Why do fish stocks collapse? The example of cod in Atlantic Canada
Date
1997-02
Authors
Myers, RA
Hutchings, JA
Barrowman, NJ
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Abstract
In 1993, six Canadian populations of Atlantic cod (Gadus morhua) had collapsed to the point
where a moratorium was declared on fishing. It has been argued that the collapses were caused by
poor recruitment of cod to the fishery. Yet we are unable to detect a difference between the
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 certainly detected an overall reduction of recruitment of 20%. There are considerable
differences in the abundance trends as determined by research surveys and reconstructed from the
commercial catch at age data (called ''virtual population analysis''
[VPA]) for each stock. VPA-based abundances consistently depict lower recruitment levels than do
survey-based estimates in recent years. More important is the observation that from the early 1980s
the VPA-based trend shows a decline where none is apparent in the survey-based trend. One
explanation of these differences would be an increase in discarding of young fish as fishing
mortality increased. We test the hypothesis that the mortality for young cod is unrelated to the
fully recruited fishing mortality. This hypothesis is rejected; in each of the six stocks, high
juvenile mortality was associated with high adult mortality. This is consistent with the discarding
hypothesis. We suggest that age-specific abundance trends estimated 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 misreporting is related to
changes in fishing mortality.
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Citation
Myers, RA, JA Hutchings, and NJ Barrowman. 1997. "Why do fish stocks collapse? The example of cod in Atlantic Canada." Ecological Applications 7(1): 91-106.