Management implications of the effects of marine-derived nutrients on salmon population dynamics

Pacific salmon populations transfer large quantities of nutrients from their marine to freshwater habitats, but the management implications of this nutrient import mechanism aren’t clear. I investigate whether nutrient effects on salmon productivity can be detected, how well management strategies that ignore nutrient effects perform given strong nutrient dependence, and under what circumstances nutrient-depleted stocks can recover. I find that stock-recruitment data don’t usually permit detection of nutrient effects. Some management strategies that ignore nutrient effects will, as they are updated, converge to policies that are similar to those that are optimal given the nutrient effects. Constant harvest rate policies will approach optimal harvesting faster than constant escapement policies. Nutrient dependence can lead to multiple stable states for a population, so that a population subject to one harvest rate can be either maintained at high abundance or, if at low abundance and nutrient-depleted, driven to extirpation.