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For seals, fewer herrings means greater risk

Where are the harbor seals going? The harbor seal population in Prince William Sound decreased roughly 63 percent between 1984 and 1997. The seals are still losing ground, and only a small portion of the decline can be attributed to the 1989 Exxon Valdez oil spill. Researchers this winter and summer braved the seas to discover why these animals are disappearing so rapidly.

With $172,886 from the North Pacific Research Board, biologists from the Simon Fraser University and the Alaska Department of Fish and Game are working to see if the lower availability of fatty fish (like herring) has increased the harbor seal’s chance of encountering predators like the killer whale and the sleeper shark.

Since 1984, the abundance of Pacific herring, an important resource used by seals, also decreased as a result of a combination of factors that included the oil spill, over-fishing, and long-term oceanographic cycles. A computer simulation model developed by Alejandro Frid, Greg Baker and Larry Dill at Simon Fraser University predicts that with decreased herring abundance, seals can’t afford to spend as much time at rocky haulouts, where there is no food.  Instead, seals must spend more time in the water hunting and risking encounters with killer whales and sleeper sharks.

“Our computer simulations predict that seals are killed by predators more often when food is scarce”, states Frid, a doctoral student and lead author of the work.  “Put simply, less food may lead to a drop in fat stores essential for survival and reproduction.  Seals get stuck between a rock and a hard place.  According to the model if the seals are too cautious they will either starve or be unable to reproduce, so instead they must work harder to make a living.  This increase in foraging effort is predicted to raise the risk of predation from sleeper sharks and killer whales beyond that experienced when herring or other resources are more available.”

Frid elaborates that data collected by Richard Thorne, a collaborator from the Prince William Sound Science Center, indicates walleye pollock numbers have been relatively stable and abundant during the herring decline.  “Pollock remained an alternative food to herring, but only in a limited sense because there may be a predation risk attached.  Pollock are found much deeper than herring, at depths where seals are more likely to encounter sleeper sharks.  Diving deeper means diving longer, and that means spending more time at the surface recovering oxygen between dives.  It is during those surface intervals that seals might be most vulnerable to attack from killer whales.  Relying on pollock is probably not a first choice if there is food closer to the surface.” Shallow foraging though has been unreliable for the last fifteen years due to the herring decline.  “Theoretically, the best alternative for a hungry seal concerned with increasing energy stores for reproduction, is to dive deeper and risk greater danger from sharks and killer whales.  But the risk may be well worth it for seals lucky enough to not become someone else’s meal.  Our model predicts that, except when fish are very scarce, seals that evade predators reach the reproductive season in good body condition and are successful in producing pups.  The decline in Prince William Sound might reflect moderate food availability and high predation risk, possibly influenced by recent increases in sleeper sharks.”

In the field Frid collaborates with Gail Blundell from the Alaska Department of Fish and Game.  Their team gathers data on the seal’s diet, use of haulouts, diving behavior, foraging success, and the seal’s body condition. This work complements work being done by the Alaska Sealife Center (go to for more information).  Richard Thorne also plays a key role in measuring fish abundance.  The data shows that some seals dive mostly between 150 to 300 meters, depths where both pollock and sleeper sharks are likely to be found.  Other seals, however, dive between 20 and 50 meters, which are depths where herring can be found in large schools, but only sporadically.

Why would some seals risk death from predation more than others?  Frid intends to find out. The final analysis will use the field data to test whether individuals diving deeper and taking greater risks, have lower fat reserves than those feeding in shallower depths.  “The computer model predicts such individual differences.  Finding similar patterns in nature would be quite important for conservation and management of the ecosystem” states Frid. It would make us think harder about conditions in which over-fishing or other human influences on fish populations might indirectly increase predation rates on harbor seals or other declining populations of marine mammals in the Gulf of Alaska.”

The North Pacific Research Board supports research on fisheries and marine ecosystems in the North Pacific, Bering Sea and Arctic Ocean.

Learn more about the NPRB mission to develop a comprehensive science plan for the North Pacific at their Web site.

[Read more at North Pacific Research Board]

| Posted 10.27.04 at 12:13 am

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