Gulf Sturgeon Ecological Investigations Active

The Science Issue and Relevance: The Gulf sturgeon, Acipenser oxyrinchus desotoi, has been federally 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, Florida, to help fill gaps in our knowledge of the species. These gaps include population abundance, spawning location and timing, recruitment under varying river conditions, foraging locations, impacts of changing climate on sturgeon populations, and niche partitioning among multi-species assemblages of large marine vertebrates. The Gulf sturgeon research program currently focuses on assessing the population and its relationship with environmental parameters. Additional data collected through captures, tagging, tracking, and hydrologic monitoring will lead to a better understanding of their habitat use and life history and will provide natural resource managers with data to guide effective decision-making and restoration, and conservation strategies.

Methodology for Addressing the Issue: To address these data gaps, we are using a wide range of tools and techniques. We have maintained a tagging program since 1986, capturing fish in a wide variety of net types and tagging them with T-bar and PIT tags. Spawning grounds have been verified using egg collecting mats. To document movements, we initially used pop-off satellite tags and active acoustic telemetry but migrated to passive acoustic 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, Florida, to Lake Pontchartrain in Louisiana.

Currently, we maintain a tagging program for mark-recapture population analysis that includes 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 Gulf sturgeon, with more than 13,000 observations of 9,400 individuals. We use mark-recapture models to estimate survival and recapture probabilities over time. Recent (2014) 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, 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 as such we will continue mark recapture studies. In addition, we will continue our participation in a multi-agency tracking and mark-recapture study to monitor sturgeon populations across the northern Gulf of Mexico following the Deepwater Horizon oil spill. Sturgeon forage in areas that are also important to other imperiled marine vertebrates such as sea turtles and manatees. A new focus of the program will be to utilize our long-term mark-recapture and acoustic tracking database to examine multi-species foraging areas in the northern Gulf of Mexico and assess how changing environmental variables such as temperature and red tide events impact these populations.