Lipid Content of Fraser River Chinook Salmon Is Lipid-rich and Energy Dense, Which Is Important for Killer Whales’ Diet

Not all Chinook salmon are created equal, which has a significant impact on the energy levels of southern resident killer whales.

A recent study quantified the lipid content in Fraser River Chinook salmon, the preferred meal of southern residents, and discovered that spring-run Chinook salmon, the first to arrive in the Salish Sea, is lipid-rich and energy-dense, which is important for the killer whales that prey on them.

Later-season Fraser River Chinook salmon have a lower energy density.

Food quality matters for southern resident killer whales

(Photo : OLIVIER MORIN/AFP via Getty Images)

"This research helps us quantify the energetic requirements of the southern residents," said Jacob Lerner, lead author of the study and a doctoral student at the Institute for Oceans and Fisheries' Pelagic Ecosystems Lab, as per ScienceDaily.

For example, if southern residents only ate low-lipid salmon, they would have to consume approximately 80,000 more Chinook salmon per year than if they only ate high-lipid salmon.

Southern resident killer whales are a well-known species in British Columbia's Salish Sea and along the Pacific Northwest coast.

These marine mammals, which have black and white markings, can weigh up to 12,000 pounds and measure up to 26 feet in length.

They are fearsome, social creatures who hunt in family groups. Unfortunately, there are only 73 of them left in the world.

They are critically endangered due to a variety of anthropogenic factors, including noise pollution and high levels of water contaminants, and their decline is primarily due to the scarcity of their preferred prey, Chinook salmon.

However, there are numerous distinct populations of Chinook salmon available all year, some with stock-specific differences in energy density and not all of which are declining.

They started with the assumption that these salmon were all created equal, that they all have the same value to resident killer whales, and we quickly realized that this is not the case, Lerner explained.

Quantifying that lipid content is important because it directly relates to a salmon's caloric value, which assigns its value as prey.

Specific estimates of lipid content for Chinook populations with varying distributions, or run-timings, could be used to inform killer whale population trends, properly time fishery closures, or even decide which hatcheries to augment to increase high-quality food availability for southern residents, according to Lerner.

This is especially important because southern resident killer whales are migratory and often spend the winter elsewhere.

When they return to the Salish Sea in the spring and summer, they frequently coincide with the arrival of Fraser River Chinook salmon.

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Behavior and diet of killer whale

Killer whales are highly social creatures, and the majority of them live in social groups known as pods (groups of maternally related individuals seen together more than half the time), as per NOAA.

Individual whales tend to stick with their birth pods.

Pods are typically made up of a few to twenty or more animals, though larger groups may form for temporary social interactions, mating, or seasonal concentrations of prey.

Killer whales feed, communicate, and navigate using underwater sound.

Members of the pod communicate with one another using clicks, whistles, and pulsed calls.

Each pod in the eastern North Pacific has its own set of calls, which are learned and passed down through generations. These calls serve as family badges and maintain group cohesion.

Quantifying that lipid content is important because it directly relates to a salmon's caloric value, which assigns its value as prey.

Specific estimates of lipid content for Chinook populations with varying distributions, or run-timings, could be used to inform killer whale population trends, properly time fishery closures, or even decide which hatcheries to augment to increase high-quality food availability for southern residents, according to Lerner.

This is especially important because southern resident killer whales are migratory and often spend the winter elsewhere.

When they return to the Salish Sea in the spring and summer, they frequently coincide with the arrival of Fraser River Chinook salmon.

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