Abstract
AbstractAmerican shad Alosa sapidissima are in decline in their native range, and modeling possible management scenarios could help guide their restoration. We developed a density‐dependent, deterministic, stage‐based matrix model to predict the population‐level results of transporting American shad to suitable spawning habitat upstream of dams on the Roanoke River, North Carolina and Virginia. We used data on sonic‐tagged adult American shad and oxytetracycline‐marked American shad fry both above and below dams on the Roanoke River with information from other systems to estimate a starting population size and vital rates. We modeled the adult female population over 30 years under plausible scenarios of adult transport, effective fecundity (egg production), and survival of adults (i.e., to return to spawn the next year) and juveniles (from spawned egg to age 1). We also evaluated the potential effects of increased survival for adults and juveniles. The adult female population size in the Roanoke River was estimated to be 5,224. With no transport, the model predicted a slow population increase over the next 30 years. Predicted population increases were highest when survival was improved during the first year of life. Transport was predicted to benefit the population only if high rates of effective fecundity and juvenile survival could be achieved. Currently, transported adults and young are less likely to successfully out‐migrate than individuals below the dams, and the estimated adult population size is much smaller than either of two assumed values of carrying capacity for the lower river; therefore, transport is not predicted to help restore the stock under present conditions. Research on survival rates, density‐dependent processes, and the impacts of structures to increase out‐migration success would improve evaluation of the potential benefits of access to additional spawning habitat for American shad.Received March 19, 2011; accepted August 22, 2011
Highlights
U.S Geological Survey, North Carolina Cooperative Fish and Wildlife Research Unit, Department of Biology, North Carolina State University, 127 David Clark Labs, Campus Box 7617, Raleigh, North Carolina 27695-7617, USA
American shad in the Roanoke River.— Water flow in the Roanoke River is regulated by six dams, with the most downstream one located in Roanoke Rapids, North Carolina, at river kilometer 221 (Rulifson and Manooch 1990; Walsh et al 2005; Figure 1)
Population Parameters The estimated number of adult female American shad in the Roanoke River between 2002 and 2008 ranged from 1,965 in 2006 to 8,449 in 2004, with an average of 5,224 individuals (Figure 3)
Summary
We developed a density-dependent, deterministic, stage-based matrix model to predict the population-level results of transporting American shad to suitable spawning habitat upstream of dams on the Roanoke River, North Carolina and Virginia. Density-dependent processes, and the impacts of structures to increase out-migration success would improve evaluation of the potential benefits of access to additional spawning habitat for American shad. Mortality due to downstream passage through dam turbines could cause population reductions, whereas access to additional spawning habitat could lead to higher production, which would lead to population increases The balance between these negative and positive factors is likely to be system specific and determines whether access to upstream habitats would benefit the population in question
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