- 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.