Poor growth of rainbow trout fed New Zealand mud snails Potamopyrgus antipodarum

The New Zealand mud snail (NZMS) Potamopyrgus antipodarum is rapidly invading North American freshwaters, leading to speculation that native fisheries, especially those involving trout, will be negatively impacted. To assess whether trout would consume NZMSs and could assimilate nutrients from them, we conducted a laboratory N-15 tracer study, a laboratory feeding study, and bioenergetics modeling with rainbow trout Oncorhynchus mykiss; we also evaluated 5 years of diet and condition data describing rainbow trout and brown trout Salmo trutta collected from a river colonized by NZMSs. The 15N tracer study showed that rainbow trout consumed and to a lesser extent assimilated NZMSs. Rainbow trout fed N-15-labeled NZMSs had muscle isotopic signatures that were 80% higher than those of fish fed unlabeled NZMSs and 30% lower than those of fish fed N-15-labeled amphipods. The feeding study showed that rainbow trout fed an exclusive and unlimited amount of NZMSs lost 0.14-0.48% of their initial body weight per day. Collection of rainbow trout feces showed that 8.5% of the NZMS shells were empty, and these snails were assumed to have been digested; 53.8% of NZMSs passed through the digestive system alive. Bioenergetics modeling showed that starved rainbow trout would have lost between 0.52% and 0.54% of their initial body weight per day, depending on their initial weight. Based on observed fish weight changes during three trial periods, mean modeled digestibility of NZMSs for rainbow trout fed an unlimited supply of the snails (proportion of maximum consumption = 1.0) was 63, 65, and 27%. Field survey data supported our laboratory experiments: we observed a sharp annual increase in the number of brown and rainbow trout consuming NZMSs between 2001 and 2005 in the Green River, Utah, downstream from Flaming Gorge Dam; moreover, the condition of brown and rainbow trout with NZMSs in their guts was significantly lower than that of fish without NZMSs in their stomachs. Our results confirm that North American trout fisheries face potential negative impacts from NZMS invasion.