G. Lough, J. Manning, E. Caldarone, L. Buckley, L. Incze, D. Townsend
It is a general feature of continental shelf spawning regions that fish larvae can be concentrated in the vicinity of tidal-fronts, the transition zone between well-mixed and thermally-stratified waters, and tidal jets can result in increased downstrean advection. There also is the possibility of retention to shoal nursery areas through cross-frontal exchange, a mechanism whose details are unknown. Enhanced growth and survival of larvae have been observed in convergence zones where peak copepod abundance and production coincide with aggregations of larval fish. However, sampling of the physics and various trophic levels is difficult because of the short time- and space-scale physical processes. Therefore in April and May 1999, we will focus on the following objectives: (1) describe and quantify the three-dimensional circulation in the vicinity of the tidal front when cod and haddock larvae are normally found along the southern flank of Georges Bank; (2) determine the small-scale distribution and temporal variability of fish larvae and their prey in relation to the physical frontal processes; (3) estimate vital rates in the different environments; and (4) evaluate the interrelations and relative importance of larval swimming behavior, physiological condition, and cross-frontal exchange and retention processes.
The field sampling strategy and instrumentation include a combination of grid surveys, cross-front transects, and drifter studies which have been designed to make measurements on the time- and space- scales relevant to larval fish and their copepod prey. Neither the fish nor their prey are likely, by themselves, to be measured with sufficient resolution to document the exchange process. Consequently, we plan to make high-resolution measurement of the current/shear fields with ship-mounted Acoustic Doppler Current Profiler (ADCP) and sample hydrographic structure, nutrients and chlorophyll concentrations as tracers of water movements contemporaneously with larval fish and prey sampling. Not only will the ADCP/CTD/fluorescence/nutrient data provide a scale of measurement not possible with larval fish, but they will provide a common denominator of measurement between our cruises and the detailed physical oceanographic process studies and other modeling components of GLOBEC. When these data and processes are combined with our other sampling systems (ARGOS/VHF drifters, MOCNESS/VPP, CTD/pump), we hope to be able to describe the interaction between tidal front processes and the transport, retention and growth of cod and haddock larvae and their prey prior to and during the transition to stratified water conditions.
