Browsing UAF Graduate School by Subject "mortality"
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Navigating the predator gauntlet: consumption of hatchery- and wild-born juvenile chum salmon (Oncorhynchus keta) by common nearshore marine fishes in Southeast AlaskaJuvenile 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.
Wasting disease and environmental variables drive sea star assemblages in the northern Gulf of AlaskaSea stars are ecologically important in rocky intertidal habitats. The recent (starting 2013) sea star die-off attributed to sea star wasting disease throughout the eastern Pacific, presumably triggered by unusually warm waters in recent years, has caused an increased interest in spatial and temporal patterns of sea star assemblages and the environmental drivers that structure these assemblages. This study assessed the role of seven potential static environmental variables (distance to freshwater, tidewater glacial presence, wave exposure, fetch, beach slope, substrate composition, and tidal range) influencing northern Gulf of Alaska sea star assemblages before and after regional sea star declines. For this, intertidal surveys were conducted annually from 2005 to 2018 at five sites in each of four regions that were between 100 and 420 km apart. In the years leading up to the regional mortality events, assemblages were different among regions and were structured mainly by tidewater glacier presence, wave fetch, and tidal range. The assemblages after wasting disease were different from those before the event, and there was a partial change in the environmental variables that correlated with sea star structure. In these recent years, the environmental variables most highly correlated with sea star assemblages were slope, wave fetch, and tidal range, all of which relate to desiccation, attachment, and wave action. This indicates that the change in sea star density and structure by wasting disease left an assemblage that is responding to different environmental variables. Understanding the delicate interplay of some of the environmental variables that influence sea star assemblages could expand knowledge of the habitat preferences and tolerance ranges of important and relatively unstudied species within the northern Gulf of Alaska.