UW Aquatic & Fishery Sciences Quantitative Seminar

Merrill Rudd

PhD Student / IGERT Fellow, SAFS, UW

Estimating demographic parameters for an updated Gulf sturgeon stock assessment

Abstract

Gulf sturgeon (Acipenser oxyrinchus desotoi) are an anadromous species that were commercially fished throughout the northeastern Gulf of Mexico in the early 1900s. In the subsequent century, several rivers supporting Gulf sturgeon spawning populations were extensively modified, leading to uncertainty in the species’ population viability and its classification as threatened under the Endangered Species Act. In recent decades, fisheries biologists have conducted ongoing Gulf sturgeon capture-recapture programs to assess stock status and fish behavior. However, uncertainties persist in important demographic parameters, such as movement and mortality rates, mostly due to low capture probability from physical recaptures. My Master’s research utilized acoustic telemetry data of Gulf sturgeon detected acoustic receivers at the mouths of nine major river drainages known to support Gulf sturgeon populations across the Gulf of Mexico. Using a simulation-tested multi-state model in Program MARK, I empirically estimated spatially explicit natural mortality rates (assuming negligible fishing mortality), and fidelity and emigration rates to and from natal rivers. Detection probability using acoustic telemetry was estimated to be much greater than previous capture probabilities, increasing inference in mortality and movement rates. Combined with genetic analysis, my estimates of Gulf sturgeon demographic rates can be used to better identify stock structure. I then used a centralized database of historic mark-recapture data from the Apalachicola and Suwannee rivers, two populations that once supported commercial fisheries with the highest cumulative tagged populations, in an updated age-structured mark-recapture model to assess stock status. I found that these eastern Gulf sturgeon populations have been stable or increasing over the past 25 years. Current abundance estimates from this study, compared with riverine carrying capacity estimates, may demonstrate the drastic recovery differences between the eastern and western Gulf sturgeon populations, paving the way for “distinct population segment” designation and more efficient management.