Stock collapses and their recovery: mechanisms that establish and maintain life-cycle closure in space and time

Experience has established that the recovery of many collapsed stocks takes much longer than predicted by traditional fishery population models. We put forward the hypothesis that stock collapse is associated with disruption of the biological mechanisms that sustain life-cycle closure of intrapopulation contingents. Based on a review of case studies of nine marine fish stocks, we argue that stock collapses not only involve biomass loss, but also the loss of structural elements related to life-cycle diversity (contingents), as well as the breakdown of socially transmitted traditions (through a curtailed age range). Behavioural mechanisms associated with these structural elements could facilitate recovery of depleted populations. Migratory behaviour is argued to relate to phenotypic plasticity and the persistence of migration routes to social interactions. The case studies represent collapsed or depleted populations that recovered after a relatively short period (striped bass, capelin), after more than a decade (herring and sardine), or not at all (anchovy, cod). Contrasting the population dynamics from these stocks leads us to make a distinction between a depleted and a collapsed population, where, in addition to biomass depletion, the latter includes damage to contingent structure or space-use pattern. We also propose a mechanism to explain how lost habitats are recolonized.