Abstract:
Seasonal emergent wetlands in the Pacific Northwest have not been regarded
traditionally as fish rearing habitat, despite access to such habitat when river flows
overtop riverbanks and connect to the floodplain. As a result, restoration and
enhancement projects to remediate for the loss of such wetland habitat are being
implemented for waterfowl and other wildlife with little consideration for fishes such as
juvenile salmon (Oncorhynchus spp.). The objectives of this study were to examine the
degree to which fish utilize emergent wetlands and to determine the influence of wetland
enhancement on fish communities in the Chehalis River floodplain. Furthermore, I
quantified the influence of enhanced wetlands on juvenile coho salmon. A minimum of
18 fish species utilized floodplain wetlands, and the most abundant were three-spine
stickleback (Gasterosteus aculeatus) and Olympic mudminnow (Novumbra hubbsi).
Both enhanced and unenhanced emergent wetlands had higher abundances of nongame
native fishes than oxbow habitats. Coho salmon (Oncorhynchus kisutch) was the
dominant salmonid at all sites and enhanced wetlands had significantly higher
abundances of yearling coho salmon than unenhanced wetlands. Dissolved oxygen
concentrations decreased in emergent wetlands throughout the season and were near
lethal limits for juvenile salmon by June each year. Survival of fishes utilizing emergent
wetlands was dependent on movement to the river before water quality decreased and/or
the wetland became isolated and stranding occurred. Emigration patterns suggested that
coho salmon yearling and young-of-the-year emigrated as habitat conditions declined.
This was further supported by the results of the experimental release of yearling coho
salmon. Yearling coho salmon benefited from rearing in enhanced wetland habitats
where their growth and survival were comparable to studies of juvenile coho off-channel
habitats.
