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.
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.
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.
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.
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.
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.”
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.
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.
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.
said that study is in progress (https://soundcloud.com/cbcvictoria/how-many-salmon-are-the-seals-eating).
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).
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
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
CBB, June 23, 2017, “Puget Sound Study: Pinniped Predation On Juvenile Salmon
Making Salmon Recovery More Difficult,” http://www.cbbulletin.com/439144.aspx
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
the study doesn’t address potential management actions, Thomas said that
predator removal is likely just “a temporary solution, if a solution at all.”
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.”
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.
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.
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.