Genetic Evaluation of Supplementation-Assisted American Shad Restoration in the James River, Virginia

Abstract: Hatchery supplementation programs have been implemented for several populations of American Shad Alosa sapidissima, which are declining across the species’ native range due to disrupted access to spawning grounds, habitat degradation, and overfishing. The genetic impacts of stocking Pamunkey River-origin larvae into the James River American Shad population since 1994 were investigated, and the effects were considered within a regional context by including American Shad populations from other Chesapeake Bay tributaries that also received interbasin stockings from various rivers over the same period. Levels of genetic diversity for microsatellite markers were high in all populations except the Susquehanna River population, which showed a significant decline in diversity between the 1990s and 2007. Before supplementation of James River American Shad, the James and Pamunkey River populations exhibited subtle standardized differentiation among groups (F′_CT = 0.012), whereas differentiation was reduced after supplementation (F′_CT = 0.007), indicating that supplementation contributed to homogenization of population structure within the two rivers. Chesapeake Bay tributaries also displayed higher levels of differentiation in the 1990s (F′_CT = 0.063) than in contemporary, supplemented samples (F′_CT = 0.004). Bayesian analyses of population structure among 1990s Chesapeake Bay samples only identified the Susquehanna River as having a distinguishable population, and no population structure was detected among samples collected in the late 2000s. In light of the fact that Chesapeake Bay American Shad populations are not rebounding in response to supplementation, our observation of reduced genetic differentiation among populations is a likely signal of substitution by hatchery-origin fish rather than increasing natural recruitment. As such, spawning habitat improvement in conjunction with continued baywide fishing regulation may be a more beneficial strategy for restoring viable American Shad populations than continued reliance on supplementation.

Received June 5, 2013; accepted February 3, 2014