THE ROLE OF FRONTAL CURRENTS IN LARVAL FISH TRANSPORT ON GEORGES BANK

The hydrography and distributions of cod larvae on Georges Bank were s urveyed during two research cruises in April and May 1993 in order to relate larval drift between cruises to the vernal intensification of t he frontal component of the residual circulation. We observed the tran sport of two patches of cod larvae. One patch, which had maximum larva l cod densities of 45 larvae 100 m(-3) in April, appeared to have been advected south about 75 lan between surveys, while the other, which h ad maximum larval cod densities of 20 larvae 100 m(-3) in April, appea red to have been advected north-northeast about 25 km. Maximum larval densities in each patch sampled during the second cruise in May were 1 5 and 18 larvae 100 m(-3), respectively, and mean growth in total leng th for larvae in the two patches was approximately 5.5 mm and 4.5 mm, respectively, between the two cruises. During the April cruise there w as a large volume of anomalous cold, fresh water, of Scotian Shelf ori gin, which occupied much of the eastern third of Georges Bank. During May, relatively cold, fresh water appeared in a band from the Northeas t Peak along the Southern Flank, between Georges Bank water on the top of the Bank, and upper Slope Water offshore. The distribution of cold , fresh water suggests its participation in the general clockwise circ ulation around the Bank. The transport of co larvae comprising the fir st patch appeared to become organized within, and move along, the fron tal boundary established by the Scotian Shelf-like watermass, while la rvae in the second patch, which we assumed to have moved to the north, may have been transported northward in an on-Bank how of warmer and s altier upper Slope Water, which may have originated from a Gulf Stream Ring. Based upon observed transport of the first patch of larvae in r elation to the frontal boundary, we present a conceptual model of fron tal mixing currents on Georges Bank, where current velocities may reac h 5 cm s(-1) at the depth of the pycnocline. We suggest that this fron tal component of the residual circulation, which is in addition to tha t resulting from tidal rectification, may be important in the transpor t of fish larvae, and that interannual variability in the degree of in trusion of extrinsic water masses may contribute to variable larval co d drift patterns, to variable larval cod retention on the Bank, and ul timately, to variable larval fish recruitment to the early juvenile st age. Copyright (C) 1996 Elsevier Science Ltd