Redefining spatial population structure of winter flounder (Pseudopleuronectes americanus): implications for stock assessment and management

Abstract

The southern New England – mid-Atlantic (SNE–MA) stock of winter flounder (Pseudopleuronectes americanus) collapsed in the 1990s without discernable recovery to date. Owing to the lack of recovery, consideration of population subcomponents, which are currently ignored in fisheries stock structure definitions, may be necessary for rebuilding. We used the otolith chemistry tracer manganese–calcium ratios (Mn/Ca) to estimate inshore- versus ocean-nursery contributions of 77.3%/22.7% in SNE–MA, 15.7%/84.3% in the Gulf of Maine (GOM), and 60.0%/40.0% in Georges Bank (GB). In addition, we used strontium–calcium ratios (Sr/Ca) to estimate migratory- and resident-contingent membership of nursery-classified fish. Across all stocks, 30.2% of fish were classified as bay residents, 25.2% as bay migrants, 25.8% as ocean residents, and 18.8% as ocean migrants. Finally, model selection indicated that both nursery-specific and contingent-specific growth models were more appropriate than a common model. Nursery-specific models exhibited increasing deviations in length with age. Contingent-specific model reflected moderate differences at the youngest ages but convergence at older ages. These findings are informative for the population structure and migration ecology of winter flounder; however, simulation is required to determine whether partial migration and substock structure are necessary inputs for sustainable fisheries management.