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

Abstract

Pacific salmon ( Oncorhynchus spp.) populations transfer large quantities of nutrients from their marine habitats to their freshwater habitats, but the management implications of this nutrient import mechanism are not clear. I investigate whether the effects of these nutrient imports on salmon productivity can be detected, how well management strategies can perform if they ignore the effects of marine-derived nutrients when productivity is strongly dependent on nutrient levels, and under what circumstances nutrient-depleted stocks can recover. I find that stock-recruitment data do not usually permit the detection of the effects of nutrient imports. Some management strategies that ignore nutrient-related 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. While the importance of nutrient effects on a salmon stock may be difficult to ascertain, managers can employ harvest strategies that are robust to this possibility.