WAS AN INCREASE IN NATURAL MORTALITY RESPONSIBLE FOR THE COLLAPSE OF NORTHERN COD

The collapse of the northern Atlantic cod (Gadus morhua) fishery off s outhern Labrador and to the northern Grand Bank of Newfoundland, once the largest cod fishery in the world, was a social and economic disast er for the region. An analysis of traditional catch-at-age data in con junction with research surveys, which assumed that research survey est imation errors of abundance by age and year were independent, led asse ssment biologists to the conclusion that the collapse was caused by an increase in natural mortality in the first half of 1991. We construct ed a statistical model to test this hypothesis. The results do not sup port the hypothesis. There is ambiguous evidence that natural mortalit y has increased since 1991; however, these results are found only in a model that has extraordinary patterns in the residuals. Our analysis suggests that even if natural mortality has been higher in recent year s (as estimated using a model with correlated errors for research surv eys), overfishing was sufficiently high to cause a collapse of this po pulation. We also demonstrate that the usual assumption that estimatio n errors from research trawl surveys are independent is not valid, and can lead to invalid inference and unreasonable estimates of abundance .