Why are killer whales fighting for survival in the Pacific?

Scientists have recently argued that lack of access to salmon is the main reason for the population decline of southern resident killer whales – also known as orcas – in the Pacific. However, a new study has found that such assumptions may actually be wrong.

To reach these conclusions, Burak Saygili and Andrew Trites, two marine mammal scientists from the University of British Columbia, consulted recreational anglers and whale-watching teams to better understand orcas’ access to Chinook salmon.

Mysterious decline of killer whales

Previous studies have shown that the number of killer whales living in the southern Pacific is declining, with both the number of adults and offspring declining dramatically.

“Two populations of fish-eating killer whales (Orcinus orca) are commonly found in the coastal waters of British Columbia, where they prefer to feed on Chinook salmon (Oncorhynchus tshawytscha). One of these populations, northern resident killer whales, occurs from southern British Columbia to southeastern Alaska – and has tripled to over 300 individuals since monitoring began in the early 1970s,” the researchers write.

“In contrast, the southern resident killer whale population, which lives in the waters between southern British Columbia and California, has not experienced sustained growth during this time. Their number varied between 66 and 98 people and amounted to 74 in December 2023.”

Killer whales fight in the Pacific

During the warmer months, southern resident killer whales swim north, and when winter comes they move south again. However, they are currently fighting for survival.

Strangely, they appear to be the only marine mammal species in their ecosystem to face such problems. Other species such as seals, porpoises, sea lions and various species of whales also thrive.

Some experts suggested the most likely problem was a lack of access to salmon, the orcas’ main food source. However, fishermen who were also looking for salmon reported that there were no problems with the catch, and they also saw whales that live to the south doing the same.

In addition, whale-watching teams told scientists that they often see the endangered orcas swimming among large groups of salmon.

The role of intensive maritime transport

According to the researchers, this suggests that the orcas are having difficulty catching them. They suspect this could be caused by the noise created by boats, especially since the region where they live has much more maritime traffic than areas further north where northern resident orcas thrive.

“Research on southern resident killer whales carrying sound and motion tags with suction cups has found that the likelihood of killer whales catching prey increases as salmon abundance increases, but is negatively affected by the movement and noise of nearby vessels,” they explained Authors.

“Ship noise can obscure communication between group members and impair foraging and navigation – while the physical presence of ships can reduce foraging for resident killer whales.”

According to the researchers, killer whales are more likely to encounter larger numbers of vessels in the Salish Sea than in North Island waters, which could mean that salmon are less accessible to southern residents than northern residents, even though there are there is a higher abundance of Chinook.

Seasonal salmon availability

Another reason could be that the orcas living in the south have problems eating at different times of the year. For example, while they find enough salmon in summer, the situation can be completely different in spring. This may explain why a calf was found to have starved to death this spring.

“Contrary to expectations, we found that the areal density of prey (large salmon) available to killer whales was higher in the southern habitat. “Our results suggest that southern resident killer whales are no more nutritionally restricted in late summer than northern resident killer whales,” the study authors said.

“This implies that the difference in the growth rates of the two populations is either due to other factors (e.g. inbreeding, disturbance, pollutants, competition) or that the southern resident killer whales are experiencing food shortages outside the Salish Sea during the winter or spring suffer.” ”

The study is published in the journal PLUS ONE.

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