The Utility of Hydroacoustic and Radio Telemetry Monitoring to Determine Tributary Adfluvial Migration and Thermal Refugia Use by Native Salmonids in Boundary Dam Reservoir

As part of the Boundary Dam FERC relicensing process, radio and hydroacoustic telemetry monitoring was conducted within the Boundary Reservoir to gain insight into tributary use by native salmonids. Specific questions of interest were fish use in relation to seasonal adfluvial migration, as thermal refugia, and the influence of reservoir fluctuations.  This presentation will focus on challenges with methodologies and the results. Fish were implanted with either a radio transmitter or with a large or small combined dual acoustic and radio transmitter (CART). The CART had temperature and pressure sensors that were used to monitor fish in deep water habitats as well as within thermal refugia, while the radio transmitters allowed rapid detection of fish in shallow water habitats and fast flowing sections of the reservoir. Tagged fish were located during bi-weekly mobile tracking surveys, and from 10 shore-based radio telemetry stations. Three of the shore-based stations had dual antennas and were deployed at the mouth of each of the three main reservoir tributaries to detect adfluvial migration. In addition in 2007, three twenty-four hour thermal refugia monitoring sessions were instituted to assess detailed fish use. In 2008 continuous monitoring stations at 3 sites monitored use of thermal refugia. In total, ten westslope cutthroat trout and 30 mountain whitefish in the reservoir were surgically implanted with telemetry tags over both years. Five westslope cutthroat trout and eleven mountain whitefish provided the majority of fish movement data due to substantial tag loss during the study. During the two year study, there was no evidence of substantive adfluvial migration; tributary use was limited to the confluence area. When reservoir temperature exceeded 22°C, both the radio and hydroacoustic tracking indicate a strong fidelity of native salmonids to tributary mouths.  These fish used shallow water habitat in the daily varial zone of the reservoir, during which time fish typically maintained a body temperature 2 to 6°C below ambient reservoir temperature. Fish depth and position in these thermal refugia changed with reservoir fluctuations, and likely was influenced by other behaviours such as by diel movement, predation avoidance, feeding, and territoriality.