Distribution shifts and overfishing the northern cod (Gadus morhua): a view from the ocean

Research on northern cod (Gadus morhua) from 1983 to 1994 indicated that a southward shift in distribution in the early 1990s was real and not an arti fact of sequentially fishing down local populations. In the early 1990s, se asonal fishery and survey data showed distribution changes where there was no fishery, and large tonnage and densities (450 000 t, densities fourfold higher than 1980s levels) appeared in the south concurrent with declines in the north. All fishery, acoustic, and trawl survey indices increased in th e south, while the stock declined. Southern-caught cod in the early 1990s e xhibited northern characteristics: (i) antifreeze production capacities abo ve historical norms and equivalent to those of northern fish, (ii) vertebra l counts above historic norms and equalling northern counts, and (iii) decl ines in size-at-age to levels associated with northern fish. The cause of t he shift is thought to be a combination of abiotic (climate) and biotic (ca pelin (Mallotus villosus)) environmental changes and cumulative long-term f isheries effects on cod behavior. The shifted distributions increased vulne rability to Canadian and foreign fisheries and led to a rapid decline in ab undance, both before and after the moratorium on fishing in Canadian waters in 1992. Rebuilding will occur in three steps: environmental restoration, recolonization by adults, and enhanced recruitment across the shelf.