Distribution of Fish and Crayfish and Measurement of Available Habitat in the Tualatin Basin Inside the Urban Growth Boundary 1999-2001

Oregon Department of Fish and Wildlife (ODFW) conducted fish, habitat, and water quality surveys in sixteen tributaries of the lower Tualatin River as part of an effort to assess the biotic health of the watershed. We surveyed lower, middle, and upper reaches of most streams. Habitat surveys were conducted during the summers of 1999 and 2000, whereas fish and water quality surveys were conducted in summer, fall, winter and spring, 1999-2001. 

This project is a follow up to similar work conducted by the ODFW from 1993-95. Industrial and residential developments, as well as efforts to restore water quality and riparian habitat have continued since the 1993-95 surveys; therefore, periodic monitoring is needed to ensure that important habitat and existing populations of native fish are protected. Compared with the 1993-95 surveys the number of species collected decreased from 25 to 24 in the sixteen streams. The number of native species decreased from 13 to 11, whereas the number of introduced species increased from 12 to 13. The native reticulate sculpin Cottus perplexus remains the most abundant and widely distributed species throughout the basin. Introduced species contributed 18.3% of the total catch. Native and introduced species tolerant of habitat degradation accounted for 17.7% of the total catch, whereas native species sensitive to habitat degradation accounted for only 3.4% of the catch. Native and introduced species moderately or very tolerant of habitat degradation made up 96.6% of the total catch. 

Using the fish assemblage data collected, we calculated seasonal index of biotic integrity (IBI) scores for each stream reach sampled. Biotic integrity scores were used to evaluate the need for restoration and enhancement within Tualatin tributaries. When summer IBI scores were compared with those from the previous ODFW study, we found that 29 of 34 reach scores had increased. Of 154 biotic integrity scores calculated, none were considered acceptable, fifteen were marginally impaired, and the remaining scores were considered severely impaired. Habitat measurements changed little from previous surveys. Glides were the most common habitat type, and soil was the most common substrate. Of the streams surveyed, very little woody debris was found.