Environmental drivers of fish communities and food webs in Gulf of Alaska estuaries

Abstract

The coastal Gulf of Alaska (GOA) is experiencing rapid, climate-driven ecological change. Climate forecasts predict increased temperatures and more precipitation as rainfall, but these changes will not have uniform effects across nearshore ecosystems. Estuarine habitats will be dynamically affected by changes in neighboring watersheds as glaciers melt and recede. Because estuaries provide critical habitat for many fishes, including some that support fisheries, it is important to understand how changing estuarine conditions may impact nearshore fish communities. The overall goal of this thesis was to investigate how environmental conditions, fish communities, and food webs vary across estuaries fed by watersheds with varying glacial coverage (0-60%). We conducted monthly beach seining and measured environmental conditions from April to September 2019 at ten estuary sites in two regions of the GOA, Lynn Canal in southeastern Alaska and Kachemak Bay in southcentral Alaska. The goal of Chapter One was to characterize differences in estuarine fish communities along the glacial gradient, between regions, and throughout the sampling season. We then focused on two abundant species in Lynn Canal, starry flounder (Platichthys stellatus) and Pacific staghorn sculpin (Leptocottus armatus), and used multiple years of data (2014, 2016-2017, 2019) to determine environmental drivers of size structure for each species. Fish communities showed the greatest differences between regions and across months, and temperature and salinity were significant drivers of variation in species composition. Variation in mean length of Pacific staghorn sculpin was best explained by year and the interaction of site and month, whereas variation in mean length of starry flounder was best explained by temperature, salinity, and turbidity. The goal of Chapter Two was to provide foundational information on the diet of juvenile coho salmon (Oncorhynchus kisutch) during the estuarine life stage and characterize variation in diets between years and regions. Juvenile coho salmon have a diverse diet of terrestrial and marine invertebrates and fishes, and they exhibited a shift to piscivory during this transitional period in nearshore habitats. Site differences accounted for most of the variability in diet, while temperature and salinity only accounted for a total of 12% of the variability in diet. Overall, we found that fish communities in GOA estuaries vary with environmental and habitat conditions, but that the glacial to non-glacial watershed gradient was less important in explaining variation in fish community structure than regional and interannual differences.