Habitat use of the central mudminnow (Umbra limi) and yellow perch (Perca flavescens) in Umbra-Perca assemblages: the roles of competition, predation, and the abiotic environment

We compared spatial distributions and diel activity patterns of three fish populations (co-occurring central mudminnows and yellow perch, and a mudminnow population in a single-species assemblage) throughout the year in two small, northern Wisconsin dystrophic lakes. In winter, all three populations were more active offshore and during the day. During spring and summer–fall all were concentrated inshore, near the bottom, and were more active around dawn and dusk. Winter distributions appeared to be responses to abiotic conditions (ice cover and low oxygen availability); overall patterns during open-water periods appeared to be linked to prey availability (e.g., chironomid emergences). Although the three populations were generally similar, some aspects of habitat use differed between species, between lakes, or between dominant and subdominant populations, with the direction of differences varying seasonally. The yellow perch population was recovering from a size-selective winterkill and between spring and summer underwent an ontogenetic niche shift, first being competitors with mudminnows, then becoming predators on mudminnows. Mudminnows did not alter their vertical or diel activity patterns in the presence of these predatory perch. They did become more concentrated inshore in summer–fall, near structural refuge provided by the bog mat, and, unlike the other two populations, failed to shift their activities offshore at twilight. Our study suggested that a simple habitat in the study lakes contributed both to overall similarities in habitat use and strong interactions between co-occurring populations. However, the ecological flexibility demonstrated by mudminnows may prevent their total exclusion from lakes inhabited by perch and allow them to respond to changing biotic environments.