The Southern Redbelly Dace, Phoxinus erythrogaster (Rafinesque), in Clark County, Illinois

Population and feeding ecology of the southern redbelly dace, Phoxinus erythrogaster (Rafinesque), was studied at Rocky Branch Nature Preserve, sections 29 and 30, T12N, R12W in Clark County, Illinois. Three stations designated along the eastern portion of Rocky Branch stream were seined bimonthly July through September, 1987. The length/frequency distribution curve constructed for the 358 dace collected determined Age class 0 to be dace less than 39mm, age classes I and II were indistinguishable because of overlap of fish lengths, and age class III was determined to be dace between 69 and 74mm. Other species encountered in abundance during this two month period were Ericybma buccata, Semotilus atromaculatus, Campostoma anomalum, Etheostoma nigrum, and Rhinichthys atratulus. Thirty-five southern redbelly dace were collected for age and sex determination and for a food analysis study. Scales and otoliths were collected from the dace, annuli were read, and dace were then sexed based on pectoral fin length. No more than a 2 mm difference existed between the two age-determination methods for age classes I and II. Males averaged smaller than females for both age classes I and II. Age class III otolith results more closely matched length/frequency than did scales, suggesting this may be a more accurate method for aging older dace. Eight southern redbelly dace intestines were examined for diet analysis. Thirteen out of the 15 most common food organisms belonged to the division Bacillariophyta. The diatom genus Navicula, along with many other known epipelic genera were found to be the most abundant food organisms, suggesting the dace feeds largely on bottom debris. Gomphonema, Cocconeis, and several other genera found in the diet were known to be epiphytic or epilithic, suggesting the dace graze on rocks and filamentous algae. The other diatom genera found in the diet were known plankters. These, along with the presence of Pediastrum, suggest the dace consume plankton, if only during respiration. Analysis of plankton samples taken prior to seining on 10 September, 1987 indicated that Navicula, Amphora, and Nitzschia were the most common algal genera. Because sampling was done in shallow water where the stream bottom was soft, it is thought that these epipelic organisms enriched the plankton samples. Ranked fourth was Ankistodesmus. Because this truly planktonic organism was not found in any of the intestines examined, it is concluded that plankton is not an important part of the diet