CHARACTERISTICS OF DISPERSAL IN THE EASTERN MOSQUITOFISH GAMBUSIA-HOLBROOKI

The demographic and genetic characteristics of dispersing Gambusia holbrooki were examined relative to changes in stream flow velocity, flow direction, and rapid saltwater inundation. Dispersal of all G. holbrooki sex classes decreased as Bow velocity increased, dispersal for a given Bow velocity being greater in female, than male and juvenile fish respectively. Juveniles and males dispersed at low (9 mm s(-1)) but not high (109 mm s(-1))emales dispersed at both flow rates. Dispersing adults were not larger than non-dispersing adults of the same sex. Direction of dispersal was sex and size dependent, but independent of flow velocity. Larger females (>22mm) moved preferentially downstream, while smaller females (17-22mm) remained stationary or moved in the upstream direction. Males dispersed equally in both directions. No relationship was found between dispersal characteristics and the presence of high salt concentrations. Movement patterns at high flow were independent of glucose-6-phosphate isomerase (GPI-2(*)) alleles carried by dispersing individuals. significant non-random dispersal of sex or size cohorts, and the interaction of dispersal with flow velocity have significant implications for the maintenance of genetic diversity in stream ecosystems, particularly where localized selection or genetic drift may after the genetic constitution of specific sex or size cohorts. The applicability of these results to field populations is discussed.