Non–indigenous brook trout and the demise of Pacific salmon: a forgotten threat?

• Published in: Proceedings of the Royal Society. B, Biological sciences Biological sciences, 2002-08, Vol.269 (1501), p.1663-1670
• Main Authors: Levin, Phillip S., Achord, Stephen, Feist, Blake E., Zabel, Richard W.
• Format: Article
• Language: English
• Subjects: Anadromous fishes; Animals; Chinook Salmon; Conservation of Natural Resources; Creeks; Ecosystem; Endangered Species; Freshwater; Freshwater fishes; Geography; Habitat Quality; Marine fishes; Non-Indigenous Species; Oncorhynchus tshawytscha; Population Dynamics; Population Growth Rate; River basins; Salmon; Salmon - physiology; Salvelinus fontinalis; Streams; Survival Rate; Trout; Trout - physiology; Wildlife habitats; Young animals
• ISSN: 0962-8452; 1471-2954
• DOI: 10.1098/rspb.2002.2063

Non-indigenous species may be the most severe environmental threat the world now faces. Fishes, in particular, have been intentionally introduced worldwide and have commonly caused the local extinction of native fish. Despite their importance, the impact of introduced fishes on threatened populations of Pacific salmon has never been systemically examined. Here, we take advantage of several unique datasets from the Columbia River Basin to address the impact of non-indigenous brook trout, Salvelinus fontinalis, on threatened spring/summer-run chinook salmon, Oncorhynchus tshawytscha. More than 41 000 juvenile chinook were individually marked, and their survival in streams without brook trout was nearly double the survival in streams with brook trout. Furthermore, when brook trout were absent, habitat quality was positively associated with chinook survival, but when brook trout were present no relationship between chinook survival and habitat quality was evident. The difference in juvenile chinook survival between sites with, and without, brook trout would increase population growth rate (λ) by ca. 2.5%. This increase in λ would be sufficient to reverse the negative population growth observed in many chinook populations. Because many of the populations we investigated occur in wilderness areas, their habitat has been considered pristine; however, our results emphasize that non-indigenous species are present and may have a dramatic impact, even in remote regions that otherwise appear pristine.