Separation of Catches of Two Atlantic Cod Stocks in an Otter Trawl Fishery by Application of the EM Algorithm

Abstract

Abstract Recent changes in the location of fishing effort have resulted in significant quantities of Atlantic cod Gadus morhua from the southern Gulf of St. Lawrence stock being caught along with fish from the resident eastern Scotian Shelf stock during the winter fishery (January–April) on the eastern Scotian Shelf. Since 1986, much of the winter fishery has occurred in an area where the two stocks were known to mix. The Atlantic cod from the eastern Scotian Shelf stock were larger at age than those fi-om the southern Gulf of St. Lawrence stock. The EM (expectation–maximization) algorithm was used with the length-at-age distributions from the two stocks when isolated to separate landings of the two stocks in mixed-stock catches. Landings of fish from the southern Gulf of St. Lawrence stock in the eastern Scotian Shelf winter fishery increased from less than 1,800 metric tons (mt) 1980 and 1985 to almost 8,800 mt in 1991, coincident with changes in the location of fishing effort during the winter fishery....