Testing the impact of plant colonization on fish communities in agricultural fitches of the Ottawa River, northwest Ohio

Background/Question/Methods

Colonization by wetland plants of channelized stream banks and the resulting fluvial geomorphology are surmised to produce more-stable channel cross-sections and improve water quality. I tested the impact of such colonization on the structure and composition of fish communities in farm ditches of the Ottawa River, a western Lake Erie tributary, by comparing twelve 20-m stream segments with and without plant intrusion (heterogeneous or Ht and homogeneous or Ho, respectively). Fish communities were sampled by sweep and block seine in each segment eleven times between June 2005 and October 2006. Measurements of pH, temperature, turbidity, DO₂, conductivity, canopy cover and discharge were comparable between Ht and Ho segments at each sampling event.

Results/Conclusions

A total of 10,501 fish representing 24 species were identified and released.  Only 0.6% of the total catch belonged to non-native species. The average Shannon diversity, species richness and number of trophic guilds were significantly higher in Ht segments (p=0.028, p=0.029, p=0.008, respectively). Moreover, Ht segments appeared to host greater abundance (124.9 ± 1.0) than Ho sites (52.3 ± 0.9) although that difference was only significant at the p=0.074 level due to large inter-annual variability in fish abundance within each habitat type. The Index of Biological Integrity was not significant different between habitats (Ht= 21.8 ± 0.1; Ho=21.2 ± 0.2). My census included 1,615 least darters, Etheostoma microperca, a previously undocumented population and listed as a State Species of Concern in Ohio. E. microperca was three times more abundant in Ht than in Ho stream segments. Seines were an effective, non-lethal tool in our fish sampling protocol. These farm ditches showed a surprisingly robust, species-rich fish community that may be maintained by allowing wetland plant colonization along stream banks.