New Study Challenges Assumptions About Killer Whales’ Decline

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

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

Mysterious decline in killer whales
Previous studies have shown that the southern resident killer whales of the Pacific are declining, with both adult and offspring numbers dropping dramatically.

“Two populations of fish-eating killer whales (Orcinus orca) frequent the coastal waters of British Columbia where they preferentially consume Chinook salmon (Oncorhynchus tshawytscha). One of these populations, northern resident killer whales, ranges from southern British Columbia to Southeast Alaska – and has tripled to over 300 individuals since monitoring began in the early 1970s,” wrote the researchers.

“In contrast, the southern resident killer whale population, which inhabits the waters between southern British Columbia and California, has not experienced any sustained growth during this time. They have fluctuated between 66 and 98 individuals, and numbered 74 as of December 2023.”

Killer whales struggling in the Pacific
During the warmer months, southern resident killer whales swim north, and, as winter arrives, they head south again. However, they are currently struggling to survive.

Strangely, they seem to be the only marine mammal species in their ecosystem facing such problems. Other species such as harbor seals, porpoises, sea lions, and different types of whales are all thriving.

Some experts suggested that the most likely problem is a lack of access to salmon, the orcas’ main source of food. However, fishermen also looking for salmon have reported no problems capturing them and have also seen southern resident whales doing the same.

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

The role of intense sea traffic
According to the researchers, this indicates that the orcas are having trouble catching them. And that, they suggest, could be caused by the noise made by boats, particularly since the region where they live has far more sea traffic than areas farther north where northern resident orcas are thriving.

“Studies of southern resident killer whales carrying suction cupped sound and movement tags have found the probability of killer whales capturing prey increases as salmon abundance increases, but is negatively affected by the movements and noise of nearby vessel,” the authors explained.

“Vessel noise can mask communication between pod members and interfere with foraging and navigation – while the physical presence of vessels can reduce the foraging effort of resident killer whales.”

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

Seasonal salmon availability
Another cause could be that the southern resident orcas have trouble eating at different times of the year. For instance, while they may find enough salmon during summers, the situation may be completely different in the spring. This could explain why one calf was found to have starved to death this spring.

“Contrary to expectations, we found that the areal densities of prey (large salmon) available to killer whales were higher in the southern habitat. Our findings suggest that southern resident killer whales are no more food limited during late summer than northern resident killer whales,” noted the study authors.

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

The study is published in the journal PLoS ONE.