Macrophyte Density Determines the Response of Native Fishes to an Introduced Predator

Freshwater fish species have been stocked extensively outside their native ranges to create and enhance fishing opportunities. Trout stocked into headwater lakes, however, often have severe negative effects on native species, including reduced abundances or even extirpations.  In contrast, recent research from productive lakes has observed little effect of stocked trout on native macroinvertebrates and forage fishes.  Macrophyte beds in these productive lakes may dampen effects of trout by providing prey species with a refuge.

Our objective was to determine how the availability of macrophyte beds mediates the response of dace (Chrosomus spp.) and fathead minnow (Pimephales promelas) to the introduction of rainbow trout (Oncorhynchus mykiss) in lakes in Alberta, Canada.  We first used surveys with minnow traps to contrast the habitat use of dace over two years in three stocked and three unstocked lakes. We then determined if macrophytes would alter the response of forage fishes to trout by stocking trout into 12 experimental enclosures constructed in littoral areas of one unstocked lake in May 2010.  Six reference enclosures were maintained at natural densities of macrophytes and six enclosures experienced reduced densities of macrophytes; all enclosures permitted unimpeded access to forage fishes. In mid-August, we added one trout (SL ± SE; 254 ± 5 mm) to each enclosure.  Trout inhabited enclosures for 3 days, and we monitored the use of enclosures by forage fishes before, during, and after trout presence.  We predicted that forage fishes would use vegetated inshore habitat more frequently in stocked relative to unstocked lakes and would limit use of reduced macrophyte enclosures when trout were present relative to the before-trout period and relative to natural macrophyte enclosures.

As predicted, a greater proportion of dace were caught inshore in stocked versus unstocked lakes (treatment*location; F_1,18.8= 11.2, p= 0.003), and dace used reduced macrophyte enclosures significantly less often in the presence of trout than before trout were added (F_2,15= 6.89, p= 0.008).  Trout presence had no effect on dace catch in natural macrophyte enclosures.  In contrast, fathead minnow showed no response to trout in either natural or reduced macrophyte enclosures. Consistent with previously published studies, these results suggest that dace, but not fathead minnow, recognize and avoid a novel predator, occupying refuge habitat when available.  To help prevent adverse effects of non-native trout, managers should consider stocking in lakes with extensive littoral macrophyte beds that provide refuges for native fishes.