Southern resident killer whales encounter higher prey densities than northern resident killer whales during summer

The decline of southern resident killer whales (Orcinus orca) may be due to a shortage of prey, but there is little data to test this hypothesis. We compared the availability of prey (Chinook salmon, Oncorhynchus tshawytscha) sought by southern residents in Juan de Fuca Strait during summer with the abundance and distribution of Chinook available to the much larger and growing population of northern resident killer whales feeding in Johnstone Strait. We used ship-based multifrequency echosounders to identify differences in prey fields that may explain the dynamics of these two killer whale populations. Contrary to expectations, we found that both killer whale habitats had patchy distributions of prey that did not differ in their frequencies of occurrence, nor in the size compositions of individual fish. However, the density of fish within each patch was 4–6 times higher in the southern resident killer whale habitat. These findings do not support the hypothesis that southern resident killer whales are experiencing a prey shortage in the Salish Sea during summer and suggest a combination of other factors is affecting overall foraging success...

...Northern and southern resident killer whales have a strong mutual reliance on Chinook salmon, but do not appear to have been equally affected by range-wide declines of Chinook salmon. The population of northern resident killer whales (which ranges from Southeast Alaska to southern British Columbia) has increased to ∼300 individuals (Ward et al. 2009; Towers 2015), while southern resident killer whales (which range from southern British Columbia to California) numbered 75 individuals as of February 2021 (Orca Network 2021).

https://cdnsciencepub.com/doi/full/10.1139/cjfas-2020-0445
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Chinook numbers in the Salish Sea have been increasing in the past couple of years but the number of Southern Resident Orcas have declined. I really think the increased amount of ship traffic in the Strait of Juan de Fuca and along the Pacific coast is impacting their ability to hunt. There are now 74 Southern Residents, but the last I heard, the calf born this year is in bad shape and it isn't known if it will survive.