Predator-prey interactions between introduced trout and long-toed salamanders and ways to mitigate nonconsumptive effects

Abstract

Predators can increase prey through mortality, but also have the capacity to alter behavior, morphology, and life history through nonconsumptive effects. In many historically fishless lakes in western North America, trout have been introduced for recreational fishing and are associated with reducing and extirpating populations of amphibians, including long-toed salamanders (Ambystoma macrodactylum). Salamanders and trout may coexist in some lakes, as larvae are able to alter foraging behavior by avoiding open water, foraging at night in shallow water, and hiding in cover to avoid predation. However, salamanders may experience nonconsumptive effects due to these behavioral changes. We sought to estimate the nonconsumptive effects of trout on morphology and life history of larval salamanders. We caught salamander larvae using minnow traps in northwestern Montana during the summers of 2012 and 2013 and compared body morphology measurements and size at and timing of metamorphosis between lakes with and without trout. Salamanders in lakes with trout were smaller: they weighed less, had shorter body lengths, and had shorter and narrower tails. Salamanders in lakes with trout were also less likely to metamorphose, did so later in the summer, and had smaller total and tail lengths at metamorphosis. These changes in morphology and life history likely were a result of reduced foraging to avoid predator attacks. We conducted a field experiment in 2013 to investigate whether adding vegetation structure could reduce nonconsumptive effects of trout on salamander larvae by providing refugia and reducing perceived risk of predation. We constructed field enclosures in lakes with and without trout and quantified changes in salamander growth and differences in size at metamorphosis with and without added structure. Salamanders appeared to detect trout cues because they grew more slowly in lakes with trout, even though trout had no ability to consume salamanders. Added vegetation structure did not influence growth rates, but did increase the probability of salamanders that reached metamorphosis. Future research efforts should investigate whether adding vegetation structure to whole lakes can mitigate the nonconsumptive effects of trout, provide a feasible alternative to fish removal, and facilitate coexistence between salamanders and trout.