Occurrence, abundance, and associations of Topeka Shiners and species of greatest conservation need in streams and oxbows of Iowa and Minnesota
Since settlement, the landscape of the Midwestern United States has undergone many changes. In Iowa and southern Minnesota, approximately 70-80% of land is in agricultural production. Agricultural practices often involve substantial modifications to existing streams. Many streams in Iowa and southern Minnesota have been channelized to aid in removal of excess water from crop fields. Channelization results in not only more homogenous habitat in the stream channel, but also leads to less connectivity between the stream and its floodplain. Many fish species have evolved to prefer particular habitat features, such as riffles, pools, and off-channel habitat. Because these habitats are often removed or made less accessible through channelization, populations of many native stream fishes in Iowa and Minnesota have declined over several decades. The Topeka Shiner (Notropis topeka) is the only fish that is federally listed as endangered in the interior streams of Iowa and Minnesota. About two decades ago, Topeka Shiners were found to commonly persist in oxbows. As a result, several agencies are now involved in restoring oxbows that have filled with sediment over time to improve habitat for Topeka Shiners and a range of other fishes and terrestrial wildlife. However, a greater understanding of what characteristics of restored and unrestored oxbows influence the presence of Topeka Shiners is needed. Furthermore, Topeka Shiners and other species of conservation need can only populate oxbows if they first are present in the adjacent stream channel. Thus, a greater understanding of characteristics that influence the presence of several of Iowa and Minnesota’s rare fish species in streams is needed.
I used electrofishing and seining to sample fish assemblages at stream sites, whereas I only used bag seines to sample the fish assemblage at oxbow sites. At each site, I also measured dozens of potentially influential abiotic (habitat and water quality) characteristics. Examining streams and oxbows separately, I then used nonmetric multidimensional scaling and logistic regression models to determine which predictor variables were most important for the presence of Topeka Shiners and other species of greatest conservation need (SGCN). In 2016-2017, 111 stream sites and 98 oxbows were sampled in the Boone River, Beaver Creek, North Raccoon River, and Rock River watersheds. Topeka Shiners were present at 40 stream sites and 13 SGCN were sampled but only six were common enough (present at 20-67% of sites) to include in statistical modeling. The presence of four of six SGCN was positively associated with species richness, and three species were associated with either wetted width and/or gravel substrate. In general, important variables for predicting species presence in streams varied across models and species. Topeka Shiners were present at 40 oxbows, being sampled more often and with a higher average relative abundance in restored oxbows compared to unrestored oxbows. Logistic regression models indicated that Topeka Shiner presence was positively associated with species richness, Brassy Minnow Catch Per Unit Effort (CPUE), Orangespotted Sunfish CPUE, dissolved oxygen, and turbidity in an oxbow, while also being negatively associated with oxbow wetted length. The results of this project add to our understanding of associations of rare fish species and will be used to help guide the restoration process for Topeka Shiners and other SGCN.