Most work on climate change and trophodynamics has focused on temporal match and mismatches between prey and predators, but it is more complicated than just “quantity.” The HAMC program is working to understand how the nutritional content of walleye pollock, Pacific ocean perch, and Pacific herring diets varies with climatic conditions and how those differences subsequently influence production. Up until now we have tended to think that prey abundance drove production. However, NOAA’s work under the auspices of the Bering Sea Integrated Ecosystem Project has demonstrated that the modes by which climate affects fish production may be more nuanced than previously appreciated.
Typically the assumption is that climate alters the phenology of prey production and, if fish larvae are feeding when prey are scarce, the larvae fail to survive. However, another sort of mismatch occurs when the prey available to fish do not match their nutritional needs. Just because humans can eat celery does not mean that a diet of celery will meet our nutritional needs. Similarly, there is growing evidence that climate influences on prey quality can have demonstrable effects on fish production. For example, warm and cold conditions in the Bering Sea result in differences in the prey field available to juvenile walleye pollock. These differences result in variations in amount of lipid in the diets of young-of-the-year pollock. In turn, reductions in the rate of lipid ingestion have a direct effect on their survival, as measured by the number of age-1 recruits per female spawner. This latter relationship is likely related to resource depletion in winter.
Similar effects may occur with more specific nutritional components. Much of the aquaculture literature from the 1980s and 1990s aimed at identifying essential macronutrients that fish larvae cannot synthesize de novo. Among these were essential fatty acids. The availability of many of these fatty acids varies in the Strait of Georgia in response to the relative abundance of diatoms and dinoflagellates in the spring bloom. Certain climatic conditions can produce a phytoplankton bloom that does not supply sufficient levels of nutrients to upper trophic levels. This may have important implications for larval groundfish such as Pacific cod whose growth rates and survival can be maximized by offering them diets replete with specific fatty acids. Thiamine deficiencies offer yet another example of how nutrient limitations can affect fish production. Populations of adult salmonids with thiamine deficiencies can experience complete year-class failure.