The Importance of Stock Structure for Understanding Stock Recruitment Relationships

Accurate understanding of recruitment dynamics and prediction of future recruitment largely rely on identification of the spawning stock.  However, many fishery management units do not reflect the underlying population structure.  Recent stock identification of New England groundfish demonstrates how stock structure and recruitment interact.  Coastal spawning and larval connectivity justify the regional stock-recruitment approach for winter flounder that combines several estuarine spawning components.  Geographic differences in temporal patterns of recruitment suggest separate stocks of yellowtail flounder on Georges Bank and off southern New England, with a mixing zone near Nantucket Shoals.  Furthermore, incorporating movement into stock assessment models of yellowtail flounder changes the perception of recruitment events and rebuilding targets derived from recruitment patterns.  Inferred stock-recruitment relationships of cod and resulting MSY reference points from stock assessments of current management units are considerably different than those expected from a new paradigm of biological stock structure.  Understanding of cod recruitment processes also depends on the recognition of seasonal spawning components.  These case studies demonstrate that subdividing a single stock or combining distinct stocks into a single stock assessment can produce misleading estimates of biomass, recruitment and the stock-recruitment relationship as well as inaccurate estimates of reference points for overfishing or rebuilding.