Juvenile and Adult Striped Bass Mortality and Distribution in an Unrecovered Coastal Population

Abstract

Striped Bass Morone saxatilis fisheries have been important in the eastern United States since the 1700s, but many populations have declined from historic levels. Enhancement programs, harvest reduction, water quality improvements, and habitat restoration have led to successful recoveries for specific stocks. However, these efforts have not been successful for the Striped Bass population in the Neuse River of North Carolina. Possible mechanisms inhibiting recovery of this population include overharvest, high discard mortality, poor water quality, and altered flow regimes. These mechanisms and their impacts on the Neuse River population are unclear; therefore, to gain insight, we estimated mortality and distribution of the population. Specifically, we tagged 100 hatchery-reared phase II juveniles (202-227 mm TL) and 111 resident adults (349-923 mm TL) with acoustic transmitters (a subset of 50 adults was also tagged with external high-reward tags). We used telemetry to monitor movement and seasonal distribution from December 2013 until September 2015. Telemetry and tag reporting data informed mortality models, and we estimated that annual discrete total mortality of phase II stocked juveniles was 66.3% (95% credible interval [CI] = 47.4-82.4%). Annual discrete total mortality of adults was 54.0% (95% CI = 41.5-65.4%). Adult discrete natural mortality was 20.1% (95% CI = 8.7-39.1%), and neither juvenile nor adult natural mortality was correlated with seasonal variation in dissolved oxygen, temperature, or salinity. These results show that poststocking mortality is significant and that juvenile mortality should be considered when establishing stocking goals. Additionally, adult natural mortality is within the range predicted by maximum age and by previous studies; however, adult total mortality is higher than targeted rates. These results can help to inform management decisions and develop measures to rebuild depressed Striped Bass populations like that in the Neuse River.

Modeling the Effects of Vital Rate Manipulation and Management Scenarios to Predict the Population Impact of Restoration Programs on an Unrecovered Coastal Population of Striped Bass

“…Vital rate parameter estimation.-Stage-and source-specific mortality rates were available for the Neuse River Striped Bass population (Table 1; NCDMF and NCWRC 2013;Bradley et al 2018). Mortality of phase II hatcheryreared juveniles, total mortality of adults, and natural mortality of adults were reported by Bradley et al (2018). We calculated fishing mortality by assuming that total mortality was a fully additive form of natural and fishing mortality, which occurred continuously throughout the year:…”

“…Ongoing research programs have estimated stage-and source-specific mortality rates in an effort to identify mechanisms associated with the lack of recovery of the Neuse River Striped Bass population. For example, Bradley et al (2018) acoustically tracked 100 hatchery-reared phase II juvenile Striped Bass and 111 resident adults in the Neuse River. They concluded that 66% (95% credible interval [CI] = 47-82%) of juveniles died within the first year after stocking and that adult mortality was lower (annual discrete total mortality = 0.54, 95% CI = 0.43-0.66; annual discrete natural mortality = 0.20, 95% CI = 0.09-0.39).…”

“…The discrepancy between predicted and observed harvests is likely due to an overestimation of population abundance as an artifact of underestimating juvenile mortality. Bradley et al (2018) censored all mortality that occurred within the first month after stocking. Although this assumption eliminated surgery-related mortality, it likely eliminated natural mortality that occurred during the first month poststocking-the period when stocked juveniles are youngest and most naïve.…”

“…Low abundance and a truncated age distribution may be contributing to the lack of recovery of the Striped Bass population in the Neuse River by limiting the number and fecundity of individuals that spawn. Bradley et al (2018) found that fish size affected whether adult fish migrated upriver during the spring, presumably to spawn. Not only are larger fish more likely to make spawning migrations, they also have higher fecundity and can produce offspring with higher survival than smaller and younger individuals.…”