Unobserved diversity, depletion and irreversibility The importance of subpopulations for management of cod stocks

Diversity is often associated with resilience but in this model, unobserved genetic or behavioral diversity can explain the collapse of supposedly regulated fish stocks such as cod. Recent studies have shown the existence of separate sub stocks of cod even at a very fine geographical scale. We show that modeling a group of distinct stocks as if it were one large stock will tend to over-estimate the growth and harvest potential. If quotas are based on such over-estimates, the unobserved stock diversity can explain sudden stock collapses and unexpectedly slow recovery as observed for Canadian cod. It is a lack of information concerning diversity that leads to irreversibility. The differences between the various stocks may be behavioral or genetic but cannot be observed by the fishermen or regulators who believe that there is a gradual decline in one big stock while in fact they are witnessing the successive disappearance of a series of sub stocks.