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Lack of prey is changing the behavior of killer whales

The Salish Sea surrounding the San Juan Islands has traditionally been a hotspot for Southern resident killer whales, which would spend the summers feeding on Chinook salmon belonging to the Fraser River stock that passes through the islands on its way to spawning grounds upriver. However, based on 17 years of whale sighting data, a team of scientists led by Oregon State University (OSU) has found that, as salmon populations dropped, the time spent by Southern residents around the San Juan Islands decreased by more than 75 percent. 

Since the whales’ main summer feeding grounds are becoming less productive and reliable, they have to search for prey elsewhere, raising concerns about the health of an already threatened species.

“This is an endangered population that is in decline with only 73 whales remaining, and prey limitation appears to be an important factor. A huge part of these whales’ time used to be spent feeding in this area,” said study lead author Joshua Stewart, an assistant professor of Quantitative Ecology at OSU.

The Southern resident killer whale population consists of three matriarchal pods – known as J, K, and L – which have traditionally been spotted in the Salish Sea between April and October. While the K and L ponds usually covered a wider geographic range, especially in winter and spring, the J pod was frequently seen in the Salish Sea to feed on returning salmon.

This iconic whale species has declined since 1995 and is currently officially listed as endangered. Previous studies have argued that the possible drivers of their decline include limited availability of their preferred prey, vessel disturbance in the Salish Sea, and high levels of pollutants in their core habitat.

As Chinook salmon populations were becoming less abundant around the San Juan Islands during summer, the number of days each pod of Southern residents was present in the area declined each year from 2004 to 2017. While the J pod was present in the region more frequently – with a high of 164 days in 2005 and a low of 36 days in 2017 – the L pod was spotted there for just 10 days in 2019, compared to a high of 103 days in 2004.

By comparing the whales’ presence with data on returning Chinook salmon, the experts identified a strong correlation between the whales’ presence and the salmon returns. “They went from spending the majority of their time in this habitat, to just a fraction of their summer. This shift is likely an effort to find alternative food sources,” Stewart explained.

According to co-author John Durban, a population ecologist at OSU, this lack of salmon is already impacting whales’ health. “This further suggests that lack of prey is likely the biggest stressor for these whales. The other stressors – pollutants and vessel disturbances – are compounded by the lack of prey. If there are fewer fish, disturbance by vessels may become more disruptive to their foraging success,” he concluded.

The study is published in the journal Marine Mammal Science.

By Andrei Ionescu, Staff Writer

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