Subadult bull trout out-migration in the Thompson River drainage, Montana

Abstract

Bull Trout populations in the Thompson River drainage have declined over the past century. Declines have been attributed to habitat fragmentation, habitat degradation, and non-native species. Out-migration characteristics (e.g., temporal and spatial origins, abiotic cues, and movement) of subadult Bull Trout (100 - 300 mm TL) were evaluated throughout the drainage to increase our understanding of local populations and better inform conservation efforts. In autumn 2014, 53 subadult Bull Trout were tagged with passive integrated transponder (PIT) tags; 29 were also surgically implanted with acoustic transmitters. Minimal Bull Trout out-migration (N = 7) was observed in 2014. In summer 2015, 566 subadult Bull Trout were PIT-tagged in the Fishtrap Creek and West Fork Thompson River drainages (Thompson River tributaries). Stream-width PIT antennas were used to monitor out-migration at the confluences of the Thompson River tributaries and at the mouth of the Thompson River. Out-migrating Bull Trout (N = 135) were sampled using directional weir traps at the tributary confluences, PIT-tagged, and implanted with acoustic- (N = 29) or radio-tags (N = 14) in autumn 2015. From July through December 2015, 10.1% of all PIT-tagged Bull Trout out-migrated from the Thompson River tributaries (11.4% of fish in the Fishtrap Creek drainage [N = 420] and 6.2% of fish in West Fork Thompson River [N = 146]), with peak out-migration occurring in late October. Highest predicted probabilities of Bull Trout out-migration occurred at lengths of 179 mm in Fishtrap Creek (30.4%) and 165 mm in West Fork Thompson River (29.3%). Only 13.5% of all Bull Trout that entered the Thompson River (N = 192) entered Thompson Falls Reservoir, with peak out-migration occurring in December. Median daily water temperature, minimum daily atmospheric pressure, and lunar illumination were weakly associated with an increase in the number of out-migrants. Radio-tagged out-migrants were randomly distributed throughout the Thompson River and exhibited long periods of site fidelity between intermittent downstream movements. Bull Trout demonstrated low out-migration rates in the Thompson River drainage and prolonged habitation of the mainstem Thompson River, which was contrary to the a priori hypothesis of clustered out-migration by subadult Bull Trout.