Extreme increases in natural mortality prevent recovery of collapsed fish populations in a Northwest Atlantic ecosystem

Improved understanding of the dynamics of populations at low abundance is needed in the face of global biodiversity loss. We examined the dynamics of depleted demersal fish populations in the southern Gulf of St. Lawrence, Canada. Twenty years ago, a number of these populations collapsed due to overexploitation. Since then, others have declined to low abundance. Despite negligible levels of fishing mortality and strong rates of production of small juvenile fish, these populations have shown no sign of recovery and some continue to decline. Lack of recovery is due to dramatic increases in the natural mortality of larger individuals in these populations. In some of these fishes, natural mortality has risen to levels typical of high-turnover forage fishes rather than long-lived demersal fishes. We hypothesize that these high levels of mortality reflect a 'predator pit' or predation-driven Allee effect, resulting from the severely depleted abundance of these fishes and the high and rising abundance of their marine mammal predators, in particular grey seals. Recovery of collapsed demersal fish populations does not appear to be possible under current conditions in this ecosystem, even in the absence of fishing. Our results indicate a need for more precautionary management regimes in order to avoid population collapses that are not reversible by reducing exploitation.