S1 Appendix -

• Main Authors: Fanny Couture, Villy Christensen, Carl Walters
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2024
• Subjects: Ecology; Inorganic Chemistry; Science Policy; Environmental Sciences not elsewhere classified; Biological Sciences not elsewhere classified; northern british columbia; northeast pacific coast; model provided evidence; model also revealed; low levels since; orcinus orca </; oncorhynchus tshawytscha </; oncorhynchus kisutch </; salmon species targeted; pacific salmon populations; coho salmon (<; chinook salmon populations; chinook salmon (<; killer whale population; salmon population declines; main limiting factor; div >< p; >) smolts increased; multiple factors may; marine mammals could; fishing mortality since; 2020 mortality ranging; marine mammals; resident </; >); could; 2020 may; total mortality; mortality pattern; srkw population; study evaluated; stock productivity; relatively stable; previous research; ocean productivity; mainly driven; important large; important driver; food availability; fishing activities; ewe ); eating southern; combined effects; climatic patterns; 6 times; Pacific; Killer Whale; Orca; Orcinus orca; Killer whale
• Online Access: https://doi.org/10.1371/journal.pone.0296358.s001

Along the northeast Pacific coast, the salmon-eating southern resident killer whale population (SRKW, Orcinus orca ) have been at very low levels since the 1970s. Previous research have suggested that reduction in food availability, especially of Chinook salmon ( Oncorhynchus tshawytscha ), could be the main limiting factor for the SRKW population. Using the ecosystem modelling platform Ecopath with Ecosim (EwE), this study evaluated if the decline of the Pacific salmon populations between 1979 and 2020 may have been impacted by a combination of factors, including marine mammal predation, fishing activities, and climatic patterns. We found that the total mortality of most Chinook salmon populations has been relatively stable for all mature returning fish despite strong reduction in fishing mortality since the 1990s. This mortality pattern was mainly driven by pinnipeds, with increases in predation between 1979 and 2020 mortality ranging by factors of 1.8 to 8.5 across the different Chinook salmon population groups. The predation mortality on fall-run Chinook salmon smolts originating from the Salish Sea increased 4.6 times from 1979 to 2020, whereas the predation mortality on coho salmon ( Oncorhynchus kisutch ) smolts increased by a factor of 7.3. The model also revealed that the north Pacific gyre oscillation (NPGO) was the most important large-scale climatic index affecting the stock productivity of Chinook salmon populations from California to northern British Columbia. Overall, the model provided evidence that multiple factors may have affected Chinook salmon populations between 1979 and 2020, and suggested that predation mortality by marine mammals could be an important driver of salmon population declines during that time.