Abundance of Gulf sturgeon in the Apalachicola River, Florida

Gulf sturgeon Acipenser oxyrinchus desotoi, a subspecies of Atlantic sturge on A. oxyrinchus, were once abundant in coastal rivers of the eastern Gulf of Mexico but have declined substantially due to habitat loss and overexplo itation. Because relatively little is known about their population status i n the Apalachicola River, Florida, we used capture-recapture data collected during 1982-1991 to assess the population of Gulf sturgeon at the Jim Wood ruff Lock and Dam, which limits upstream migration. Wt, estimated that abou t 100 fish greater than 45 cm total length were present below the dam, alth ough the estimates were biased to an unknown degree because of violations o f capture-recapture model assumptions. To obtain a less biased estimate, we conducted an intensive 10-week capture-recapture experiment in 1993 that a ccounted for most of the assumption violations. We also used radiotelemetry to test the assumption that the population remained closed to immigration and emigration during sampling. The 1993 results also indicated a populatio n of about 100 Gulf sturgeon below the dam; however, movement in and out of the sampling area occurred, so the population at the dam was not closed. U sing simulation, we found that Jolly-Seber and Schnabel capture-recapture m odels were generally unbiased when fish had a high probability of returning to the study area after temporary emigration. However when fish had a low probability of returning to the study area after temporary emigration, subs tantial bias occurred in both models. Length composition data from 1982 to 1991 and for 1993 suggested that low recruitment may account for the failur e of the population to rebuild. We recommend identifying all areas of conce ntration of Gulf sturgeon in the Apalachicola River and developing a standa rdized sampling program for monitoring this threatened species.