Estimating Fishing Mortality and Movement of Greater Amberjack in the Gulf of Mexico and the Atlantic Ocean off of the Southeastern United States

Abstract

Greater Amberjack (Seriola dumerili) are an important marine species for sport and commercial fisheries across the southeastern United States. Two Greater Amberjack stocks are recognized for assessment and management off the southeastern US: the South Atlantic and Gulf of Mexico (GOM) stocks. Currently, the South Atlantic stock is neither overfished nor undergoing overfishing, but the GOM stock is experiencing both conditions. Obtaining independent estimates of exploitation and movement for these stocks would aid management by supplementing and potentially corroborating existing stock assessments. We conducted a large-scale multi-investigator tagging study to estimate regional fishing mortality, length-based vulnerability, and large-scale movements of Greater Amberjack. From May 2022 to October 2023, we tagged 1029 Greater Amberjack with conventional reward tags; 401 fish were also implanted with an internal acoustic transmitter. The study area was divided into three regions: the Western GOM, Eastern GOM, and the South Atlantic (ATL). We assumed 100% reporting of recaptured tags from the recreational fishery by using a $250 reward incentive. Detections of fish tagged with acoustic transmitters and external tag return data were incorporated into a Bayesian multistate mark-recapture model to produce estimates including sector-specific fishing mortality, natural mortality, tag loss, detection probability, and discard rates of Greater Amberjack. Two separate scenarios of the model were run with differing commercial reporting rates, the first set at 70% and the second at 50% reporting. Estimates for fishing mortality in the Atlantic were similar to the most recent stock assessment, producing estimates of 0.19 and 0.20 yr-1 for recreational and 0.11 yr-1 and 0.17 yr-1 for commercial fisheries between the 0.7 and 0.5 commercial reporting rate models, respectively. In the Gulf, estimates were slightly lower than the last stock assessment, but a rebuilding plan has been limiting fishing due to the stock status of this region. My first model scenario generated estimates of 0.145 yr-1 for average annual recreational fishing mortality and 0.04 yr-1 for commercial fishing and under the second scenario, 0.15 yr-1 and 0.06 yr-1 for recreational and commercial, respectively. Natural mortality was very high, 1.38 yr-1, possibly due to permanent emigration that cannot be accounted for in the model. Length-based vulnerabilities did not show a trend, differing from the SEDAR stock assessment estimates for age/size-specific gear selectivity. There were no recorded movements of tagged Greater Amberjack between the GOM and the ATL and limited movements between the EGOM and WGOM. Results from this study will help provide supplemental information on critical estimates that fisheries management agencies can use to make informed regulatory decisions for future fishing seasons to help maintain a sustainable Greater Amberjack fishery.