Many tributary streams of Lake Ontario have become important spawning habitats for introduced Pacific salmon. We examine how phosphorus released through salmon decomposition and other environmental factors potentially limit primary productivity in two Lake Ontario tributaries in New York State. Contribution of phosphorus (measured as total phosphorus) from salmon carcasses, computed as the percentage of the estimated phosphorus discharged from the stream, was very low (< 1%∙yr−1), but of modest importance (> 50%∙d−1) during restricted periods in the spring. Experimental results from three stream sites demonstrated that phosphorus, which is present naturally at high concentrations, is not limiting primary productivity during the period that the salmon are in the streams. Finally, results from an analysis of 15 free-water oxygen studies indicate that the streams are productive (gross primary production 0.9–10.2 g O2∙m−2∙d−1) and that light appears to limit stream primary productivity more than nutrients. We conclude that salmon migrations are unlikely to substantially increase the rate of primary productivity in these already fertile streams.