Bioenergetic Ontogeny: Linking Climate and Mass-Specific Feeding to Life-Cycle Growth and Survival of Salmon
David A. Beauchamp
Abstract.—Size-selective mortality is a dominant variable regulating the dynamics of salmon populations. Body size, growth rate, and energy state during one life stage influence survival during that and subsequent life stages. Therefore, simultaneously examining allometric processes, foraging, and thermal constraints on growth within and among life stages can provide a powerful analytical framework for identifying critical periods and sizes during the life cycle of salmon, and for understanding the processes that contribute to the specific ecological bottlenecks confronting different species or stocks of salmon. A bioenergetics model was used to simulate generalized growth responses to a factorial combination of body size, daily feeding rate, and prey energy density over a continuous range of temperatures (0–24oC). The results of these simulations indicated that: 1) smaller salmon benefit from higher potential scope for growth or activity than larger salmon, based on the different allometric relationships for maximum consumption, metabolism, and waste; 2) optimal temperatures for growth decline with increasing body size; 3) optimal temperatures for growth also decline as daily rations decline; 4) thermal tolerances (temperature thresholds beyond which weight loss will occur) also shift to cooler temperatures for larger salmon and when ration sizes decline; 5) increasing the composite energy density of the diet can increase both optimal growth temperature, and thermal tolerance, especially at larger body sizes; 6) after spawners enter freshwater, the amount of energy and days available to migrate, and successfully spawn at a given upstream location was very sensitive to ambient river temperature, and the swimming speed required to reach the spawning grounds. When placed in the context of climate variability, seasonal shifts in temperature, and food availability, these simulations suggest that growth will be more frequently limited by feeding rate (prey availability) and prey quality than by temperature, especially for smaller, younger life stages. Larger salmon should be more sensitive to temperature change, but reductions in optimal growth temperature and thermal tolerance would be magnified for all life stages, if either feeding rate or prey quality were to be reduced. Given intense size-selective mortality during one or more early life stages, this simulation framework could be adopted to identify the key factors limiting growth to critical sizes during critical periods in the life cycle of specific salmon stocks.