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Study: Harbor Seals Target Salmon Juveniles Of Conservation Concern In Salish Sea
Posted on Friday, July 07, 2017 (PST)

Harbor seals eat both adult and juvenile salmon, but the adults they target in the autumn are from healthier stocks of fish (of less conservation concern) than the juveniles they target in the spring, according to a recent study of prey preferred by harbor seals in the Straits of Georgia in British Columbia.

 

Harbor seals tend to eat adult chum and pink salmon during the fall season, but they target larger juvenile chinook, coho and sockeye salmon, even though there is a higher abundance of chum salmon available in the spring. Those species, according to the study, are of greater conservation concern.

 

Harbor seals are abundant, year-round predators in the Salish Sea, that have high energetic needs for fish resources with a potential that can significantly impact salmon species that either reside or migrate through the inland marine waters of the Pacific Northwest.

 

With about 40,000 of the seals in the Straits of Georgia, the impact on juveniles is huge. Harbor seals consume about 2 kilograms of fish per day (4.4 pounds) and if those are all juvenile salmon, the total impact for coho salmon alone amounts to 5.7 million in one month (assumes the average hatchery coho smolt weighs 20 grams (0.044 pounds) and a seal diet is 4.8 percent coho smolts). This may be “driving regional survival patterns of Chinook and coho salmon,” the study concludes.

 

“While most people suspect that seals devour adult salmon of concern (like that seal that stole the prize chinook off your fishing line), we found that harbor seals mainly eat adult chum and pink salmon in the Strait – species that are doing pretty well overall,” said researcher Austen Thomas PhD, research scientist with Smith-Root, Inc. “However, when we looked at the juvenile salmon they were eating, seals appeared to consume more chinook, coho and sockeye salmon than they did juvenile pink and chum.

 

“This, combined with the fact that most of the juvenile salmon available to seals are chum, suggests that harbor seals are selective of the juvenile salmon they choose to consume – preferring species that are of conservation concern.”

 

Researchers drew these conclusions by developing two molecular methods of determining a harbor seal’s diet that use advancements in DNA sequencing technology to quantify the amounts of both juvenile and adult salmon that the seals eat, according to Thomas. Both utilize scat samples.

 

They sampled four sites, collecting 1,258 samples. Three of the sites were in river estuaries near the Fraser River on mainland British Columbia and on Vancouver Island at Comox and at Cowichan Bay. The fourth site was near the Gulf Islands.

 

“Some estimates generated from these data suggest that seals may consume up to 60 percent of all juvenile chinook/coho that reach saltwater (both hatchery and wild) within the first four months after ocean entry,” Thomas said.

 

He said that study is in progress (https://soundcloud.com/cbcvictoria/how-many-salmon-are-the-seals-eating).

 

The study, “Harbour seals target juvenile salmon of conservation concern,” was published in the Canadian Journal of Fisheries and Aquatic Sciences (http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2015-0558#.WVp9e4jyvIU).

 

Thomas’ co-authors are Benjamin Nelson, PhD student, University of British Columbia; Monique Lance, seabird and marine mammal research biologist, Washington Department of Fish and Wildlife; Bruce Deagle PhD, molecular ecologist, Australian Antarctic Division; and Andrew Trites PhD, professor, University of British Columbia.

 

There is some evidence that the preference seals have for juvenile chinook, coho and sockeye is happening in other regions of the Northwest, for example in south Puget Sound where a recent study of harbor seal predation came to the same conclusion.

 

See CBB, June 23, 2017, “Puget Sound Study: Pinniped Predation On Juvenile Salmon Making Salmon Recovery More Difficult,” http://www.cbbulletin.com/439144.aspx

 

“Therefore, it is reasonable to assume that seals in the Columbia River may also have preferences for juveniles of certain salmon species,” Thomas said. “Our fish management strategies often involve intensive supplementation of salmon resources that result in abundant, energy-rich juvenile salmon that strongly resemble the ideal harbor seal prey (forage fishes like herring and sardine). So, we should not find it surprising when seals and other marine mammals take advantage of those resources.”

 

Although the study doesn’t address potential management actions, Thomas said that predator removal is likely just “a temporary solution, if a solution at all.”

 

“While it is tempting to conclude that a simple removal of the predators will result in increased survival of these salmon populations, that neglects the possibility that the ultimate cause of mortality could be something other than the predator, i.e. the ‘dead fish swimming hypothesis,’” Thomas said. “The thought is, even if you removed one predator from the equation, those fish would either be gobbled up by another predator or die of other causes because they simply weren’t fit to survive.”

 

He said that seals are opportunistic predators that eat salmon only when their preferred food is not abundant. That preferred food is forage fish, such as the energy rich northern anchovy.

 

“There is strong preliminary evidence in Puget Sound that when an alternative prey item is available in high abundance, survival of juvenile steelhead is much greater in the Salish Sea,” Thomas said. “The concurrent diet data we have for seals in Puget Sound also supports this idea.

 

“I think the best solution is to put significant additional effort and resources into resorting the food web and the forage fish/gadoids that likely reduced the predation pressure on salmon populations in the past,” he concluded.

 

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