Gulf Sturgeon Ecological Investigations

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The Gulf sturgeon, Acipenser oxyrinchus desotoi, has been listed as Threatened since 1991. Beginning in 1986, USGS has been investigating sturgeon population abundance and ecology throughout its range, but mostly in the Suwannee River.

Weighing sturgeon prior to tagging

Weighing sturgeon prior to tagging

The Science Issue and Relevance: The Gulf sturgeon, Acipenser oxyrinchus desotoi, has been listed as Threatened since 1991. Beginning in 1986, USGS has been investigating sturgeon population abundance and ecology throughout its range, but mostly in the Suwannee River. Initially, fairly little was known about the life history. USGS has been working over the years to fill in the unknowns, ranging from the size of the population, where and when they spawn, what river conditions lead to successful recruitment, and where juveniles and adults feed during the winter. Unknowns about the Gulf sturgeon currently under investigation include the sex ratio of the population, the size and structure of the population, and the environmental parameters leading to selection of habitat by fish during the summer period. Additional data through captures, tagging, tracking, and hydrologic monitoring will lead to better understanding of their habits and life cycle and will give natural resource managers information for effective decision-making, restoration, and conservation.

Sturgeon are carefully weighed and measured

Sturgeon are carefully weighed and measured

Methodology for Addressing the Issue: To address these unknowns, we are using a wide range of tools and techniques. We have maintained a tagging program since 1986, capturing fish in a wide range of net types and tagging them with t-bar and PIT tags. Spawning grounds have been verified using egg collecting mats. To document movement, initially we used pop-off satellite tags and active sonic telemetry, but migrated to passive sonic telemetry when that technology matured. Estuarine use by juveniles was investigated by deploying an array of 54 receivers in Suwannee Sound, while offshore use by adults was investigated by a multi-agency/multi-investigator cooperative research project utilizing an array of 148 receivers distributed from Cedar Key to Lake Pontchtrain.

Researchers pulling nets with captured sturgeon

Researchers pulling nets with captured sturgeon

Currently, we are still maintaining a limited tagging program to keep a pool of tagged fish in the population for mark-recapture population analysis. We also maintain a pool of active acoustic tags in the population, adding new tags as the battery life expires on older ones. We maintain and utilize an array of telemetry receivers along the Suwannee River, from the river mouth to the spawning grounds. The configuration of the array changes annually depending on monitoring needs. We maintain a database of marked fish, with more than 13,000 observations of 9,400 individuals. We use mark-recapture models to examine survival and recapture probabilities over time. Recent analyses indicate that the population has increased from approximately 2,200 net-vulnerable individuals in 1986, of which only 30% were reproductive adults, to approximately 9,500 net-vulnerable individuals currently, of which 60% are reproductive adults. Analyses also show that the population size has been essentially static since 1996, indicating that the Suwannee River population may be at carrying capacity.

Future Steps: The Suwannee River is an index river under the Gulf Sturgeon Recovery Plan, and will undergo mark-recapture sampling for three consecutive years, then two years with no m-r sampling in a five year rotation. We are experimenting with the use of side scan sonar technology to generate an abundance index for use in population estimation that can be more easily obtained than mark-recapture sampling. Telemetry data collected from 2012-2015 are being used for occupancy modelling during summer residency. This information, in addition to side scan sonar imagery, will give natural resource managers insight into where and why Gulf sturgeon jump and may allow for an improved and predictive risk warning system during jumping activity.