The idea that fertilizing streams -- deliberately or inadvertently -- is beneficial needs a complete evaluation (HCN, 6/27/11). The stream section immediately below the outfall from a sewage treatment plant may be more productive, but that can contribute to low dissolved oxygen. This means that it is less suitable for spawning; developing eggs and fry are more likely to suffocate when the overlying water is low in oxygen or rich in small particulate organic matter, which can infiltrate the gravels and consume oxygen there.
Changing the nutrient status of a stream is likely to produce multiple changes in stream ecology. Increasing the year-round productivity is more likely to benefit resident species than seasonal visitors. That is, competitors to the juvenile salmon may benefit more from the addition of nutrients. This may also result in a change in the species present in the community, especially when non-natives are in the neighborhood. Higher productivities are likely to support larger individual predators and larger numbers of predators on juvenile salmon. A common explanation for the lifestyle of salmon is that the juveniles escape the predation pressure of estuaries and oceans by starting life in low-productivity water, which doesn't support abundant predators. Before we go about changing the productivity of natural streams, we need to understand all of the potential consequences. Diving into a stream without first determining if it is deep enough can result in a major headache.