A study of northern Atlantic cod (Gadus morhua) of eastern Newfoundland and Labrador as a metapopulation

I present a model of intrapopulation structure in northern Atlantic cod (Gadus morhua) inhabiting the coastal and continental shelf regions off northeast Newfoundland and Labrador based on metapopulation dynamics. I suggest that the metapopulation concept may best explain the historical patterns and present day observations of northern cod distribution. Evidence for subpopulation structure is assembled from new and published data sources, including studies of spawning time and location, migration behaviour, and genetic variation. A theoretical model of northern cod metapopulation dynamics is derived from the simple model of Levins (1970). I consider the various spawning components of northern cod historically centered on the offshore banks and in coastal bays as subpopulations of a northern cod metapopulation. I modify the Levins (1970) model by separating the probability of subpopulation extinction into two component probabilities representing: (1) natural processes and (2) the effects of fishing. Metapopulation theory predicts that fewer suitable areas (spawning grounds) will be occupied as population abundance declines. The corollary prediction is that as population abundance recovers, spawners will recolonize the currently unoccupied spawning areas. The agents of colonization may be migratory adult fish and straying larvae. The model supports the current view that overfishing may have played an important role in the extinction of northern subpopulations off Labrador