Transient dynamics of an altered large marine ecosystem

Will cod make a comeback? Cod populations in the northwest Atlantic suffered a sudden and dramatic collapse about 20 years ago, and despite the subsequent ban on cod fishing there has been no appreciable recovery in the intervening years. Frank et al . report on the nature of the collapse and the reasons for its persistence. They find that the prolonged duration of the alternate (collapsed) state of the ecosystem was governed by the oscillatory, runaway consumption dynamics of the predator's former prey — forage fish. These prey species are now in decline, however, having outstripped their zooplankton food supply. The trend now is towards an increase in large-bodied zooplankton abundance, so a gradual return to the earlier ecosystem state is possible — a conclusion that bodes well for other collapsed fisheries. Overfishing of large-bodied benthic fishes and their subsequent population collapses on the Scotian Shelf of Canada’s east coast 1 , 2 and elsewhere 3 , 4 resulted in restructuring of entire food webs now dominated by planktivorous, forage fish species and macroinvertebrates. Despite the imposition of strict management measures in force since the early 1990s, the Scotian Shelf ecosystem has not reverted back to its former structure. Here we provide evidence of the transient nature of this ecosystem and its current return path towards benthic fish species domination. The prolonged duration of the altered food web, and its current recovery, was and is being governed by the oscillatory, runaway consumption dynamics of the forage fish complex. These erupting forage species, which reached biomass levels 900% greater than those prevalent during the pre-collapse years of large benthic predators, are now in decline, having outstripped their zooplankton food supply. This dampening, and the associated reduction in the intensity of predation, was accompanied by lagged increases in species abundances at both lower and higher trophic levels, first witnessed in zooplankton and then in large-bodied predators, all consistent with a return towards the earlier ecosystem structure. We conclude that the reversibility of perturbed ecosystems can occur and that this bodes well for other collapsed fisheries.