Salmonid foraging below dams

In regulated rivers, temperature and flow from dam releases can have a profound effect on how salmon and steelhead feed.  Take a look at some diet analysis performed on the Mokelumne River, California.



A young steelhead feeding on an ant                  Gastric lavage performed on an adult steelhead
In this figure stomach fullness of juvenile Chinook salmon correlates well with mean river temperature over a two year period.
Here we see that the amount of zooplankton (primarily Daphnia p.) in the diets of juvenile Chinook salmon correlates well with reservoir release. 
These two relationships are important because reservoir release has a strong relationship with the water temperature downstream and the amount of zooplankton available to juvenile Chinook salmon. 

Trout and salmon in streams typically feed more on drifting larvae and pupae of aquatic insects than on insects dwelling on substrates.  Terrestrial insects and other aquatic invertebrates can also be quite important.
We observed no strong correlation between the length of steelhead and the size of their prey items.

However, the steelhead length correlates with the number of prey items ingested.  Note that 1999 was a significantly cooler water year.

Therefore, the need to increase prey item consumption as a steelhead grows can greatly influence where it is located in the stream.  This is further compounded by water temperature.