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

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.

Migration diversity, spawning behavior, and habitat utilization of winter flounder

Winter flounder (Pseudopleuronectes americanus) have declined to less than 11% of their historical abundance in offshore areas and have experienced severe declines and inbreeding in coastal subpopulations. Understanding metapopulation structure and disparate migratory behavior is vital to understanding the species’ dynamics. We used acoustic telemetry to evaluate migratory types, homing behavior, and spawning behavior in adult winter flounder (n = 72) in Mattituck Creek, New York. Telemetry results showed that 17% of the tagged individuals displayed resident behavior and remained in the creek year-round despite mean summer temperatures reaching 28 °C. Alternatively, the migratory group (∼83%) started to leave the system when the mean water temperature reached 12 °C. A dominant driver of migration was not indicated; instead, migration out of or into the creek appeared complex and related to date, temperature, lunar cycle, photoperiod, and year. Approximate spawning was estimated to occur as early as November and continued through April based on aggregative behaviors of adults and back-calculations of dates using young-of-the-year otoliths. Our results support the emerging view that winter flounder’s dynamics reflect local life history and population diversity, which are currently unaccounted for in management.