|dc.description.abstract||Streams with intact floodplain connections are important to juvenile salmonids during their freshwater residence, providing refuge during periods of high flow as well as prey produced in emergent marsh and terrestrial riparian habitats. Habitat restoration was undertaken in 2009-2010 on lower Hansen Creek, Washington, with the goal of recovering these important lower elevation freshwater floodplain functions. The project
converted 140 acres of isolated floodplain into 53 acres of alluvial fan and 87 acres of flow-through wetlands. To quantify the initial biological responses at the Hansen Creek alluvial fan restoration site and provide a baseline of data for future comparisons, we conducted invertebrate and fish sampling at the restored habitats. The study was conducted from September 2010 through September 2011 in three areas within the restoration site, and in one reference area outside the project area. We collected diets from juveniles of Coho salmon (Oncorhynchus kisutch) and Steelhead trout (O. mykiss), the two dominant salmonids in the creek. Salmonid abundances and diets were also sampled during periodic seasonal flooding that occurred on the restored floodplain. Insects were sampled with fallout traps once-monthly March through September, and neuston invertebrates were sampled once on the floodplain during an inundation event in April and once at the in-channel sites in June. Visual snorkel fish surveys were attempted January through May, and successfully completed June through August. Sampling to assess the Biological Index of Biotic Integrity (B-IBI) was completed in September 2010 and September 2011.
Diets of both Steelhead trout and Coho salmon consisted of aquatic drift, terrestrial insects, and benthic prey items. In general, higher instantaneous rations, a measure of feeding intensity, were recorded from diets of juvenile Coho salmon collected during periodic inundation events than during regular monthly sampling in the channels. Despite relatively warm water temperatures and decreased visibility due to high turbidity, the floodplain appeared to provide favorable feeding opportunities to salmonids, likely due to greater availability of drift and emergent insect prey. Terrestrial insect numbers peaked in July and August. The floodplain site had consistently higher insect abundances, and
always had significantly different fallout trap assemblages compared to the other sites. Neuston organisms collected in the main channel habitats were dominated by chironomid larvae while those collected during inundation consisted of other types of insects and planktonic organisms. The three reaches sampled for B-IBI all scored in the fair range in both 2010 and 2011, except for one reach, which scored in the good range in 2010.
One of the goals of this study was to evaluate methods for future sampling at the Hansen Creek restoration site. Insect fallout trap and B-IBI sampling are common techniques that will provide data that is comparable to other sites, and our results from the neuston nets indicate that they can provide information that the other methods do not. The methods we used for catching salmonids were arrived at after trying several techniques early in the study. High flows and turbidity during much of the year precluded visual collection methods, and a combination of dip nets with block-and-sweep net samplings was conducted. These methods were probably not completely effective at capturing larger more evasive salmonids or at quantifying salmonids in complex habitats. In the future, a better method would be multiple-pass depletion electrofishing paired with visual snorkel surveys when visibility allows, such as the Basin-wide Visual Estimation Technique.
Pole seining in the inundated areas was effective for obtaining juvenile salmonids for diets, but we were not able to generate densities using this method. Also, fishing with pole seines in the floodplain area will likely become more difficult as the vegetation community matures. In this case, additional fishing methods such as fyke nets or traps could be used.||en_US