the presence of fewer fatty fish such as herring, capelin and sandlance and a greater abundance of less fatty fish such as Pacific cod and pollock.

Researchers at the SeaLife Center hypothesize that fewer fatty fish could make a real difference at critical times during the life cycle of harbor seals, potentially resulting in poorer body condition during the winter, slower growth rates, delays in ovulation or reproduction, or reduced immune system function. Less fat in the diet could also affect harbor seal pup health and development by reducing or inhibiting the ability to stay warm, grow fast, or expend energy for sustaining themselves in the elements or for escaping predators.

“Oceanic regime shifts may be important in linking the increasing climate variability in ecosystems to how animals in coastal and oceanic environments are thriving. We are working to discover how regime shifts in climate can change the availability of certain types of fish available to marine mammals,” says Dr. Shannon Atkinson, science director at the Alaska SeaLife Center.

While human-induced climate change is a factor often considered when discussing wide ranging declines in Alaskan wildlife, the Pacific Decadal Oscillation is considered by many climate researchers to be a recurring natural phenomenon that alters storm patterns that affect the flow of nutrients and circulation in near shore environments. Climate is not static in Alaska. According to data from quality controlled weather monitoring stations summarized at the Alaska Climate Research Center website (, the average annual temperature for 20 Alaska stations located throughout the state has risen 2.69 degrees Fahrenheit from 1971 to 2000.

Named in indigenous languages of Alaska, the harbor seals will help researchers understand the diet and nutritional needs of Alaska’s harbor seals throughout their life cycle. Measurements will evaluate the effects of diet on health, growth, and reproduction. Specific measurements will include growth rates, blood chemistry, blubber accumulation, metabolic rates, and rates of sexual development. High technology methods such as 3D photogammetry will document three-dimensional growth; stable isotopes will be used to provide very specific measures of body condition and development, such as blubber or lean muscle mass.

“This study will allow us to mimic changes in diet typical of harbor seals in the outside world and see if the physiology of the animal can cope with ecological changes that have been observed in the Gulf of Alaska, “ says Anne Hoover-Miller, harbor seal program manager. “Observing animals in the wild only gives us a superficial snapshot of the environmental conditions that affects seals,"says Hoover-Miller. “Monitoring harbor seals in the wild setting does not allow us the same freedom to have a closer look at subtle physiological changes,” she adds.

Harbor seals are well adapted to life in the sea. Harbor seals are the only true seal that typically swim within minutes of birth. While most dives are less than fifty meters and under five minutes, harbor seals are capable of diving to depths exceeding 600 feet and can remain submerged for more than 20 minutes. Oxygen-conserving adaptations in harbor seals allow such dives. These adaptations include reduced peripheral circulation, a reduced heart rate, and high levels of myoglobin in the seal’s blood, which improves efficiency of oxygen use in muscles. Harbor seals are efficient swimmers that use their hind flippers for propulsion and foreflippers as rudders. Movement on land is slower and more laborious due to the fact that harbor seals lack opposable feet. On average, harbor seals in the wild only have about a 50 percent chance of surviving the first year of life and 20 percent chance of survival to adulthood. 

[Read more at Alaska SeaLife Center]

Alaska SeaLife Center | Posted 11.24.04 at 7:00 am

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