Recruitment of Fusconaia ebena (Bivalvia: Unionidae) in Relation to Discharge of the Lower Ohio River

Abstract

Demographically complete sampling of a large population of Fusconaia ebena (Lea) in a mainstream shoal in the lower Ohio River (LOR) from 1983 through 1998 revealed two extremely successful recruitment years—1981 and 1990. Dominance of the 1981 and 1990 cohorts allowed length-to-age relationships to be estimated directly from length-frequency histograms. Two linear relationships adequately described growth rates from age 2 through 17 y. The first model applied to ages 2 through 10 y when annual growth averaged 6.1mm. The second model applied to ages 10 through 17 y when annual growth averaged only 1.1 mm. A survivorship curve was based on density of the 1981 cohort from age 2 through 17 y. During that period a constant proportion (17%) of the cohort died each year. Only 9% of the 1981 cohort alive in 1983 were still alive in 1998. In both 1981 and 1990 rapid and large spring rises in LOR discharge were immediately followed by rapid and large declines. These rises coincided with the expected spawning peak of Alosa chrysochloris, the only known fish host for F. ebena glochidia. The rapid return to low flow and depositional conditions was appropriately timed to enhance successful settlement of juvenile F. ebena after their parasitic stage on A. chrysochloris gills.