Major changes in the Georges Bank ecosystem, 1980s to the 1990s

A major change in surface layer salinity and in zooplankton community structure occurred on Georges Bank between the 1980s and 1990s. Around 1990, salinity began to significantly decrease and the zooplankton community shifted to a greater dominance by smaller sized species. Annual estimates of the number of cod Gadus morhua and haddock Melogrammus aeglefinus eggs hatched on Georges Bank for the years 1979 to 1987 and 1995 to 1999 allowed calculation of a firstyear survivorship index for both species by dividing the number of new recruits from a virtual population analysis by the hatched egg abundance. The survivorship index for each species also exhibited a major shift between the 2 decades, with cod survivorship declining by a factor of 2 from the 1980s to the 1990s, and haddock survivorship increasing by about the same amount. The strong trend in (or regime-shift nature of) each of the relatively short physical and biological time series makes establishing statistically significant relationships problematic. However, the co-occurrence of major shifts in all of the parameters suggests that a major change in the Georges Bank ecosystem occurred between the 1980s and the 1990s. Identifying the causes of that change and the mechanisms connecting the different components of the system during that change will require additional analyses and modeling.