Effects of intraspecific density and environmental variables on electrofishing catchability of brown and rainbow trout in the Colorado river

We investigated electrofishing catchability (q) for brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss in the Colorado River, Grand Canyon National Park, Arizona, over a range of fish densities, water temperatures, turbidities, conductivities, shoreline types, and seasons. The covariance of q with rainbow trout density strongly resembled random distributions, thereby suggesting no relationship between q and rainbow trout density. The catchability of rainbow trout was greater in turbid water (greater than or equal to480 nephelometric turbidity units [NTU]) than in clear water (less than or equal to10 NTU), although lower water temperature may have contributed to this effect. The catchability of rainbow trout was greatest over sand-silt shorelines. The catchability of brown trout increased sharply to levels above those predicted from random chance up to about 0.025 fish/m(2) and then assumed an asymptotic or declining relationship with intraspecific fish density. In contrast to the situation with rainbow trout, the catchability of brown trout was higher over rocky shorelines (cobbles, boulders, and bedrock) than sand-silt shorelines, suggesting that the variability of q in relation to shoreline type is species specific. We hypothesize that the catchability of rainbow trout is influenced more by environmental variables than by density. We also hypothesize that brown trout catchability varies with density because a greater proportion of fish occur in shallow, nearshore areas (where electrofishing is most effective) when fish density is high. This effect is enhanced by high catchability over rocky substrates. Our findings emphasize the need to understand the biological and environmental factors affecting electrofishing catchability, especially in monitoring programs that rely on catch-per-unit-effort data to accurately represent fish population status and trends.