Population Dynamics of a Migration Limited Shortnose Sturgeon Population

Upstream migration of the Cooper River, South Carolina, population of shortnose sturgeon (Acipenser brevirostrum) is severely limited at Pinopolis Dam (river km 77). Shortnose sturgeon congregating and spawning in the tailrace of the dam has been documented but data is lacking on the group's population dynamics. From 1995 through 1999, the population was sampled in the tailrace with 91.4 m long bottom-set gillnets during the spawning season when fish were concentrated in the area. Fish captured during 1996-1998 were tagged for a multiple census mark-recapture population estimate using the modified Schnabel method. Most captured fish were measured and weighed, and the data were fit to a weight-length regression. Pectoral fin rays were removed from a sub-sample of 35 fish for determination of the spawning population's age structure. Length-at-age data were then used to develop a von Bertalanffy growth equation. Population viability was modeled with the MOCPOP 2.0 age-structure model. Overall, 220 adult shortnose sturgeon were captured in the Pinopolis Dam tailrace from 28 January through 30 March when water temperature ranged from 8.0 to 17.8 C. The average catch per gillnet-hour was 6.8 fish. Mark-recapture population estimates ranged from 87 to 301 fish. Observed total lengths ranged from 705 to 1,164 mm (x¯ = 938 mm, SD = 85.6) and observed weights ranged from 2.68 to 19.36 kg (x¯ = 7.02 kg, SD = 2.83). Ages ranged from 5 to 18 yr. Estimated annual instantaneous mortality was 0.148. The population model reached equilibrium when annual recruitment was approximately 50 age-1 fish. The population may be depressed to an unsustainable level as a result of poor recruitment that is linked to obstructed migration to favorable spawning habitat.

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