FISH ENTRAINMENT INVESTIGATIONS AT THE FORT SHAW DIVERSION 2003-2004, SUN RIVER, MONTANA

Abstract

Fish entrainment associated with irrigation has had long-term consequences for many species in the western United States. Investigation of entrainment and attempts at its prevention through physical or behavioral barrier installation are common management practices.  However, the factors affecting entrainment are rarely quantified. The purpose of this study was to quantify fish entrained from the Sun River mainstem into the Fort Shaw diversion canal by species using an entrainment monitoring system; and second, to analyze the factors that contributed to this fish entrainment. These factors included: year, photoperiod, Sun River flow, Fort Shaw Canal diverted flow, Fort Shaw lag flow, ratio of diverted flow to Sun River flow, temperature, and moon phase. There were 6536 fish captured in 2003 and 2004 comprising 13 species and unknowns. We found significant differences in entrainment rates associated with Sun River flow, ratio of diverted flow to Sun River flow, and photoperiod. Our data indicated that at this site, fish entrainment is directly related to volume of water diverted, especially when diverted flows exceed mainstem river flows, and that fish entrainment at this site was highest during dark periods. The fish entrainment netting system used at the Fort Shaw Diversion was considered to be both economical and effective for documenting occurrence and determining the composition and number of fish entrained.