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Posted January 11, 2000

The Effect of Seals on Maine Atlantic Salmon

By, James R. Gilbert,, Department of Wildlife Ecology

Seal populations in Maine waters have tripled in the past two decades. Seals do eat Atlantic salmon as well as herring, cod, hake, and other species. However, no information is available as to what, if any, impact seal predation has on wild salmon populations. Seals have been implicated in damage to salmon aquaculture pens and escape of cultured salmon into the wild. Despite these observations, there is little factual information on the roles seals play in the decline of wild salmon in Maine.

Three species of seals are abundant in the Gulf of Maine . . .

The three species of seals are commonly found in the Gulf of Maine are the harbor seal, the gray seal, and the harp seal. Of these, the most common is the harbor seal. The population estimate has increased from 10,500 in 1981 to approximately 30,900 being counted in Maine in 1997. The harbor seal is a year-round resident that has its pup on coastal ledges in May and early June. In winter, some portion of the population moves offshore into the Gulf of Maine, a portion moves south to Cape Cod, and a portion remains in coastal waters of Maine. The harbor seal matures at age 3, and has one pup a year. In the wild, many seals live into their mid teens. The gray seal has been an infrequent summer resident of Maine for many years, but more recently has established pupping sites on a few islands in Penobscot Bay. Gray seals have a single pup in mid January. Most of the gray seals in Maine are born in Canada, especially Sable Island, Nova Scotia. Gray seal populations on Sable Island have increased over the last 35 years, with over 20,000 pups being born in a recent season. The harp seal is a winter visitor to the Gulf of Maine. The harp seal has a single pup in early April on the sea ice. The harp seal population in Eastern Canadian waters recovered to 5 million animals in 1996, and has been at that level since. There has been a kill quota in Canada of 275,000. The Greenland population of harp seals has supported a take of some 75,000 over the last several years. Harp seals do enter the Gulf of Maine in winter. The number of harp seals caught in fishermen's gill nets equals the number of harbor seals caught in gill nets.

Seal damage at aquaculture pens . . .

All seals in the Gulf of Maine are protected under the Marine Mammal Protection Act of 1972, last amended in 1994. In the 1994 amendments, lethal take at aquaculture sites was prohibited. In lieu of that permission, a "Maine Aquaculture-Pinniped Interaction Task Force" was constituted to look at alternatives to lethal take of seals. According to testimony to this task force, farm salmon farmers implicated seals in 50% of the insurance loss claims. Some of these losses were escapes caused by seals making a hole in the net, and in others, salmon have been eaten through the nets. The Task force could not determine the extent of losses caused by seals when they submitted their report in 1996, and there is no information about the extent of losses due to seals since 1996. The industry has responded by double netting pens or pen sites, and using sound generators to keep seals away from pen sites. Most of the damage and escapes caused by seals has been in February and March. We do not know if the harbor seals at the pen sites are "rogue" individuals, or if many different seals are involved.

Seals eat a variety of fish species . . .

Harbor seals eat a variety of fish, generally those that are schooling species or bottom species. The most recent food habits study for seals along the coast was at Grand Manan in 1992. Of 470 seal stomachs collected, 250 contained prey remains. Atlantic herring, Atlantic cod, squid, winter flounder, alewife, and hake were the most common food items. In the off-shore Gulf of Maine, hake were the most common food item found in 75 harbor seals. Atlantic salmon were not found in either study. Harbor seals do enter rivers to feed if sufficient food fish are available. While there have been no rigorous studies of incidence of take of Atlantic salmon by seals, there is anecdotal information of salmon returning with scrapes, etc. from seals. Between 5 and 25% of the salmon returning up the Columbia River in Oregon and Washington have marks from seals. In Scotland, the predation rate on Atlantic salmon by seals was not related to the number of seals.

The Pacific Salmon issue: directions taken. . .

In California, Oregon, and Washington, the 26 runs of 7 salmonid species have been listed as endangered. The causes of the declines in these runs (stocks) were a combination of over-fishing, hydroelectric dams, and other stream habitat degradation. The National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine, reviewed this Pacific Salmon issue in 1996. Among the Natural Research Council's conclusions was: "Salmon management should be based on the premise that local reproductive populations are genetically different from each other and valuable to the long-term production of salmon." In addition, the National Marine Fisheries Service recommended to Congress (1999) that the authority for intentional legal taking of California sea lions and Pacific harbor seals be selectively reinstated to protect gear and catch and to reduce impacts of seal predation on endangered salmonid stocks. The states and the Federal Government are cooperating in the management of the stocks.

Conclusion . . .

There is little factual scientific information on the role of harbor seals in the Atlantic salmon controversy. There is much anecdotal information and opinion. One can speculate, but no one has information to implicate or not implicate seals as a major factor in the decline of Atlantic salmon.

For further information please feel free to contact:

James R. Gilbert, Department of Wildlife Ecology, 207-581-2866, gilbert@umenfa.maine.edu

Dr. Gilbert's research addresses the population dynamics of marine mammals, especially seals. He has studied seal populations in Antarctica, Alaska, Russia, and Maine. He has been active with gray and harbor seal research in Maine since 1979. The U.S. Marine Mammal Commission, the National Marine Fisheries Service, the U.S. Fish and Wildlife Service, the U.S. Minerals Management Service, and the Maine Agricultural and Forest Experiment Station, University of Maine have supported Dr. Gilbert's research.

If you would like to receive addition copies of this white paper or any other papers in the series, please contact:

Judith Round, College of Natural Sciences, Forestry, and Agriculture, University of Maine, 5782 Winslow Hall, Orono, Maine 04469-5782

Telephone: (207) 581-3229, E-Mail: judy.round@umit.maine.edu

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