For nearly 20 years, Robin Baird has been following killer whales, trying to figure out what they eat. At first, he would look to see what was in their mouths as the whales feasted on fish near the surface. But then he and his colleague, Brad Hanson, started looking for more subtle clues in their flukeprints — the orbs of placid water that appear on the surface when a killer whale dives into the sea, or flicks its tail underwater. In or around these flukeprints, you can find fish scales and fecal matter, which provide a treasure trove of information.
“If they’re catching things near the surface of the water, it’s much [easier] to be able to document than if they’re capturing things at depth,” Baird, a research biologist at Cascadia Research Collective, told Mongabay in an interview. “What the fecal samples do is they show … a higher proportion of things like lingcod and halibut and sole flounder [that] are caught deeper in the water column and less likely to be brought up to the surface.”
A new study published March 3 in PLOS ONE provides a comprehensive look at the diet of the endangered southern resident killer whales (Orcinus orca), or orcas, that inhabit the Pacific coast between the U.S. and Canada, as well as the inland waterways of Washington state, during the spring, summer and fall months.
By analyzing 150 prey and fecal samples collected between 2004 and 2017, the researchers found that endangered Chinook salmon (Oncorhynchus tshawytscha) was always the prey of choice, constituting nearly 100% of their diet in the spring, and about 70% to 80% of their diet in mid-winter and early spring. But when Chinook salmon was scarce, particularly in the fall when orcas only ate this prey about 50% of the time, they had to supplement their diet with other fish such as chum salmon (Oncorhynchus keta), coho salmon (Oncorhynchus kisutch), steelhead trout (Oncorhynchus mykiss), and also non-salmonid species like lingcod (Ophiodon elongatus) and Pacific halibut (Hippoglossus stenolepis).
“Whether or not you interpret that as a good thing, in the sense that, ‘Okay if there aren’t enough Chinook around, they’re going to be able to get by other types of prey,’ or if you view it as an indicator of, ‘Well, we need to do more to make Chinook available to them, year round,’ I think it is a bit up in the air,” said Baird, who was a co-author of the study.
Chinook salmon is one of the fattiest, calorie-rich prey foods available to resident killer whales, and the consumption of these fish has long been linked to the survivability of orcas. Yet many Chinook salmon populations are threatened with extinction due to threats like overfishing and the construction of dams, which obstruct their ability to spawn.
For instance, 12 of the 13 Chinook salmon populations in the Fraser River, a waterway that snakes through British Columbia, are considered to be at risk because of habitat destruction, overfishing, and also warming waters due to climate change. But in the lower Columbia River between Oregon and Washington, Chinook salmon populations have moderately recovered thanks to spawning and reintroduction efforts. However, these populations are still considered “threatened.”
“If returns to the Fraser River are in trouble, and Columbia River returns are strong, then prey availability to the whales potentially balances out as the whales have evolved to move rapidly throughout their range,” Hanson, lead author of the study and a wildlife biologist at NOAA, said in a statement. “But if most of the stocks throughout their range are reduced then this could spell trouble for the whales.”
The southern resident killer whale population has been declining over the years; right now, it’s estimated there are only 74 individuals in three pods. By contrast, the northern resident killer whales have been steadily increasing, and have a current population of about 300 individuals. This discrepancy could be due to the northern resident killer whales having more access to mature Chinook salmon, the study suggests.
Hatcheries for Chinook salmon provide an important source of prey for southern resident killer whales, according to the study. But these programs should be carefully managed so that the prey is available at all times of the year, especially when there are shortages in the orcas’ diet, the researchers say.
“We don’t need more cookie-cutter fish that all come back during the time when Chinook are most abundant; we need to diversify and increase availability at other times of the year,” Baird said in a statement.
Emmalai Page, a marine campaigner at the Canadian NGO Pacific Wild, who was not involved in this study, said the research confirmed what conservationists already suspected about orcas’ diets: that they supplemented with other fish when Chinook salmon wasn’t available. However, she said it was interesting to see the seasonal changes in their diet.
“We have known that SRKWs [southern resident killer whales] were most likely not getting enough Chinook salmon, so it is reassuring to know that there is supplementation of other species occurring,” Page told Mongabay in an email.
She also said this study, as well as others like it, can help inform conservation efforts to preserve salmon populations, including Chinook salmon stocks, for resident killer whales.
“It can often be difficult to align the messaging of conservation organizations, academia and government,” Page said. “Having a scientific study concluding what many have observed for years is validating in itself. Going forward, more collaboration is needed between conservation organizations, First Nations, management organizations and academia. No single group has all the resources to make all the changes necessary.”
Hanson, M. B., Emmons, C. K., Ford, M. J., Everett, M., Parsons, K., Park, L. K., … Barre, L. (2021). Endangered predators and endangered prey: Seasonal diet of southern resident killer whales. PLOS ONE, 16(3). doi:10.1371/journal.pone.0247031
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