EVIDENCE OF DIFFERENTIAL DETECTABILITY OF LOTIC ADULT RAINBOW AND BROWN TROUT - IMPLICATIONS FOR HABITAT USE INFERENCE FROM DIRECT-UNDERWATER OBSERVATION

The number of observations of brown trout Salmo trutta was significant ly lower than expected based on the ratio of estimated densities of br own trout and rainbow trout Oncorhynchus mykiss and the number of tagg ed rainbow trout and brown trout in a Sierra Nevada river. We believe, that behavioral differences as well as abundant cover composed of lar ge substrate in the study area were major factors contributing to the lack of brown trout observations relative to observations of rainbow t rout. These results should be considered by managers and researchers w ho collect habitat use data for brown trout with direct-underwater obs ervation because that data could be considerably biased.