Effects of environmental change, fisheries and trophodynamics on the ecosystem of the western Scotian Shelf, Canada

The dynamics of marine ecosystems, which are complex and non-linear, are difficult to predict. Like most marine ecosystems, NW Atlantic systems have been subjected to centuries of fisheries exploitation as well as environmental and internal trophodynamic drivers. By reducing the complexity of the western Scotian Shelf ecosystem to 57 functional living groups, we were able to reproduce many of its observed dynamics, such as major decadal trends in abundance and mortality for most groups, and explore the relative strength within the triad of drivers using a trophodynamic model. We explored potential biophysical drivers through the use of a primary production anomaly, generated by a model fitting routine. The estimated primary production series was negatively correlated with the spring sea surface temperature for the Scotian Shelf Large Marine Ecosystem, a stratification index and the Atlantic Multidecadal Oscillation (AMO) index. The aggregated biomass of the main resident fish species was also negatively correlated with the AMO index. All 3 drivers contribute to shaping the observed biological and ecological changes of the western Scotian Shelf. This has substantial implications for fisheries management: (1) climate change (global warming) may negatively affect productivity at the species and ecosystem level; (2) these effects may be magnified due to the combined effects of trophic interactions and exploitation; and (3) fisheries assessments must account for environmental and climate change, and for the broader ecosystem effects. Failing this, fisheries should be managed well below their single-species reference points.