Coherence of Atlantic Cod Stock Dynamics in the Northwest Atlantic Ocean

Abstract

AbstractThe stocks of Atlantic cod Gadus morhua in the Northwest Atlantic Ocean declined in abundance from 1965 to 2003; the declines in spawning stock biomass (SSB) have been temporally coherent. A coherent, sharp increase in SSBs from 1975 to 1985 and a subsequent decrease from 1985 to 1992 are superimposed on the general decline. The coherence suggests that cod stock variability in the Northwest Atlantic Ocean is driven by a common set of causes or that the linkages among the nominal stocks are stronger than was previously thought. The coherent increases in cod SSB from the mid‐1970s to 1985 occurred under relatively low fishing mortalities. The declines in SSB beginning in 1985 began during a period of low fishing mortalities. During the 1985‐1992 period the declines in Atlantic cod abundance were coupled with greatly reduced growth rates, increased natural mortality rates, and a lack of response to reduced fishing mortality. This suggests that the 1985‐1992 decreases were driven by a strong negative environmental signal, implying that the environment had a stronger role in affecting cod abundance than had been previously thought. It appears that the decline in SSB over most of the range of the cod was coupled with a major perturbation in the forage available to cod. Inasmuch as this perturbation involved seemingly disparate groups, such as capelin Mallotus villosus, euphausids, and Atlantic herring Clupea harengus, it appears that the change in the environment was associated with the dynamics of the plankton.