Regional Differences in Temperature Dependent Growth of Steelhead with Implications for Climate Change

Climate change will likely impact Pacific anadromous fishes already suffering from population declines. In order to understand these effects we not only need informed models of how climate change will interact with a variable landscape, but we also need empirically derived biological responses such as temperature dependent growth. We currently manage Pacific salmonids in distinct population segments yet poorly understand how these distinct populations vary in specific parameters. This is of particular concern for species like steelhead (Oncorhynchus mykiss) of which most Pacific populations in the lower 48 United States are threatened or endangered. We would expect both local and regional variability in growth response to temperature in steelhead, which express great variability in other traits such as life history. Yet laboratory results testing this hypothesis are lacking. I tested steelhead growth response to different temperatures using a controlled laboratory experiment. To test both local and regional variation I used two populations from coastal central, CA, and one from California’s central valley. I then compared my results to published results from Oregon and a more southern central valley tributary. I tested steelhead growth at 14, 20 and 24^oC water temperatures to test the hypothesis of higher relative growth optimums at warmer southern regions. Finally, I altered the sequence of temperatures experienced by individual fish to examine how temperature dependent growth is affected by previous temperatures experienced. Temperature dependent growth varied both among and within regions. I found evidence for higher relative growth optimums in more southern populations. Temperatures previously experienced had no bearing on immediate temperature dependent growth implying that individual fish will have optimal growth at optimal temperatures even after periods of low growth potentially near lethal limits. This pattern was general among populations used in my experiment. Strong patterns of temperature dependent growth highlight the need to conduct more experiments with finer temperature resolution to show the full relationship between temperature and growth. Furthermore, the strong population specificity across regional and local scales identifies the need for such research on a population specific basis. Results highlight the need for population specific management approaches when considering dam and water management, habitat criteria, and efforts to restore endangered populations of steelhead. These results emphasize the importance of considering both among and within basin variability in how populations will respond to climate change.