Preliminary Evaluation of the Use of Nitrogen Stable Isotope Ratios to Establish Escapement Levels for Pacific Salmon

Abstract

Research over the last decade has established the ecological significance of the nutrients and organic matter deposited by Pacific salmon in the freshwater habitats where they spawn. A large proportion of the nitrogen in plants and animals in streams where salmon are abundant may be derived from spawning fish, and juvenile salmonids exhibit higher growth rates at locations where carcasses are available. Currently, no method is available to establish salmon escapement goals that meet the nutritional needs of streams. We examined the relationship between the abundance of spawning coho salmon (Oncorhynchus kisutch) and the nitrogen stable isotope ratio of coho salmon parr to determine whether a saturation level for salmon-derived nitrogen could be identified. Coho parr were collected from 26 sites in western Washington in late winter. The isotope ratio in the coho parr was related to the abundance of salmon spawning at that site the previous autumn. The amount of carcass-derived nitrogen increased with increasing abundance of carcass tissue up to 0.15 kg of carcass/ m2 of streambed area but exhibited no increase above this level. These preliminary data suggest that relationships between stable isotope values and carcass abundance may provide a useful supplement to traditional methods of establishing escapement goals for Pacific salmon.