Fish community response to evolving channel complexity in an agricultural headwater system

Headwaters in many watersheds in the midwestern United States are often dominated by ditches that are dredged to drain farmland and are maintained as homogeneous channels. These ditches may provide important headwater habitat for fish but are rarely managed as such. With reduced dredging, these ditches tend to stabilize their cross-sectional profile with patchy sediment deposits and plant growth. We tested the impact of such channel complexity on the structure of fish communities in agricultural ditches of the upper Ottawa River (Ohio), a western Lake Erie tributary, by comparing twelve 20 m (66 ft) channel segments with and without such complexity (heterogeneous [Ht] and homogeneous [Ho], respectively). Fish communities were sampled at low water with block seines in each site eleven times between June 2005 and October 2006. Temperature, pH, turbidity, dissolved oxygen, conductivity, and discharge were comparable between Ht and Ho segments during each sampling event. A total of 10,843 fish representing 24 species were identified, assessed for spawning condition and age class, and released. In spite of the large variability in fish community metrics over time, Ht habitat had a higher species richness (Ht = 6.56 ± 0.63, Ho = 4.17 ± 0.63; p = 0.02), Shannon diversity (Ht = 1.33 ± 0.12, Ho = 0.90 ± 0.12; p = 0.03), and number of feeding guilds (Ht = 3.2 ± 0.20, Ho = 2.2 ± 0.20; p = 0.01). Fish abundance and Index of Biotic Integrity showed no significant effect for habitat. The community was dominated by tolerant cyprinids (Pimephales promelas and P. notatus) but also contained 1,514 Etheostoma microperca, a previously undocumented population and species listed as a “State Species of Concern” in Ohio. The majority (73%) of E. microperca were found in Ht segments. In spite of the prevalence of exotic species in other regional aquatic systems, only 0.7% of the total catch belonged to nonnative species. With the majority of headwaters in the midwestern United States consisting of dredged ditches, balancing management for efficient drainage while allowing some level of channel complexity may benefit native fishes and contribute to surprisingly rich communities.