Noturus exilis Nelson, 1876
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Basis for Listing
The Slender Madtom (Noturus exilis), also known as the Slender Stonecat, reaches its known northern range limit in southern Minnesota. The first verifiable record of this species in the state was based on three specimens collected from Otter Creek in Mower County in 1954. Specimens were collected south of the Minnesota state line in Otter Creek (Mitchell County, Iowa) in 1990 (Schmidt 1991). Additional specimens were collected in the Minnesota reach of Otter Creek in 1991 and 2008 (Schmidt 2012). Given its extreme rarity in the state, the Slender Madtom was listed as a special concern species in Minnesota in 1984. At that time, information on distribution, abundance, preferred habitats, and life history was limited. In 2013, following the completion and analyses of targeted surveys, the status of the Slender Madtom was elevated to endangered.
The Slender Madtom is a thin catfish that is 7-13 cm (3-5 in.) in total length. It is brown or black on the sides with a white belly, and its anal, dorsal, and tail fins have a black edge. It has a blunt snout with 2 short barbels pointing upwards from the nostrils as well as 4 barbels protruding from the underside of the chin. Its jaws are sub-equal with the upper jaw slightly longer than the lower, and it has numerous small teeth in broad bands. The Slender Madtom has an adipose fin that is a low keel-like ridge, not free and flap-like. The 5-8 barbs on its pectoral hard rays distinguish it from the very similar Stonecat (Noturus flavus) and Tadpole Madtom (N. gyrinus) (Eddy and Underhill 1974; Lyons et al. 2012).
Across its range, the Slender Madtom is found in riffles of small- to medium-sized permanent spring-fed creeks with moderate to swift currents. Its preferred bottom substrates include limestone slabs, rubble, or gravel interspersed with sand. The species usually stays near or under sheltering rocks in riffles at depths of less than 30 cm (12 in.) or sometimes under the cover of leaf litter in pools (Vives 1987). In Illinois, young Slender Madtoms favored shallow riffles while adults favored pools, except during the breeding season when adults swam into riffles (Mayden and Burr 1981). The only site in Minnesota where the Slender Madtom has been found is a small steam (tributary to the Cedar River).
Biology / Life History
There have been no studies of the species' life history in Minnesota. Elsewhere in its range, the Slender Madtom breeds in spring and summer in cavities under rocks (Hatch et al. in preparation). Both sexes usually mature in 2-years, though Vives (1987) suggested females can spawn in their first summer, if a critical minimum size is reached. Slender Madtoms are primarily nocturnal, spending their days seeking cover. Their diet includes caddisflies, midgeflies, other insects, and filamentous algae.
Conservation / Management
Populations across the Slender Madtom’s range have declined since the late 1970s. The species is nearly extirpated in Wisconsin, though once more widespread there than in Minnesota (Lyons et al. 2000). Causes for decline may include siltation and turbidity in farming areas and the dewatering of habitats by hydropower operations. Predation and both intraspecific and interspecific competition may also affect the survivorship of Slender Madtoms (Vives 1987). Additional research needs for this species in Minnesota include life history studies, genetic analysis, identification of habitat guilds, and the determination of specific habitat impacts and stressors.
Conservation Efforts in Minnesota
Stream surveys in 1985 and 1990 failed to discover additional Slender Madtom records in Minnesota, but the species was found two miles south of the Minnesota-Iowa border. A 1991 survey collected two specimens from Otter Creek in Mower County. Because of the small number of known locations of Slender Madtoms in Minnesota, and because the species is difficult to distinguish from Tadpole Madtoms (Noturus gyrinus) and Stonecats (N. flavus), a permit is required to take any madtoms or stonecats in Dodge, Freeborn, and Mower counties.
The recent inception of Minnesota’s Clean Water Legacy Program will eventually yield benefits to Slender Madtom habitats through nutrient and sediment load reductions.
References and Additional Information
Eddy, S., and J. C. Underhill. 1974. Northern fishes, with special reference to the Upper Mississippi Valley. Third edition. University of Minnesota Press, Minneapolis, Minnesota. 414 pp.
Lyons, J., P. A. Cochran, and D. Fago. 2000. Wisconsin fishes 2000: status and distribution. University of Wisconsin Sea Grant Institute, Madison, Wisconsin. 87 pp.
Mayden, R. L., and B. M. Burr. 1981. Life history of the Slender Madtom, Noturus exilis, in southern Illinois (Pisces: Ictaluridae). Occasional Papers of the Museum of Natural History 93, University of Kansas, Lawrence. 64 pp.
NatureServe. 2015. NatureServe Explorer: an online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. <http://www.natureserve.org/explorer>. Accessed 19 May 2016.
Phillips, G, L. American Eel Anguilla rostrata (Lesueur, 1817). In J. T. Hatch, G. L. Phillips, K. P. Schmidt, and M. McInerny, editors. The Fishes of Minnesota (in preparation).
Schmidt, K. 2012. NANFA Members Search for Minnesota's Rarest Fishes. American Currents, 37(4) 2-7: , Fall 2012.
Schmidt, K. P. 1991. Stream survey results for the Slender Madtom (Noturus exilis), Crystal Darter (Ammocrypta asprella) and Bluntnose Darter (Etheostoma chlorosomum) in southeastern Minnesota. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 12 pp.+ appendices.
Vives, S. P. 1987. Aspects of the life history of the Slender Madtom, Noturus exilis, in northeastern Oklahoma (Pisces: Ictaluridae). American Midland Naturalist 117:167-176.