No abstract

In southeastern Alaska, a large volume of freshwater flows into estuaries from thinning and retreating glaciers and from high rainfall in the surrounding coastal temperate rainforest. Freshwater runoff to the coast affects the physical and chemical properties of estuarine habitats and, in turn, the structure and dynamics of biological communities. This study quantified temporal and spatial variation in the diets of Pacific Staghorn Sculpins Leptocottus armatus as an important step toward understanding how climate‐driven changes in freshwater runoff may translate into shifts in the feeding ecology of a highly abundant consumer in estuaries along the west coast of North America. Stomach contents of Staghorn Sculpins were collected monthly (April–September 2014) from three intertidal sites at the mouths of rivers that differ in their headwater hydrology. Staghorn Sculpins (N = 402) consumed a variety of marine prey items including epibenthic invertebrates, juvenile fishes, and terrestrial and aquatic insects. Based on a multivariate analysis of similarity, diet composition showed weak to moderate differences across sampling months and sites. Diet differences reflected seasonal shifts in the contributions of freshwater‐tolerant marine invertebrates and young of the year fishes. The greatest site differences were observed between the Cowee Creek estuary and Mendenhall River estuary, the watersheds with the lowest and highest glacial coverage, respectively. Overall, the generalist feeding behavior of Staghorn Sculpins and their consumption of prey items that are well adapted to brackish and freshwater conditions suggest that these fish are resilient to changes in estuarine conditions associated with increasing freshwater runoff.

Multiple restoration actions have been implemented in response to declining salmon populations. Among these is the addition of salmon carcasses or artificial nutrients to mimic marine-derived nutrients historically provided by large spawning runs of salmon. A key assumption in this approach is that increased nutrients will catalyze salmon population growth. Although effects on aquatic ecosystems have been observed during treatments, it is unclear if permanent population increases for salmon will occur. To test this assumption and address associated uncertainties, we linked a food web model with a salmon life cycle model to examine if carcass additions in a river reach would improve conditions for salmon in the long term. Model results confirmed immediate increases in the biomass of periphyton, macroinvertebrates, and fish during carcass additions. In turn, juvenile salmon grew larger and experienced improved freshwater and smolt survival, which translated to a greater number of adults returning to spawn. However, once additions ceased, salmon abundance returned to pre-treatment levels, which, based on our model, is owing to a combination of instream and out-of-basin factors. Overall, results of this work suggest that benefits during carcass and nutrient additions may not translate into persistent productivity of salmon unless additions are sustained indefinitely, or other limiting factors are addressed