Incorporating Environmental Covariates into a State-Space Stock Assessment Model for Yellowtail Flounder

Yellowtail flounder, Limanda ferruginea (a.k.a., Pleuronectes ferruginea), inhabit the northwest Atlantic continental shelf and historically supported target fisheries off New England. The Georges Bank stock declined in recent decades and has not recovered despite catch reductions. Stock assessments of Georges Bank yellowtail flounder exhibited retrospective patterns, which may result from model misspecification, such as assumptions that omit environmental effects on population or fishery dynamics. In the northwest Atlantic, ocean waters are warming four times faster than the global average, and low productivity of yellowtail flounder has been linked to ocean warming in the region. This study accounts for a changing environment in stock assessment and examines approaches to identify environmental data for inclusion in a state-space stock assessment model. After reviewing available information on environmental drivers affecting yellowtail flounder, exploratory analyses were conducted to identify relevant variables for incorporation. A stock assessment model including these effects was developed for the Georges Bank stock. Results indicate that including environmental covariates improves diagnostics and provides a case study for integrated, ecosystem-informed stock assessments.