The US Northeast continental shelf has experienced large-scale perturbations over the last four decades due to human activities (notably harvesting) and environmental change (including climate-related impacts). The need for holistic assessments of ecosystem status in relation to natural and anthropogenic forcing is now widely recognised as a critical element in support of the development of an ecosystem approach to management. In particular, linking the pressures related to anthropogenic and natural drivers of ecosystem change to alteration in system status is an essential first step in developing effective management strategies. We have compiled key indicators of climate, physical dynamics, ecosystem state, and human activities and conditions to evaluate the status of this large marine ecosystem.
Decadal and multidecadal scale changes in climate and physical forcing factors affecting the ecosystem are evident at a number of different levels. Water temperatures have increased in coastal locations and on the continental shelf from a low in the late 1960s to the present. There has been corresponding increase in high temperature (>16°C) and a relative decline in intermediate (5-15°C) thermal habitat available for marine organisms. Increases in temperature and decreases in salinity have led to increases in water column stratification from a low in 1984 to the present that has had a profound effect on primary and secondary productivity.
The changes observed in some of the physical variables have been accompanied by clear changes in some of the biotic variables. The decreases in salinity have been accompanied by concomitant changes in an index of larger-bodied phytoplankton species (principally diatoms) in the ecosystem. A related index of water column stratification is closely related to the time series change in the total biomass of zooplankton and changes in the species composition copepod communities. There has been a pronounced shift from a demersal fish-dominated community to one dominated by elasmobranchs and pelagic fish. The fish community has also been affected by a persistent change in conditions that favour temperate-cold water fish community to one favoring warmer water species.
The overall biomass of the entire fish community as indexed by trawl surveys has increased over the last four decades as elasmobranchs and small pelagic fishes have increased in abundance even as other groups such as groundfish have undergone decreases. Some of these changes reflect apparent species replacements as heavily exploited species declined. The mean trophic level of fish in trawl surveys has fluctuated without trend. In contrast, the mean trophic level of the catch (invertebrates and vertebrates) has declined steadily since 1960, reflecting changes in the abundance of economically important species.
Estimates of the primary production required to support observed catch levels indicate that recent fisheries are probably more sustainable than those in the 1960s and 1980s for all species and for small pelagic species specifically.
The human component has been a critically important agent of change in this large marine ecosystem. Economic indicators for the groundfish fishery suggest that this resource has been in a long-term state of decline. Trends in human population and disposable income in the region suggest that human induced pressures on marine resources will remain high and perhaps continue to increase in the future.
Although marked improvement in the condition of some components of the NES LME is now evident under more effective management, the system remains classified as experiencing overfishing from an ecosystem perspective according to criteria of Murawski and Tudela et al.
Further Reading
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