Transition to an alternate state in a continental shelf ecosystem

Abstract

The collapse of Atlantic cod (Gadus morhua) stocks in the northwest Atlantic Ocean ranks among the most dramatic and widely known ecological changes of the 20th century. Less widely known are the systemic changes that occurred within these areas prior to and coincident with the cod collapse. Our analysis of a fishery-independent, long-term, standardized database collected on the eastern Scotian Shelf off Nova Scotia revealed that during the past four decades, coherent, community-level reductions in body size, biomass, and physiological condition have occurred in the resident demersal fish species. The changes occurred over large spatial (>104 km2) and short temporal (<10 years) scales, suggesting a progressive decline in the nature and extent of the energy flow through the benthic system. The unexpected and persistent poor condition of a variety of morphologically and functionally dissimilar demersal fish species, living in an environment of reduced intra- and inter-specific competition, suggests a decoupling of the benthic– pelagic systems. This decoupling appears to be attributable to a cascading series of processes involving the cumulative removal of biomass resulting from commercial fishing, compensatory and self-stabilizing increases in pelagic fish biomass, and a decline in groundfish productivity exacerbated by decadal scale variability in water temperature and stratification.

Measuring marine fish biodiversity: temporal changes in abundance, life history and demography

“…Although we cannot state with certainty that the rise in pelagic fish abundance can be attributed to a reduction in predation pressure by higher-level demersal predators, the conclusion is not unreasonable, particularly given the evidence that lower-trophic species have benefited similarly from the presumed predation release afforded by the reduction in demersal predators, e.g. increased abundance of shrimp and small pelagic fish (Choi et al 2004) following the collapse of Atlantic cod, increased abundance of flatfish following the collapse of demersal species . On Georges Bank, changes in the abundance of elasmobranchs and sand lance following declines by ground-fish and herring/mackerel, respectively, have also been attributed to reduced competition and predation (Fogarty & Murawski 1998).…”

Size-selective fishing drives species composition in the Celtic Sea

“…Local extinction or substantial depletion of most large species may require long recovery times. Experience in heavily exploited demersal fish communities such Size-selective fishing drives species composition in the Celtic Sea as that on the Eastern Scotian Shelf suggests a multidecadal process (Hutchings, 2000;Choi et al, 2004;Hutchings and Reynolds, 2004). In this context, perhaps current evidence of recovery in the North Sea LFI (Greenstreet et al, 2011) is driven by recovery of the intraspecific size structure of species of intermediate size (e.g.…”

Reconciling end-to-end and population concepts for marine ecosystems

“…2; crustaceans on the Newfoundland and Nova Scotia continental shelves (Worm and Myers 2003); and small pelagic fish in the North Sea (Heath, 2005). Some possible explanations of these patterns are phrased in terms of "cascades" (Frank et al 2005) or "regime shifts" (Choi et al 2004). These studies rely on comparisons or correlations of time series of a few selected species and do not consider the energetic implications of changes in the entire trophic web.…”