Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

esume migration in early spring to natal streams, and then spawn (Meehan and Bjornn
1991; Nickelsen, Nicholas et al. 1992) in January and February (Barnhart 1986). Winter
steelhead enter freshwater between November and April in the Pacific Northwest
(Nickelsen, Nicholas et al. 1992; Busby, Wainwright et al. 1996), migrate to spawning
areas, and then spawn, generally in April and May (Barnhart 1986). Some adults,
however, do not enter some coastal streams until spring, just before spawning (Meehan
and Bjornn 1991).
There is a high degree of overlap in spawn timing between populations regardless of run
type (Busby, Wainwright et al. 1996). Difficult field conditions at that time of year and
the remoteness of spawning grounds contribute to the relative lack of specific information
on steelhead spawning. Unlike Pacific salmon, steelhead are iteroparous, or capable of
spawning more than once before death (Busby, Wainwright et al. 1996), although
steelhead rarely spawn more than twice before dying; most that do so are females
(Nickelsen, Nicholas et al. 1992). Iteroparity is more common among southern steelhead
populations than northern populations (Busby, Wainwright et al. 1996).
After two to three weeks, in late spring, and following yolk sac absorption, alevins
emerge from the gravel and begin actively feeding. After emerging from the gravel, fry
usually inhabit shallow water along banks of perennial streams. Fry occupy stream
margins (Nickelsen, Nicholas et al. 1992). Summer rearing takes place primarily in the
faster parts of pools, although young-of-the-year are abundant in glides and riffles.
Winter rearing occurs more uniformly at lower densities across a wide range of fast and
slow habitat types. Some older juveniles move downstream to rear in larger tributaries
and mainstem rivers (Nickelsen, Nicholas et al. 1992).
Juvenile steelhead migrate little during their first summer and occupy a range of habitats
featuring moderate to high water velocity and variable depths (Bisson, Sullivan et al.
1988). Juvenile steelhead feed on a wide variety of aquatic and terrestrial insects
(Chapman and Bjornn 1969), and older juveniles sometimes prey on emerging fry.
Steelhead hold territories close to the substratum where flows are lower and sometimes
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