Southern resident killer whales are an iconic species living in British Columbia’s Salish Sea and the northeastern Pacific coast. Unfortunately, there are only 73 of these fierce, social creatures left in the world.
Now, a recent study led by the University of British Columbia (UBC) has found that, although these whales are critically endangered by many anthropogenic factors, such as water contaminants and noise pollution, their decline is mostly caused by the limited availability of their preferred prey, the Chinook salmon.
However, while scientists previously assumed that all types of Chinook salmon are equal in terms of their energy density, the UBC researchers discovered that, in fact, the spring-run Chinook salmon (the earliest to arrive to the Salish Sea) are much richer in lipids and energy density than those that arrive later in the autumn.
“This research helps us quantify the energetic requirements of the southern residents,” said study lead author Jacob Lerner, a doctoral student in Pelagic Ecosystems at UBC. “For example, if the southern residents ate just low-lipid salmon, they would have to eat around 80,000 more Chinook salmon every year than if they just ate high-lipid salmon.”
Quantifying the lipid content of various species of salmon is important since it directly relates to their caloric value. Thus, specific estimates of lipid content for different Chinook populations could be used to inform trends in killer whale populations, properly time fishing closures, and decide which hatcheries to augment in order to ensure high quality food availability for southern residents.
“We identified a spectrum of high, medium, and low-lipid Chinook populations from the Fraser that can be used to better inform energetics models and manage both species,” Lerner explained. “We also identified life history parameters for the salmon to predict where on this spectrum they may fall.”
These findings are particularly important since southern resident killer whales are a migratory species that often spend their winter months elsewhere and return to the Salish Sea for the spring and summer.
“Southern resident killer whales used to come here earlier in the spring season when they could eat early migrating Chinook salmon,” said study senior author Brian Hunt, an associate professor of Oceanography at UBC.
“Those early Chinook were very energy dense as they need to fuel their long freshwater migration back to their spawning grounds, but those stocks have been declining. With the whales coming later, they mainly have access to Chinook from the lower Fraser. These fish don’t migrate very far, and have lower energy density.”
Further research is needed to clarify how ocean conditions influence energy accumulation in Chinook salmon.
“We plan to keep monitoring Fraser Chinook salmon fat content. And one of questions we want to answer is how changing ocean conditions might be affecting their energy accumulation. Our concern is that ocean warming and food web shifts in the North Pacific Ocean are leading to lower energy accumulation in Chinook salmon. This will have implications for both the Chinook themselves – will they have enough energy for return migration and spawning? – and the killer whales that depend on them,” Hunt concluded.
The study is published in the journal Nature Scientific Reports.
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