Effects of Saltwater Intrusion and Flow Diversion on Reproductive Success of Striped Bass in the Savannah River Estuary

Abstract

Spawning by striped bass Morone saxatilis in the Savannah River estuary was monitored in 1986–1989 to determine effects of elevated salinity and altered channel morphology on reproductive success. A tide gate and its attendant channel modifications displaced the salt wedge (salinity >0.5‰) upriver and altered the pathways and rates of water flow in the estuary. Average densities of striped bass eggs at sites sampled in 1989 were 0.5–11.0% of levels reported in the late 1970s. All recently spawned eggs (< 10 h old) were found where salinity was 0.5‰ or less, an indication that adults select freshwater sites for spawning. A hypothesis of consistent recruitment failure in the 1980s is supported by parallel declines in abundance of eggs and adults on the spawning grounds, the lack of any detectable upriver displacement of spawning activity, and the low abundance of eggs in 1988–1989 despite the freshwater conditions prevalent at the historic spawning sites. Studies that used floats to simulate rates and pathways of egg dispersal after spawning suggested that rapid transport to areas with salinity levels harmful to eggs and larvae was responsible for reproductive failure. Restoration of conditions suitable for successful reproduction would include maintenance of freshwater on the spawning grounds and prevention of the rapid flushing of eggs and larvae into high-salinity areas.