Navigating the predator gauntlet: consumption of hatchery- and wild-born juvenile chum salmon (Oncorhynchus keta) by common nearshore marine fishes in Southeast Alaska

Abstract

Juvenile chum salmon (Oncorhynchus keta) undergo extensive mortality at marine entry, a period which is believed to be a potential population bottleneck. Although this early mortality has been consistently observed, our understanding of the mechanisms responsible is limited. Furthermore, the implications of large-scale salmon hatchery releases for the ecology of juvenile chum salmon and their consumers is another important knowledge gap. To better understand the predation responses of abundant consumers to hatchery- and wild-born juvenile chum salmon, we examined the diets of Pacific staghorn sculpin (Leptocottus armatus) and Dolly Varden (Salvelinus malma) near Juneau, Alaska, in 2016 and 2017. Chum salmon composed 4.5% and 19.6% of the diets of staghorn sculpin and Dolly Varden by weight, respectively, and 88% of chum salmon individuals consumed were of hatchery origin. Chum salmon prey were shorter than average when compared to chum salmon concurrently collected by beach seine and hatchery releases of chum salmon. Regression analyses indicated that occurrence of juvenile chum salmon in diets varied primarily by date and site. Predation generally occurred more frequently at sites closer to hatchery release areas. The quantity of chum salmon in staghorn sculpin stomachs was related to predator length, chum salmon catch-per-unit-effort (CPUE), and the proportion of hatchery fish present; however, date was the only important predictor explaining quantity of chum salmon in Dolly Varden stomachs. To translate diet data into consumption rate, we experimentally determined gastric evacuation rate for staghorn sculpin and implemented a field-based consumption model. Average daily consumption of chum salmon was low relative to all other prey groups. Estimates of average seasonal consumption of juvenile chum salmon by staghorn sculpins suggest that predator populations would have to be implausibly large to consume even 1% of local hatchery chum salmon production. Together, these results yield new insights into the interactions between the predators of wild-born and hatchery-born salmon during the critical stage of marine entry.