In GLOBEC investigations, causality is often inferred from observed covariability between environmental indicators and populations, but mechanisms of action (e.g., an effect on individual growth rate or survival at a certain age) are seldom known, though they are frequently hypothesized. The population dynamic effects of the mechanism of action are seldom elucidated, and investigators are often not aware of the population dynamic differences between variability at different ages or between variability in survival or growth. However, research in population dynamics is increasing awareness of the differences these make in terms of sensitivity of populations to the environment and the time scales of variability of the environmental forcing and the response. Salmon and cod are two taxa that have been of interest to GLOBEC and they span the Pacific and the Atlantic Oceans in the Northern Hemisphere. Their populations vary spatially in development rates and the consequent distribution of spawning ages, and they experience inter-annual temporal variability in both survival at various ages and development rates (and spawning age distributions). The investigators will examine the role of the differences that population dynamics makes in structuring the different responses of various salmon and cod populations to environmental variability and climate change. Specifically, they will describe how the mechanism of action (variable growth rate or survival rate at age) influence population sensitivity to environmental fluctuations at various time scales, including expected time scales of population response. Examples of similar studies include out elucidation of the differences in population responses of coho and chinook salmon to the regime shift in the mid-1970s due to differences in spawning age distributions. Discovering that the expected differences were slight re-focused attention on other potential causes of the differences in response. Another example is identification of the causes of cohort resonance in cod and the drawing of attention to the fact that increasing resonance (sensitivity to specific time scales of environmental variability) also led to increasing sensitivity to variability at very low frequencies such as might be seen in climate change. Concern was expressed that this heightened sensitivity to random noise could interfere with attempts to detect slow climate change.
A societal benefit will be derived from this investigation of how the addition of fishing mortality rate changes the basic response of populations to environmentally induced variability in development rates and survival rates at various ages. This will aid in the risk analysis associated with fishery management. Also, description of the expected scales of variability to which populations will be sensitive will aid in the design and analysis of ocean observing systems. From a human resources point of view, this project will be train one student and two postdoctoral scholars.
