Noturus exilis, also called the slender madtom, is a species of the catfish family Ictaluridae. Ictaluridae includes bullheads, madtoms, channel catfish, and blue catfish. Noturus exilis is found in the central portion of the Mississippi River basin, but is most abundant in Ozarkian streams. Slender madtoms occur west of the Mississippi River in the Ozarks of Oklahoma, Arkansas, and Missouri north to southern Wisconsin and Minnesota. It also occurs east of the Mississippi River in the uplands of Alabama, Tennessee, and Kentucky in the Tennessee, Cumberland, and Green drainages. Nelson first described Noturus exilis in 1876. The slender madtom is moderately large with a terminal to sub terminal mouth, flat head, small eyes, and black marginal bands on the median fins. Most slender madtoms are less than . Noturus flavus and Noturus nocturnus are rather similar in shape and coloration to Norturus exilis. Slender madtoms inhabit small to medium-sized streams, in riffle and flowing pool habitats with coarse gravel to slab rock substrates. The presence of a shelter object, such as a large rock, seems to be important in habitat selection.
Distribution
Slender madtoms inhabit two disjunct areas of the Central Highlands: one in the Ozark Highlands ranging from eastern Kansas and Oklahoma to the southern tip of Illinois and including most of Missouri and northwest Arkansas, and another in the Eastern Highlands, from middle Tennessee and part of southeastern Kentucky, ranging into northern Alabama and Mississippi. The slender madtom also occurs as several smaller, isolated populations in Iowa, Illinois and southern Wisconsin and Minnesota. Slender madtoms have seen a decrease in population size from their previous habitat ranges. Due to habitat alteration along the small streams of the Tennessee drainage, this species may be in great danger of extirpation from Mississippi. It has also declined in the northern part of its range in Wisconsin, due primarily to agricultural runoff or alterations in river flow due to dams. The slender madtom's range has mostly likely decreased due to dam construction, channelization, and agricultural runoff.
Ecology
The slender madtom's diet consists of midges, mayflies, caddisflies, isopods, amphipods, and copepods, but ephemeroteran naiads and chironomid larvae make up a majority of their diet. Smallmouth bass (Micropterus dolomieu) is a known predator of slender madtoms, which may be one reason why slender madtoms prefers a nocturnal lifestyle, seeking cover during the day when this visual predator is on the prowl. Embryo predation by Orconectes virilis (virile crayfish), Campostoma anomalum (central stoneroller) and Etheostoma caeruleum (rainbow darter) has also been noted. The slender madtom and the orangethroat darter (Etheostoma spectabile) also compete for and share the same diet. The slender madtom inhabits the rocky riffles and pools of creeks and small rivers, spending much of the day hiding under large rocks and emerging to feed at night. Slender madtoms feed preferably after dusk and before dawn. Slender Madtoms have also been observed in high percentages in vegetated patches and backwaters during late summer. The slender madtom can be found in streams with current speeds from 8–92 cm/s and can be found at depths between 5–42 cm. The slender madtom has never been a strong swimmer, which has reduced their dispersal over the centuries.