US GLOBEC Newsletter 13: Backward-Facing Workshop

| ICES Backward-Facing III Workshop by Michael Fogarty The third in a series of workshops under the auspices of the International Council for the Exploration of the Sea devoted to retrospective analysis was held in Woods Hole during May 4-6, 1998. The convenors of the meeting were Steve Murawski (NOAA, NMFS, Woods Hole, MA), Cisco Werner (University of North Carolina) and Keith Brander (ICES GLOBEC Coordinator). The objectives of the third backward-facing workshop were to examine the interrelation-ships among recruitment patterns of gadoid stocks in the Georges Bank, Scotian Shelf, and Gulf of Maine regions of the Northwest Atlantic and to examine physical forcing mechanisms potentially affecting recruitment. The extensive information available on recruitment fluctuations in cod and haddock populations, and the long-standing history of investigation of the physical oceanography of the region, were important factors in choosing to investigate this theme. The historical record of fisheries for cod and haddock in the Georges Bank-Scotian Shelf region is extensive with records available for the last century. Landings of the two species have varied widely over time, and dramatic changes have occurred under heavy exploitation during the last four decades. Separating the relative effects of harvesting and environmental variation on the populations holds considerable importance in man-agement of these stocks and offers an important scientific challenge. Several general conclusions emerged concerning changes in the physical characteristics of the system during the 1960s and 70s. A general pattern of cool water temperatures in the1960s with a concurrent subsurface freshening was evident throughout much of the region. Observations on nutrients and oxygen levels indicate lower nitrate and higher dissolved oxygen concentrations at depth in the region during this period. The cooling of the shelf waters in the 1960s is attributable to cold, fresher Labrador Slope waters off-shore that subsequently penetrated onto the shelf. This has been linked to an inverse relationship between the strength of the Labrador Current's southwestward transport and the NAO index. During the 1960s the North Atlantic Oscillation (NAO) was at its lowest levels since the 1880s. Long-term water temperature records at Woods Hole show a consistent warming trend from the 1960s through the 1990s, resulting in an overall 2 °C increase. One dimensional energy models for the Georges Bank region have successfully described changes in stratification in response to time dependent variability of surface heating, tidal stirring and wind mixing. Interannual stratification variability is notable for waters 60 to 100m, and is greatest during summer and fall, related to variability in the interannual heat flux. U.S. GLOBEC studies have indicated that since 1995 there has been a significant reduction in salinity in the water column on Georges Bank and the Gulf of Maine. Decreases in salinity between 1995 and 1997 do not appear to be related to advective effects of the Labrador Current. From fall 1997 through spring of 1998, however, cold Labrador Slope water did enter the Gulf of Maine. The temperature and salinity characteristics of these waters match closely those observed during the very cold 1960s. This input of cold Labrador Slope water is believed to be due to an increase in the transport of the Labrador Current, which may be linked to the low NAO index of the past two years. With respect to population fluctuations of cod, a moderate degree of concordance in recruitment and survival rate among the various cod stocks was noted. The highest correlation in recruitment was between adjacent stocks in the southern extent of the area. The high coherence in annual recruitment, combined with numerical simulations showing egg and larval drift supports the possibility of interchange between the Gulf of Maine and Scotian Shelf. For haddock, there was a moderate degree of coherence in landings, recruitment, and recruitment survival among adjacent pairs of haddock stocks. The strong 1962 and 1963 year classes in all areas suggests a common environmental factor affecting recruitment in those years. Decline in recruitment of all haddock stocks in the mid to late-1960s generally corresponds to a reduction in shelf temperatures, along with an increase in south-westward transport along the shelf break, and a reduction in the NAO index, possibly associated with advective events. Improvements in recruitment survival in the 1970s roughly corresponded to an increase in temperatures and the NAO index. These retrospective analyses show clear changes in the environment on decadal scales within this region. Plausible linkages of these signals with recruitment success of gadoid populations have been hypothesized on the basis of these studies, providing important avenues for investigation. The broad spatial and temporal context of these studies offers an important window on climate scale events and their impact on gadoid populations. Last updated: 21 February, 1999 Please address any comments, suggestions or questions to: GLOBEC Webmaster