Close Encounters with a Fish Screen: Integrating Physiological and Behavioral Results to Protect Endangered Species in Exploited Ecosystems

Protection of endangered species that are restricted to highly modified ecosystems and threatened by anthropogenic activities often includes regulatory design and operational criteria for structures and facilities located within the species' critical habitat. In many freshwater systems, loss of fishes at water diversions has contributed to population declines of multiple species. Fish screens prevent removal of fish from the habitat; however, under current design criteria, the degree of protection provided by screens is unknown for most affected species. We used a large laboratory‐based flume to observe and quantify the responses of delta smelt Hypomesus transpacificus , an endangered osmerid threatened by water diversions in California's Sacramento–San Joaquin Estuary, to a simulated fish screen. For a range of flow and environmental conditions, we measured screen contact frequency, swimming behavior, injuries, and survival. Delta smelt experienced frequent temporary contact with the screen, and contact rates were influenced by flow and time of day–light level. Contact was injurious, and postexperiment mortality rates were directly related to both contact frequency and severity as well as temperature. Quantitative models showed that, for this species, both behavioral responses such as swimming velocity and physiological responses to fish screen contact, as modified by environmental conditions, controlled the species' performance and its risk from the diversion and the screen. The results illustrate that ecologically effective protection strategies and regulatory criteria developed on the basis of multiple integrated responses of the organism to the stressor offer greater benefits and certainty to both the organism and the regulated activities.