Bull Trout Movements Match the Life History of Sockeye Salmon: Consumers Can Exploit Seasonally Distinct Resource Pulses

Abstract

Resource pulses provide consumers with opportunities to feed at high rates. The spawning and die-off of semelparous Pacific salmon provide consumers with feeding opportunities that contribute substantially to annual energy budgets. The life history of Pacific salmon also includes the downstream migration of smolts, which could provide similar opportunities for consumers to exploit. We tracked seasonal movements of adult Bull Trout Salvelinus confluentus in and out of the Chilko Lake–River outlet in interior British Columbia, which is characterized by a large Sockeye Salmon Oncorhynchus nerka population. Use of the outlet, lake, and river habitats were compared with the timing of out-migrations of Sockeye Salmon smolt and the spawning events of Sockeye Salmon, Chinook Salmon O. tshawytscha, and Coho Salmon O. kisutch. Bull Trout activity and residency in the outlet increased during the spring Sockeye Salmon smolt out-migration, and 40% of Bull Trout were found to return to the outlet in successive years. During Sockeye Salmon spawning, Bull Trout residency was increased in the river where most Sockeye Salmon spawning occurs. Therefore, Bull Trout may exploit both life history events, although direct consumption of eggs was not observed. Spawning by Chinook Salmon and Coho Salmon in the river did not appear to attract Bull Trout; Bull Trout residency increased in the lake (where spawning does not occur) during these times. Bull Trout residency in the lake also increased when outlet water temperatures were >16°C, and thus the lake may provide thermal refuge when river temperatures are above optima for this coldwater char. This research demonstrates that movements of aquatic consumers can depend upon smolt out-migrations, and such movements can allow exploitation of multiple pulses that are seasonally distinct. These results also highlight the ecological significance of Pacific salmon for consumers in freshwater systems.

Received August 18, 2016; accepted January 11, 2017 Published online March 21, 2017

ACKNOWLEDGMENTS

We thank A. Bass, V. Minke-Martin, C. Middleton, A. Lotto, and G. William for assistance in the capture and tagging of Bull Trout. The Xeni Gwet’in First Nation graciously allowed access to the study area. Funding for the study was provided by the Ocean Tracking Network, (supported by the Natural Sciences and Research Council of Canada [NSERC] and the Canada Foundation for Innovation), the Mitacs Accelerate program, and the Salish Sea Marine Survival Project (marinesurvivalproject.com) via the Pacific Salmon Foundation (for which this is Publication 7). Further funding was provided via a NSERC Discovery Grant awarded to S.G.H. and a Fisheries Society of the British Isles (FSBI) Research Grant provided to N.B.F. N.B.F. was further funded by a NSERC Vanier Graduate Scholarship. We thank the Stock Assessment Division of Fisheries and Oceans Canada (DFO) for allowing researchers to stay at the field camp. We also thank the team (including B. Leaf, K. Benner, N. Trouton, and L. Ritchie) of the DFO Fraser River Stock Assessment Program for access to spawning distribution data and estimates for spawning abundances and timing used in analyses. Field work followed animal use protocols (A11-0125) approved by the University of British Columbia and the Canadian Council on Animal Care. The manuscript was improved by the comments and edits of four anonymous reviewers.