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

Abstract

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.

Identifying drivers of demographic rates in an at‐risk population of marine mammals using integrated population models

“…When NRKW abundance is tied to density-dependence in the SRKW population, it is estimated that the 2020 population size is at 82% of carrying capacity. However, if SRKW population dynamics are not regulated by competition (or another mechanism), as suggested by a recent field study (Sato et al 2021) from NRKWs, the population is estimated to be at only 58% carrying capacity. This discrepancy may be important to acknowledge in future discussions of recovery targets, and more research on interactions between the two populations would be valuable.…”