Native fish and macrophytes minimize impacts of introduced trout on littoral invertebrates: an experimental mesocosm study

Fish and aquatic macrophytes play important roles in structuring aquatic invertebrate communities in lakes. Recent research suggests that native fishes and macrophytes can reduce impacts of introduced fish by "pre structuring" invertebrate populations and providing refuge habitat, respectively. To determine how macrophytes and small-bodied fishes may buffer against potential impacts of stocked trout in productive lakes, we conducted a mesocosm experiment in Alberta, Canada with two macrophyte treatments (natural and reduced densities) and two fish treatments (native fish present and absent) into which we stocked non-native Rainbow Trout. We sampled invertebrates from benthic and midwater habitats within mesocosms and examined effects of experimental treatments and trout presence on the abundance and sizes of invertebrates. Presence of native fishes and density of macrophytes both affected invertebrate assemblages, but the addition of trout to experimental mesocosms had no additional effects. Invertebrate taxa were commonly more abundant in the absence of fish or in the presence of natural (i.e., high) macrophyte densities, with macrophyte density having stronger effects than fish. Lengths of invertebrate taxa were mostly unaffected by either treatment. Our experiment demonstrated that native fishes and abundant macrophyte beds can interact with invertebrate populations in ways that can buffer them against impacts of non-native trout stocked into productive, fish-bearing lakes.