Competitive Interactions: Spatial and temporal niche partitioning between Brook and Brown Trout in southwest Michigan

Niche diet partitioning between Brook Trout and Brown Trout were studied in two southwestern Michigan, USA streams, one dominated by Brook Trout, the other with both Brook and Brown Trout. Brook Trout are endemic to eastern North America and have been introduced throughout the world for sport fishing. Brown Trout were first introduced to Michigan in 1883 using a genetic strain from Germany. Stomach contents were collected from a population of Brook Trout in the absence of Brown Trout in Frost Creek, while the other population was in potential competition with Brown Trout in Cedar Creek. Absent Brown Trout, Brook Trout exhibited greater niche feeding breadth compared to those in direct competition. This unique study with seasonal sampling over a one-year timeframe indicates that in Cedar Creek, niche partitioning does take place with Brook Trout focusing on benthic invertebrates in the presence of Brown Trout (Pairwise ADONIS, P = 0.006). Those not in competition with Brown Trout fed opportunistically between drift and benthos (Pairwise ADONIS, P = 0.006 drift/benthos). Ivlev’s electivity Index indicated Brook Trout in Frost Creek were found to prefer Crustacea, Diptera, and Trichoptera while avoiding Plecoptera and Ephemeroptera. Brook Trout in Cedar Creek preferred Plecoptera and Ephemeroptera to a greater degree than those within Frost Creek. Stomach sampling following a flood pulse indicated that both Brook and Brown Trout were feeding almost exclusively on terrestrial annelids, washed into the stream as flood waters subsided (Ivlev’s electivity: Brook Trout 0.96 and Brown Trout 0.97). The thermal regime was significantly warmer (17.8oC vs. 16.1oC mean daily summer temperature) in the stream where Brook and Brown Trout co-occurred vs. the stream with solely by Brook Trout. Frost Creek had a lower mean summer temperature below thermal stress levels for Brook Trout. Cedar Creek had a higher mean annual temperature, likely increasing the competitiveness of Brown Trout over Brook Trout. Cold thermal regimes give Brook Trout a competitive edge and aid in slowing Brown Trout invasions into headwater streams. In addition to thermal regimes, successful recruitment as evidenced by high young-of-the-year (YOY) abundance is an important indicator of robust Brook Trout populations. In Frost Creek 45 YOY were captured vs. 4 YOY in Cedar Creek. I hypothesize that this low recruitment is potentially due to predation pressure – possibly from larger Brown Trout, although there was no evidence of this in stomach sampling. Variables likely linked to the continued persistence of Brook Trout in headwater streams include maintenance of groundwater inputs, which contribute to cooler thermal regimes, a robust food resource, and high-quality habitat that promotes successful recruitment. Isolated Brook Trout populations in small headwater streams will likely become threatened as climate change warms streams above their thermal tolerances and through increased winter precipitation, which may negatively affect YOY abundance through scouring of redds