Temperature effects on juvenile anadromous salmonids in California's central valley: what don't we know?

摘要

The anadromous Chinook salmon (Oncorhynchus tshawytscha) (4 runs) and steelhead (rainbow trout, O. mykiss), are both native to California's Sacramento-San Joaquin River (SSJR) system, whose watershed encompasses the central valley of California. The SSJR system holds the southernmost extant Chinook salmon populations in the Eastern Pacific Ocean, whereas coastal anadromous steelhead populations are found at more southerly latitudes. Populations of both species of anadromous salmonid have experienced dramatic declines during the past 100 years, at least partly from water impoundments and diversions on most central valley rivers and their tributaries. These changes restricted the longitudinal distribution of these salmonids, often forcing the superimposition of steelhead populations and Chinook salmon populations in the same reaches. This superimposition is problematic in part because the alterations to the river systems have not only changed the historic flow regimes, but have also changed the thermal regimes, resulting in thermally-coupled changes in fish development, growth, health, distribution, and survival. Given the highly regulated nature of the system, resource managers are constantly trying to strike a balance between maintaining or increasing the population size of anadromous fish runs and with other demands for the water, such as irrigation and water quality. To do so, in this review, we summarize the published information on the temperature tolerance and growth of the stream-associated life stages of these two valuable species, which are so central to the natural heritage of the State and its cultures. We show that many of these limits and growth-related effects are specific regarding life stage and that some may be specific to distinct strains or races of Chinook salmon and steelhead within the system. Because the number of published studies on the physiology of central valley salmonids was surprisingly low, we also use this review to highlight critical areas where further research is needed. Overall, this review should assist biologists and resource decision-makers with improved understanding for the protection and enhancement of these native fishes.