Bigmouth shiner (Notropis dorsalis) COSEWIC assessment and status report: chapter 10
Limiting Factors
Tompkins (1987) concluded that the bigmouth shiner can tolerate little human disturbance. Eutrophication from shoreline development in Oneida Lake, New York was the likely cause of decreased shiner populations (Clady 1976). However, it appears that the bigmouth shiner has actually benefited from human disturbance to stream hydraulics throughout settlement of North America. As mentioned previously, Pfleiger (1971) suggested that channelization of prairie streams has created habitat conditions in Kansas that have become favourable for the bigmouth shiner, and Scarnecchia (1988) found that bigmouth shiner populations were significantly higher in channelized sections than in unchannelized sections. Additionally, besides being the most widely distributed and abundant minnow in Iowa, it has been suggested that the bigmouth shiner is becoming more widely distributed as habitats are changed into shallow streams with homogeneous width, depth and current. The bigmouth shiner is one of a few species found over the seemingly sterile sand flats that are common in medium and smaller Iowa streams (Iowa Department of Natural Resources Web Site 2003).
Starrett (1951) also found that the bigmouth shiner was less susceptible to periods of flooding than were co-existing species, as it moved from small streams into the Des Moines River, Iowa to avoid becoming isolated in backwaters. No changes in abundance were observed. Reproduction might be affected by high water levels in spring, as preferred habitats, food sources, and spawning sites could be affected. Fedoruk (1970) suggested that the high turbidity levels in the Red River had likely prevented further dispersal in Manitoba. Since the original status report, the bigmouth shiner has been found in many areas within the Assiniboine River watershed. Depending on the timing of the dispersal into these areas, the bigmouth shiner might have had to use the Red River as a dispersal route to enter the Assiniboine River system. Thus, unless the turbidity levels in the Red River were lower during the period of dispersal than they are at present, the bigmouth shiner might be more tolerant of turbid water than previously observed, at least for dispersal purposes. Stewart pers. comm., state that the bigmouth shiner in Manitoba can be found in moderately turbid to turbid water.
In Ohio, decline in some bigmouth shiner populations has been attributed to competition with the invasive silverjaw minnow (Notropis buccatus) (Trautman 1981). No such recent invading cyprinids into the Hudson Bay watershed share habitat as closely with the bigmouth shiner as does the silverjaw minnow. The spotfin shiner (Cyprinella spiloptera), with which the bigmouth shiner forms an assemblage along with the sand shiner and emerald shiner (Notropis atherinoides) in Wisconsin (Mendelson 1975), was first collected from the Roseau River in 1988, and had dispersed to the tailrace of the Portage Spillway on the Assiniboine River at the town of Portage la Prairie by July 1990 (Appendix 1 in McCulloch and Franzin 1996). The Portage Spillway, which will most likely be a barrier to spotfin shiner dispersal upstream in the Assiniboine River, is downstream of all of the reported bigmouth shiner collection locations on the Assiniboine River, and downstream of the mouth of the Cypress River. The spotfin shiner prefers moderate to large rivers of moderate to high turbidity, where it is found over sand, gravel or rubble substrates (Gilbert and Burgess 1980). Where the two species co-occur with sand and emerald shiners in Wisconsin, Mendelson (1975) found that the bigmouth shiner was the most responsive to both con- and heterospecifics. Each species appeared to coexist by utilizing different morphological adaptations.
Two large-scale land uses that occur within the bigmouth shiner’s present distribution in Manitoba are agriculture and forestry. The presence of livestock in watercourses can result in accelerated bank erosion and increased siltation, both of which could be harmful to fish habitat. In forestry operations, several stream crossings can be installed every year depending on a company’s annual operating plan. Poorly installed culverts can prevent fish passage and increase siltation into the watercourse either directly, or from sediment runoff from road infrastructure.