Environmental Sex Determination in Southern Brook Lamprey,Ichthyomyzon gagei

Sex ratios were determined for 20 populations of southern brook lamprey larvae, Ichthyomyzon gagei, collected from throughout much of their range in the southeastern United States. Ratios varied between 9 and 49% males. Differential mortality was an unlikely factor, as sex ratios were similar among age groups within each population. Environmental sex determination is suggested. Sex ratio varied with growth (expressed as length of larvae at specific ages), larval density, pH, and annual mean temperature of the natal stream. Generally, under conditions promoting rapid growth, the percentage of males varied directly with density and inversely with temperature. Where growth was slow, the percentage of males declined as larval density increased, the response being less at low than at high pH. Temperature had little effect when larval growth was slow. The percentage of males declined when growth was rapid under otherwise similar environmental conditions.