Anadromous salmonid fry are particularly vulnerable at emergence and the initiation of feeding because the fish leave the secure, low-energy environment in the interstices of streambed gravels and enter the high-energy environment of the river. Many researchers believe that in an ideal natural environment, the general behavioral tendency after emergence is for salmonids to select very quiet shallow water over a variety of substrates. Rearing salmonids require a constant food supply from “drift” organisms, principally composed of a wide variety of terrestrial and aquatic
insects. Instream habitat complexity is extremely important for fry and juvenile salmonids. Habitat structure provided by instream structure such as large woody debris (e.g., fallen trees and rootwads) and large rocks or boulders provide young salmonids areas to rear and protection from predatory fish.
- Habitat Quantity and Quality – The instream habitat attributes of complexity and
diversity previously available upstream of large dams cannot be re-created in the large
river channels downstream of dams through flow alone and cannot serve as a surrogate
for the lost habitats upstream of the dams. Certain increases in reservoir releases can
substantially reduce the number and areas of low water velocity which could displace fry
from optimal to suboptimal habitats and induce premature downstream movement.