Overfishing of large-bodied benthic fishes and their subsequent population collapses on the Scotian Shelf of Canada's east coast and elsewhere 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.

Globally, overfishing large-bodied groundfish populations has resulted in substantial increases in their prey populations. Where it has been examined, the effects of overfishing have cascaded down the food chain. In an intensively fished area on the western Scotian Shelf, Northwest Atlantic, the biomass of prey species increased exponentially (doubling time of 11 years) even though the aggregate biomass of their predators remained stable over 38 years. Concomitant reductions in herbivorous zooplankton and increases in phytoplankton were also evident. This anomalous trophic pattern led us to examine how declines in predator body size (approx. 60% in body mass since the early 1970s) and climatic regime influenced lower trophic levels. The increase in prey biomass was associated primarily with declines in predator body size and secondarily to an increase in stratification. Sea surface temperature and predator biomass had no influence. A regression model explained 65 per cent of prey biomass variability. Trait-mediated effects, namely a reduction in predator size, resulted in a weakening of top predation pressure. Increased stratification may have enhanced growing conditions for prey fish. Size-selective harvesting under changing climatic conditions initiated a trophic restructuring of the food chain, the effects of which may have influenced three trophic levels.

The effects of human exploitation on macroecological patterns have received little attention, although such investigations may highlight unique spatial and temporal changes characteristic of species and assemblages subject to persistent disturbance. In unexploited systems (mainly among temperate avifauna) positive relationships between local abundance and geographic distribution are prevalent for individual species through time (intraspecific pattern) and among species during fixed time periods (interspecific pattern). We investigated intraspecific and interspecific relationships for 24 common marine fishes on the Scotian Shelf and Bay of Fundy, Canada, some of which have been commercially exploited for several decades. Based on extensive fisheries-independent trawl survey data from 1970 to 2001, 16 of the 34 stocks, comprising 13 species, exhibited significant positive intraspecific relationships. Significant relationships were associated mainly with those stocks that demonstrated significant temporal trends in both abundance and geographic distribution. The time-averaged (32 yr) interspecific relationship was positive and significant at the largest scale examined. Significant annual interspecific relationships were also detected over 26 yr. Surprisingly, the slopes of the annual relationships increased systematically and doubled through time, probably due to size-selective exploitation, shifting target species, and associated species interactions. In contrast to previous studies, our results indicate that the contributions of individual species to the interspecific relationship can change through time, and these changes dramatically alter the interspecific abundance-distribution relationship. Temporal trends in the interspecific relationship have not previously been reported, and appear to be due to the large spatial-and temporal-scale effects of exploitation.

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