Acoustic Imaging Observes Predator–Prey Interactions between Bull Trout and Migrating Sockeye Salmon Smolts
Matthew L.H. Cheng1,2, Scott G. Hinch3, Francis Juanes4, Stephen J. Healy3, Andrew G. Lotto3, Sydney J. Mapley1, and Nathan B. Furey1
1Department of Biological Sciences, University of New Hampshire, 46 College Road, Durham, NH, 03824, USA
2Department of Fisheries at Lena Point, College of Fisheries and Ocean Sciences, University of Alaska Fairbanks, 17101 Point Lena Loop Rd, Juneau, AK 99801, USA
3Pacific Salmon Ecology and Conservation Laboratory, Department of Forest and Conservation Sciences, University of British Columbia, 2424 Main Mall, Vancouver BC V6T 1Z4 Canada
4Department of Biology, University of Victoria, PO Box 1700 Station CSC, Victoria, British Columbia V8W 2Y2, Canada
Annual migrations by juvenile Pacific salmon Oncorhynchus spp. smolts are predictable, presenting opportunities for predators to exploit these seasonal prey pulses. Directly observing predator– prey interactions to understand factors affecting predation may be possible via dual-frequency identification sonar (DIDSON) acous- tic imaging. Within Chilko Lake, British Columbia, prior teleme- try and stomach content analyses suggested that the out-migration of Sockeye Salmon Oncorhynchus nerka smolts influences the movements and aggregations of Bull Trout Salvelinus confluentus that feed extensively on smolts during their out-migration. Bull Trout captured at a government-installed counting fence exhibited high consumption of smolts, but it is only assumed that feeding occurred directly at the fence. We used DIDSON to assess fine- scale predator–prey interactions between Sockeye Salmon smolts and Bull Trout over 10 d during the 2016 smolt out-migration. We found that smolt–Bull Trout interactions were correlated with smolt densities at the counting fence, consistent with the prior diet studies in the system. Predator–prey interactions were also cou- pled with nocturnal migratory behaviors of Sockeye Salmon smolts, presumably to minimize predation risk. These results demonstrate that DIDSON technology can record interactions between predators and migrating prey at a resolution that can identify variability in space and time and provide insight on the role of anthropogenic structures (e.g., counting fences) in mediating such interactions.