PARENT SESSION
Oral Session #36: Conservation of aquatic systems: Ecology and ecotoxicology. Presiding: R. Lathrop.
Tuesday, August 7, 2001. 1:00 PM to 5:15 PM. Hall of Ideas H.
Depensatory mechanisms in the ecology of sockeye salmon and their ecosystems.
Schindler, Daniel1, Lew, Warner2, Leavitt, Peter3, 1 2 3
ABSTRACT- Traditional approaches to management of Pacific salmon have typically used single-species stock-recruit models to set harvest goals. These models generally ignore negative feedbacks to population dynamics when stocks are at low densities (i.e. depensatory effects). We will present two lines of evidence to suggest that depensatory mechanisms are apparent in the ecology of sockeye salmon. In Iliamna Lake (AK), juvenile sockeye growth is density-dependent such that juveniles grow relatively fast and tend to migrate to the ocean as 1-year old smolts following years with low densities of adult spawners. Following large spawning events, juveniles grow relatively slowly and delay migration to the ocean until they are two years old. Paradoxically, marine survival rates of the fast-growing 1-year old smolts are substantially lower than for the slow-growing 2-year old smolts. In addition, because post-spawning mortality of salmon represents a major nutrient subsidy to nursery lakes and streams, decreases in salmon densities in response to fishing and climate may reduce the productivity of freshwater nursery ecosystems. Despite these two depensatory mechanisms that may reduce salmon productivity at low population sizes, data from paleolimnological studies and from long-term monitoring of Alaskan salmon fisheries suggest that sockeye populations are exceptionally resilient to environmental change and harvesting. We suggest that life-history flexibility may reinforce the resiliency of this species.
KEY WORDS: depensatory mechanisms, salmon, climate change, aquatic ecosystems