Killer Whales Going Hungry

Jacob Lerner

The population of Canada’s southern resident killer whales in the Salish Sea off British Columbia has been declining for over 30 years. Sightings of thin killer whales have led researchers to assign much of the blame for their decline on a shortage of the killer whales’ preferred prey — Chinook salmon. But the issue is not so black and white.

Southern resident killer whales are critically endangered due to a medley of issues such as noise pollution, contaminants and low genetic diversity. Yet, their decline has been most closely linked with a concurrent decline in the abundance of their Chinook salmon prey.

However, killer whales prey on many distinct populations or stocks of Chinook salmon, not all of which are in decline. And this discrepancy begs the question of whether all Chinook salmon are equally nutritious as prey. Previous research on killer whale energetics — studies that estimate how many calories whales burn and how much food they need — had operated under the assumption that all Chinook were created equal.

Chinook salmon are critical prey for the southern resident killer whales due to their large size, year-round availability and high fat (lipid) content. Chinook salmon accumulate the highest lipid levels of any Pacific salmon species. This high lipid content leads to a high energy density and makes the salmon a high-quality food source.

However, not all Chinook salmon are the same. Their life history varies from fish to fish. Some enter freshwater in the spring, migrate hundreds of kilometres and climb hundreds of meters in altitude to reach their spawning grounds, while others spawn close to sea level in the fall, less than 50 kilometres from the sea.

The aquaculture industry commonly uses a hand-held fat metre device to measure lipid content in fish fillets. We calibrated one of these fat meters to provide a more ecologically relevant measure of whole-body lipid content.

Chinook salmon populations are classified into three categories: a low, medium and high lipid group. The differences between these categories were primarily driven by the distance and elevation of spawning grounds from the ocean.

Data showed that spring Chinook have nearly 70 per cent more lipid and 30 per cent more calories than fall Chinook. This means that, to meet their energy needs, the southern resident killer whale population would need to eat 325,000 fall chinook salmon over the year if they only ate this category of salmon. If they only ate high-lipid spring salmon, they would require 245,000 salmon to meet their energy needs, a difference of 80,000 Chinook.

Like these whales, the energy-rich spring Chinook are themselves endangered. As these Chinook populations have dropped, the killer whales have steadily been losing access to some of their richest prey sources.

The lipid content in Chinook salmon has major implications for both the killer whales that rely on the calories it offers and the salmon that need these energy reserves to return to freshwater and spawn.

Understanding the variation in the lipid content of different salmon, using tools like the fat metre, to assess the quality of the salmon can help improve management of both the southern resident and Chinook populations.

Fish hatchery production or enhancement measures could consider the energy density of salmon to ensure that resource managers are supporting high-quality Chinook and not flooding the environment with low-energy stocks.

It is crucial to understand how changing ocean conditions may impact salmon lipid accumulation, as this will affect both the Chinook salmon and the killer whales that depend on them.

- The Conversation