Preliminary evaluation of the use of nitrogen stable isotope ratios to establish escapement levels for pacific salmon

Research over the last decade has established the ecological significance o f the nutrients and organic matter deposited by Pacific salmon in the fresh water habitats where they spawn. A large proportion of the nitrogen in plan ts and animals in streams where salmon are abundant may be derived from spa wning fish, and juvenile salmonids exhibit higher growth rates at locations where carcasses are available. Currently, no method is available to establ ish salmon escapement goals that meet the nutritional needs of streams. We examined the relationship between the abundance of spawning coho salmon (On corhynchus kisutch) and the nitrogen stable isotope ratio of coho salmon pa rr to determine whether a saturation level for salmon-derived nitrogen coul d be identified. Coho parr were collected from 26 sites in western Washingt on in late winter. The isotope ratio in the coho parr was related to the ab undance of salmon spawning at that site the previous autumn. The amount of carcass-derived nitrogen increased with increasing abundance of carcass tis sue up to 0.15 kg of carcass/ m(2) of streambed area but exhibited no incre ase above this level. These preliminary data suggest that relationships bet ween stable isotope values and carcass abundance may provide a useful suppl ement to traditional methods of establishing escapement goals for Pacific s almon.