Species- and origin-specific susceptibility to bird predation among juvenile salmonids

Juvenile salmonids often experience high mortality rates during migration and bird predation is a common source of mortality. Research suggests that hatchery-reared salmonids are more prone to predation than wild salmonids, and that Atlantic salmon (Salmo salar) experience lower predation than Sea trout (Salmo trutta), yet telemetry studies have displayed equivocal results. Here, using a large data set on passive integrated transponder (PIT) tagged hatchery-reared and wild juveniles of Atlantic salmon and Sea trout (25,769 individuals) we investigate predation probability by piscivorous birds (mainly Great Cormorants Phalarocorax carbo) on salmonids originating from River Dalalven in Sweden. Bird colonies and roosting sites were scanned annually (2019-2021), and the temporal dynamics of bird predation on salmonids released in 2017-2021 was assessed. Hatchery-reared trout was clearly most susceptible to cormorant predation (0.31, 90% credibility interval [CRI] = 0.14-0.53), followed by wild trout (0.19, 90% CRI = 0.08-0.37), hatchery-reared salmon (0.13, 90% CRI = 0.07-0.23), and wild salmon (0.08, 90% CRI = 0.04-0.14), in subsequent order. This order in predation probability was consistent across all studied tag- and release-years, suggesting that the opportunistic foraging of cormorants affects the overall survival of juvenile salmonids, but that the inherent predation risk between different salmonid types differs systematically.