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Fewer Salmon Means Fewer Orcas

As a key food supply declines, the endangered population of Southern Resident killer whales, known to frequent the Salish Sea off the coasts of Washington and British Columbia, is spending far less time in that region, a new study shows. The Salish Sea around the San Juan Islands has traditionally been a hotspot for the whales. The Southern Residents would spend the summer months feeding on Chinook salmon, much of which belonged to the Fraser River stock that passes through the islands on its way to spawning grounds upriver. But 17 years of whale sighting data shows that as the Fraser River Chinook salmon population dropped, the time spent by the Southern Residents around the San Juan Islands also declined ­– by more than 75%, said Joshua Stewart, an assistant professor with Oregon State University’s Marine Mammal Institute and the study’s lead author.

“This is an endangered population that is in decline with only 73 whales remaining, and prey limitation appears to be an important factor,” Stewart said. “A huge part of these whales’ time used to be spent feeding in this area.” This new study shows that as the whales’ primary summer feeding grounds are becoming less reliable and productive, they are having to search elsewhere for prey, raising further concerns about the health of the population. The Southern Resident killer whale population is comprised of three matriarchal pods – J, K and L – that have traditionally been seen in the Salish Sea region between April and October. The J pod is more frequently found in the Salish Sea throughout the year, while the K and L pods cover a wider geographic range, particularly in winter and spring.

“Research we are conducting on body condition using drones is revealing that the summer is an essential feeding period when the Southern Residents load up on returning salmon before the slimmer winter months,” said Fearnbach. The Southern Resident population has been in decline since 1995 and is listed as endangered under both the U.S. Endangered Species Act and the Canadian Species at Risk Act. Past research has shown three possible drivers of the whales’ decline: limited availability of their primary prey, Chinook salmon; vessel disturbance in the Salish Sea; and high levels of pollutants in their core habitat, Stewart said.

Understanding the Southern Residents’ foraging behavior is important for developing strategies to support the recovery of the species. Fraser River Chinook salmon are the largest and highest quality salmon in the Southern Residents’ foraging range. The fish help the whales build up blubber stores in the summer to get through the winter and early spring when prey are of lower quality and harder to find, Stewart said. In an effort to learn more about the links between the Southern Resident whales’ foraging behavior and the abundance of Fraser River Chinook salmon, Stewart analyzed nearly 20 years of whale sighting data compiled through reports from naturalists and researchers throughout the Salish Sea.

Comparing the whales’ presence with data on Chinook salmon returning to Fraser River tributaries for the same period of time showed a strong relationship between the whales’ presence and the salmon returns. In years of higher salmon returns, whales were present more often; when salmon returns were lower, the Southern Residents spent less time in the area. “They went from spending the majority of their time in this habitat, to just a fraction of their summer,” Stewart said. The Marine Mammal Institute is part of OSU’s College of Agricultural Sciences and based at Hatfield Marine Science Center in Newport.

Story by Michelle Klampe OSU HMSC Photo provided by OSU HMSC

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