title: Increased natural mortality at low abundance can generate an Allee effect in a marine fish.
abstract: Negative density-dependent regulation of population dynamics promotes population growth at low abundance and is therefore vital for recovery following depletion. Inversely, any process that reduces the compensatory density-dependence of population growth can negatively affect recovery. Here, we show that increased adult mortality at low abundance can reverse compensatory population dynamics into its opposite-a demographic Allee effect. Northwest Atlantic cod (Gadus morhua) stocks collapsed dramatically in the early 1990s and have since shown little sign of recovery. Many experienced dramatic increases in natural mortality, ostensibly attributable in some populations to increased predation by seals. Our findings show that increased natural mortality of a magnitude observed for overfished cod stocks has been more than sufficient to fundamentally alter the dynamics of density-dependent population regulation. The demographic Allee effect generated by these changes can slow down or even impede the recovery of depleted populations even in the absence of fishing.
authors.affiliation: Department of Environmental Sciences , University of Helsinki , PO Box 65, 00014, Finland.
authors.forename: Anna
authors.initials: A
authors.lastname: Kuparinen
authors.affiliation: Department of Biology , Dalhousie University , 1355 Oxford St., PO Box 15000, Halifax, Nova Scotia, B3H 4R2, Canada ; Centre for Ecological and Evolutionary Synthesis, Department of Biosciences , University of Oslo , Oslo 0316, Norway.
authors.forename: Jeffrey A
authors.initials: JA
authors.lastname: Hutchings
keywords: Atlantic cod; density-dependence; fisheries; overfishing; population growth rate; recovery