Zooplankton community composition in relation to environment and juvenile salmon diets in Icy Strait, Southeast Alaska

Abstract

Zooplankton in the nearshore marine habitat function as an important prey resource for many pelagic fishes, are a major component of the lower tropic level, and serve as a vital ecosystem indicator. Understanding how the zooplankton community changes in response to fluctuations in biophysical factors is critical in a changing climate and is important to understanding the dynamics of commercially important upper-trophic level species that depend nutritionally on zooplankton. The Alaska Fisheries Science Center's Southeast Coastal Monitoring project has surveyed the pelagic ecosystem in eastern Icy Strait monthly from May to August since 1997 to understand how environmental variation affects the pelagic food web and the sustainability of salmon resources. I used this long-term dataset (1997-2017) to address the goals of this study: 1) to investigate the influence of temperature on the Icy Strait zooplankton community; and 2) to understand how juvenile salmon utilize zooplankton prey in relation to temperature driven fluctuations in the zooplankton community. In Chapter 1, I noted that the composition of the zooplankton community varied in years with anomalously high or low temperatures. I observed shifts in the timing of development in many key taxa during these anomalous years. For example, in anomalously cool years, several taxa were found in higher densities later in the summer than in anomalously warm years. In Chapter 2, I examined how oceanographic factors influenced the diet composition and quality of four species of juvenile Pacific salmon (Oncorhynchus spp.) in Icy Strait (Southeast Alaska) from 2013 to 2017. In 2015 I observed a change in diets, including zooplanktivorous (pink salmon O. gorbuscha, chum salmon O. keta, and sockeye salmon O. nerka) and piscivorous (coho salmon O. kisutch) species, from typically diverse diets to diets dominated by euphausiids. This year was notable for warm waters, deep pycnoclines, and below average zooplankton nutritional quality. Juvenile salmon appeared to supplement their lipid intake and meet nutritional requirements by switching to larger euphausiid prey. The results from these studies increase our understanding of zooplankton community dynamics, salmon trophic relationships, and the resilience and flexibility of the food web during climate-driven reorganizations of the pelagic marine ecosystem.