Cyclical, multi-trophic-level responses to a volatile, introduced forage fish: Learning from four decades of food web observation to inform management

Abstract Species introductions can have significant effects on recipient ecosystems. Anticipating potential ecosystem change in response to introduced species based on historical information can help managers prepare for future conditions. Rainbow Smelt Osmerus mordax have been introduced widely to improve sport fish growth. As intended, Walleye Sander vitreus growth in Horsetooth Reservoir, Colorado increased after Rainbow Smelt introduction, but poor Walleye recruitment occurred as well. Additionally, opossum shrimp Mysis diluviana became absent from both predator diets and intermittent surveys, the dominant Daphnia species in Horsetooth Reservoir shifted and Daphnia densities declined significantly. These patterns were repeated during two different time periods of increased Rainbow Smelt abundance, suggesting that Rainbow Smelt have a strong influence on multiple components of the ecosystem. The repetition of responses to Rainbow Smelt offered the opportunity to evaluate indicators to anticipate potential ecosystem regime shifts that restructure predator–prey dynamics across trophic levels. Three predictors (i.e., high estimated Rainbow Smelt abundance, high catch rates of large Walleye, and low Daphnia densities) were associated with poor Walleye recruitment. Simple indicators like these could inform timely management decisions to take advantage of the benefits Rainbow Smelt offer, while lessening their undesirable effects. For example, management decisions could be made, such as preparing for Walleye egg collections, rearing and stocking of Walleye, increasing availability or quality of Walleye spawning habitat, allowing more protective or liberalized adult Walleye harvest to promote natural recruitment, and limiting Rainbow Smelt access to their spawning habitat.