Quantifying food web interactions and limitations for native salmonids in Ross Lake, WA and implications for the introduction of anadromous salmonids

Abstract

Food web function influences ecosystem productivity and plays a critical role in ecosystem services and conservation of species of concern. In reservoirs, existing and future changes to these ecosystems (e.g., invasive species, climate change, water operations) may alter their capacity to support native species. As introducing anadromous fish upstream of migration barriers is gaining traction as a conservation strategy, these food web dynamics can also influence the success or failure of such programs. In this dissertation, I explore these ideas using Ross Lake, a hydropower reservoir in the Upper Skagit River, Washington, USA, as a case study. I used a bioenergetics-based approach to quantify food web interactions to identify potential limitations (e.g., prey supply, predation, temperature) to growth and production of native species and proposed introductions of anadromous salmonids in the lake and examined these processes within the context of seasonal thermal constraints on habitat use and prey availability. In Ross Lake, the proliferation of an unauthorized introduction of redside shiner Richardsoniu balteatus approximately 20 years ago has fundamentally altered food web interactions by providing a novel prey source for piscivorous fishes and becoming the dominant zooplanktivore/invertivore. A bioenergetics model did not exist for this species; therefore, my first objective was to parameterize and evaluate this model using controlled laboratory experiments (Chapter 1). With this model in our toolbelt, I then quantified food web interactions to identify the relative roles of direct competition with redside shiner (Chapter 2) versus predation (Chapter 3) in limiting growth and production of native salmonids in Ross Lake. This baseline assessment of the bottom-up and top-down limitations in the Ross Lake food web provided the foundation to then examine these food web dynamics in the reservoir as limitations to the feasibility of introducing anadromous salmonids above the Upper Skagit River dams (Chapter 4). Collectively, this dissertation provides a framework for assessing the food web constraints to native species conservation in regulated river systems, including important considerations for future feasibility studies in Ross Lake and other regulated rivers where anadromous salmonid introduction programs are considered.