The slender madtom is a small, slender catfish with the rear margin of the tail fin rounded or squarish, and the adipose fin forming a low, keel-like ridge, rather than a free, flaplike lobe. In the western and northern Missouri Ozarks, it is the most common madtom in small and medium-sized streams that have gravel bottoms, clear water, and permanent flow. It is scarce in the southern Ozarks, however.
Madtoms, as a group, are small, secretive catfishes that most people never see. The key identifier for madtoms has to do with the adipose fin (the small, fleshy fin that is present on the midline of the back just ahead of the tail fin). In madtoms, the adipose fin forms a low, keel-like ridge without a free, flaplike lobe along the trailing edge. The adipose may be connected to the tail fin, or it may have (at most) a slight notch in between. (In our other catfishes, the adipose fin forms a free, flaplike lobe, widely separate from the tail fin.)
The slender madtom may be identified by the following: the pectoral spine has well-developed sawlike teeth along the rear margin. The dorsal, tail, and anal fins are prominently dark-edged or at least darkest along the outer margins; otherwise, the body and fins are nearly plain, without definite dark blotches, bars, or speckles. The upper jaw does not project beyond the lower jaw (the two are nearly equal). The tooth pad on the upper jaw lacks backward extensions. The notch between the adipose fin and the tail fin is closer to the tip of the tail fin than to the dorsal fin base.
The back and sides are yellowish brown, often faintly mottled with darker brown. The underside of the head and body is white or pale yellow. The dorsal, tail, and anal fins have a prominent black fringe in populations from the southern Ozarks; in other Missouri populations, the black fringe is variably developed and is sometimes indistinct. The fins are otherwise yellowish or creamy white.
Most madtoms possess a mild venom that is associated with the pectoral and dorsal spines. When introduced into a puncture wound produced by the spine, the venom causes a painful reaction. The spines are often erected and locked in place when the madtom is alarmed, increasing the chance of a puncture. The venom is not considered dangerous to people, and the chances of being “spined” are not great if the possibility is kept in mind when handling a madtom. If you’ve been jabbed by a madtom spine and think you’re having a severe reaction, seek medical attention.
Similar species: In shape and coloration, the stonecat (N. flavus) and freckled madtom (N. nocturnus) are most similar, but both of them have the upper jaw projecting well beyond the lower jaw. The tadpole madtom (N. gyrinus) also does not have distinct dark body blotches, bars, or saddle marks, and its lips appear even (lacking the “overbite”), but it is a much chubbier fish and lacks dark edges to the fins.
There are about 30 species of madtoms (in the genus Noturus), and all occur in the central and eastern United States and nearby parts of Canada. In Missouri, 10 species of madtoms have been recorded. It can be difficult to separate the different species of madtoms using the traditional methods of fish ID (counting fin rays, for instance, or comparing ratios of body-part measurements). Noting differences in pigmentation (such as dark bars or patches) can help, but such coloration often varies by particular locality and habitat (such as amount of vegetation, turbidity, or different substrates). Color can also vary by a fish’s health, mood, breeding condition, sex, and individual genetics, and dead fish may show little coloration at all. Molecular (DNA) date is being used more and more as a way to separate the species; of course, it is not very useful in the field. Geography can be a good clue for species IDs, since different species may be restricted to certain stream systems and never occur in others.