Evans, A.F., Q. Payton, A. Turecek, B. Cramer, K. Collis, D.D. Roby, P.J. Loschl, L. Sullivan, J. Skalski, M. Weiland, and C. Dotson. In review. Avian predation on juvenile salmonids: spatial and temporal analysis based on acoustic and passive integrated transponder tags. Transactions of the American Fisheries Society 145:860-877.
Abstract
We evaluated the impact of avian predation on juvenile steelhead (Oncorhynchus mykiss), yearling Chinook salmon (O. tshawytscha), and subyearling Chinook salmon by piscivorous waterbirds from 11 different breeding colonies in the Columbia River Basin, USA, during 2012 and 2014. Fish were double-tagged with both acoustic and passive integrated transponder (PIT) tags and tracked via a network of hydrophone arrays that provided detections of fish at various spatial and temporal scales during out-migration. Recoveries of PIT tags on bird colonies, coupled with last known detections of live fish passing arrays, were used to quantify where predation occurred, when it occurred, and the impact of birds on the survival of tagged fish. Results indicated that avian predation was a substantial source of steelhead mortality, with predation probabilities (proportion of available fish consumed) ranging from 0.06–0.28 for fish traveling through the lower Snake, lower Columbia, and middle Columbia rivers. For yearling Chinook and subyearling Chinook salmon, estimates ranged from 0.03–0.09 and 0.01–0.05 of the available tagged fish, respectively. Predation was generally highest on smolts by gulls (Larus spp.) near hydroelectric dams and by Caspian terns (Hydroprogne caspia) within the reservoirs. No predation hotspots were identified for foraging double-crested cormorants (Phalacrocorax auritus) or American white pelicans (Pelecanus erythrorhynchos). A comparison of total mortality (1-survival) relative to mortality from colonial waterbirds indicated that avian predation was one of the greatest sources of mortality affecting survival of steelhead and yearling Chinook salmon during out-migration. Predation by colonial waterbirds on subyearling Chinook salmon, however, was generally low and a minor component of total mortality. Results indicate that the use of both acoustic and PIT tag technologies can be used to quantify where and when mortality occurs and what fraction of that mortality is due to colonial waterbird predation relative to other, non-avian mortality sources.