ABSTRACT: We examine alternative hypotheses for the decline of 20 cod Gadus morhua stocks in the North Atlantic. The year of the lowest observed biomass of spawners did not correspond to low juvenile survival for the cohorts that should have contributed tothe stock in that year. However, fishing mortality was very high for the years preceding the collapse. The collapse of the cod stocks was not caused by a lack of resilience at low population abundance because all spawners were able to produce manypotential replacements at low population size. We show that as populations collapsed, fishing mortality increased until the populations were reduced to very low levels. We conclude that increased fishing mortality caused the population declines, and oftenthe collapses, of the cod stocks.