Allee effects and the allee-effect zone in northwest atlantic cod

According to the theory of compensatory dynamics, depleted populations
should recover when the threat responsible for their decline is removed
because per capita population growth is assumed to be highest when
populations are at their smallest viable sizes. Yet, many seriously
depleted fish populations have failed to recover despite threat
mitigation. Atlantic cod (Gadus morhua) off Newfoundland despite thirty
years of dramatically reduced fishing mortality and numerous fishery
closures has not recovered suggesting that drivers other than fishing
regulate the growth of collapsed fish populations, inhibiting or
preventing their recovery. Here, using Bayesian inference, we show strong
evidence of Allee effects in a south Newfoundland cod population, based on
data on recruitment and spawning stock biomass. We infer the Allee-effect
threshold, below which recovery is impaired. We demonstrate the necessity
of data at low population sizes to make inferences about the nature of
low-abundance dynamics. Our work indicates that Allee effects are not
negligible in commercially exploited fish populations, as commonly
projected, and that they represent an inhibitory force that can
effectively prevent recovery from overfishing. Our findings contrast with
prevailing fisheries management practices that assume compensatory
dynamics at low abundances with potential to seriously overestimate the
recovery potential of collapsed populations.