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.
Return
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.