Dispersion and seasonal movements of American plaice (Hippoglossoides platessoides) tagged in waters of the Gulf of St. Lawrence, Scotian Shelf and Bay of Fundy

Approximately 17 tagging studies were conducted on American plaice between 1958 and 1997 throughout the Southern Gulf of St. Lawrence, Scotian Shelf and Bay of Fundy. Only about half these studies were formally analysed and discussed in the literature, and many of those represented in the literature were characterised by appreciable recoveries made subsequent to publication. Analyses of previously untreated data and re-analysis of older studies with updated recoveries was conducted to create a synopsis of likely population identities and movements. These tagging studies demonstrate that American plaice in the Gulf of St. Lawrence are distinct from Scotian Shelf plaice, and support the hypothesis of discrete southern and northern sub-populations of American plaice within the southern Gulf of St. Lawrence. American plaice in Sydney Bight are also distinct from plaice in the Gulf of St. Lawrence, but it remains unknown if they are a discrete spawning group or derive from plaice spawning on Banquereau. Movements of plaice tagged in the vicinity of Passamaquoddy Bay provide some evidence that plaice in the Bay of Fundy might be associated with the Scotian Shelf, possibly by a spawning group on Browns Bank. All the groups of plaice addressed in this study were combined as a single “Designatable Unit”, and assessed as “Threatened in a Species at Risk” context, by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC), due to excessive declines in abundance of mature individuals over 2–3 generations (~86% decline in the Gulf, ~67% decline on the Scotian Shelf). Observed long-term dispersion indicates that plaice has some potential to recolonize depleted areas, but such movements are displayed by fish older than are commonly found in the population in recent decades. Seasonal protection of spawning grounds might safeguard components of the population in the short-term until numbers of these older fish can be rebuilt.