Life history of the least madtom Noturus hildebrandi ( Siluriformes: Ictaluridae) with comparisons to related species.

Aspects of reproduction, age, growth, development and food habits of 403 specimen's of Noturus hildebrandi were studied from collections made in the North Fork Obion River, Tennessee. The least madtom lived a maximum of 18 months and reached a maximum SL of 52 mm; 12-month-old individuals averaged 39 mm. Individuals grew at a decreasing rate in SL and at a constant rate in body weight. Growth rates were highest from hatching to the 1st month and decreased steadily to only 1 % during the reproductive season. Both sexes matured at ca. 35 mm SL and spawned in June and Jul\ of their second summer. Females produced 17-38 (x = 29.9) mature oocvtes (2.3 mm diam) and had a mean relative fecundity (30.0 oocytes/g adjusted body weight) higher than most other ictalurids. Large females spawned earlier than smaller females and produced more oocvtes. Six clutches of captive-spawned embryos averaged 19.2; chorionic diam ranged from 3.1-3.8 mm (x = 3.4) and yolk diam ranged from 2.6-3.2 mm (x = 2.9). Mesolarvae hatched in 193-209 hr at 25 C and averaged 6.3 mm TL (range 5.6-6.6 mm). Principal food items were larvae of midges, caddisflies, stoneflies and mavflies. Least madtoms were parasitized by nematodes and flukes. INTRODUCTION The least madtom Voturus hildebrandi, prior to the discovery of the pygmy madtom Noturus stanauli (Etnier and Jenkins, 1980), was the smallest known ictalurid. Knowledge of its biology consists only of comments by Cook (1959), Taylor (1969), Burr and Mayden (1979) and LeGrande (1981) on habitat, species associations and relationships to other madtoms. Bailey and Taylor (1950) speculated that N. hildebrandi was related to N. elegans. Taylor (1969) later viewed the least madtom as a composite of two subspecies, closely related to N. baileyi of the upper Tennessee River System. These two species comprised the N. hildebrandi species group and formed the sister taxon to the N. elegans species group composed of N. albater, N. elegans, N. trautmani and N. taylori (Douglas, 1972). Etnier and Jenkins (1980) placed N. stanauli in the hildebrandi group. LeGrande (1981) related the least madtom to N. elegans and N. albater. The least madtom inhabits small eastern tributaries of the Mississippi River below the fall line from the Obion River drainage of Kentucky (Burr and Mayden, 1979) to the Homochitto River System of southwestern Mississippi (Bailey and Taylor, 1950; Taylor, 1969). The subspecies discussed herein, Noturus hildebrandi lautus, occurs from the North Fork of the Obion River to the Hatchie River Svstem in southwestern Tennessee and northern Mississippi. Besides the need for basic ecological data on warmwater fishes (Larimore, 1981; Mavden, 1983), life history data on Notorus hildebrandi are important for interspecific comparisons and interpretations of the ecologies of madtoms. Noturus baileyi, N. stanauli and A'. trautmani, species allied to N. hildebrandi, are now rare; knowledge of the biology of the least madtom max help in determining why these species are threatened with exPresent address: Department of Zoology, University of Florida, Gainesville 32611.