Comparisons of fish assemblages and habitat associations in littoral zones of reservoirs in the Willamette Basin, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/765374581

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  • The littoral zones of seven reservoirs in the southern Willamette Valley of Oregon were sampled with a boat electroshocker during the summer months of 1995 and 1996. Shoreline substrates were inventoried before sampling sites were randomly selected. Sampling sites consisted of 9 of 12 possible habitat types including four substrate types (bedrock, fines, gravel to small boulders, and large boulders), each with or without overlying vegetation or wood. Fish assemblages occupying these specific habitat types were compared. Various aspects of substrate distribution and dimensions were utilized to help explain differences in fish assemblages. Significant differences in richness, diversity, mean length, mean weight, and catch per unit effort (CPUE) of fish assemblages occupying the specific habitat types were found. Fish assemblages using vegetated substrates were greatest in richness, diversity, and CPUE, while bedrock and unvegetated fines were lowest. Gravel to small boulder substrates tended to be intermediate for these variables. Large boulder substrates were consistently on the higher end of the range of these values. The largest and oldest fish occupied habitats with wood and large boulder substrates; smaller, younger individuals tended to be found in fine substrates with vegetation. Largemouth bass, bluegill, and coarsescale suckers were well distributed across habitat types. Other species such as the yellow bullhead, white crappie, reticulate sculpin, longnose dace, and speckled dace were much more restricted in their use of habitat. Overlying structure tended to increase fish diversity and richness for fine but not for coarse size substrates. Comparison of fish species between reservoirs, using regression analysis, was also performed. Fish sampled in Lookout Point Reservoir showed the overall greatest weight for length values, whereas, fish from Hills Creek and Green Peter Reservoirs were lowest in these values of the seven study reservoirs. Regression analysis of specified substrate parameters to fish species richness and diversity indicated bedrock was generally a negative influence. The occurence of gravel to small boulder substrate was a positive influence over the range of segment sizes encountered. Large expanses of fines resulted in depressed richness and diversity. Conversely, smaller segments of fine habitat appear to have a strong positive effect on fish richness and diversity. Large boulders were found in too small a quantity to impact reservoir littoral zone fish assemblages. The exotic/native ratio of fish abundance averaged about 3/1 in all habitats sampled except fine substrates without overlying structure. Only here were native fish more abundant than exotics.
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