WHY DO FISH STOCKS COLLAPSE - THE EXAMPLE OF COD IN ATLANTIC CANADA

In 1993, six Canadian populations of Atlantic cod (Gadus morhua) had c ollapsed to the point where a moratorium was declared on fishing. It h as been argued that the collapses were caused by poor recruitment of c od to the fishery. Yet we are unable to detect a difference between th e recruitment of year classes that should have contributed most to the spawning stock at the time of the collapse and recruitment levels in earlier years. A power analysis shows that we would have almost certai nly detected an overall reduction of recruitment of 20%. There are con siderable differences in the abundance trends as determined by researc h surveys and reconstructed from the commercial catch at age data (cal led ''virtual population analysis'' [VPA]) for each stock. VPA-based a bundances consistently depict lower recruitment levels than do survey- based estimates in recent years. More important is the observation tha t from the early 1980s the VPA-based trend shows a decline where none is apparent in the survey-based trend. One explanation of these differ ences would be an increase in discarding of young fish as fishing mort ality increased. We test the hypothesis that the mortality for young c od is unrelated to the fully recruited fishing mortality. This hypothe sis is rejected; in each of the six stocks, high juvenile mortality wa s associated with high adult mortality. This is consistent with the di scarding hypothesis. We suggest that age-specific abundance trends est imated from research surveys and VPA should be compared for all stocks where such data exist, and that high priority should, be given to the measurement of discarding levels and the extent to which catch misrep orting is related to changes in fishing mortality.