Assessment of Effects of Freshwater Runoff Variability on Fisheries Production in Coastal Waters

Abstract

The effect of freshwater runoff on the fisheries production of the Gulf of St. Lawrence is reviewed. In addition, the impact of the runoff from the Gulf of St. Lawrence on the fisheries production downstream (the Scotian Shelf and the Gulf of Maine) is considered. The relative importance of local freshwater runoff and larger scale ocean circulation on continental shelf fisheries production is evaluated. Conclusions are drawn relating to the impact of freshwter runoff variability on fisheries production in the northwest Atlantic as well as on the possible mechanisms of impact. Within the Gulf of St. Lawrence there is persuasive evidence that inter-annual variability in runoff has an impact on fisheries production. The mechanisms by which the variability is induced, however, are not understood. Even though there is entrainment of nutrients into the surface layer of the Gulf of St. Lawrence due to freshwater runoff, and some evidence for relatively high primary production, there is little evidence that suggests that the zooplankton, benthos, and fisheries production are higher than on contiguous shelf areas. Regulation of the Gulf of St. Lawrence system of rivers does not seem to have had a measurable impact on fisheries production. Non-local ocean circulation may have a greater impact than freshwater runoff from the Gulf of St. Lawrence on at least the cod fisheries production outside the Gulf of St. Lawrence (on the Scotian Shelf and in the Gulf of Maine). It is inferred that there is limited support for the freshwater runoff-driven food-chain hypothesis of Sutcliffe for generating fisheries production variability. There is accumulating evidence that the physical oceanographic processes of advection and diffusion can have a direct impact (i.e. without links through primary production) on population abundance of zooplankton as well as on fish populations during their early life history stages.