Environmental Consequences of Tidal-Front Entrainment in Larval Fish Along the Southern Flank of Georges Bank

Lough, G.1, E. Broughton1, J. Manning1, L. Buckley2, L. Incze3, N. Wolf3, F.E. Werner4, K. Pehrson Edwards4, R. Converse4, and A. Aretxabaleta4
1Northeast Fisheries Science Center, NMFS, NOAA, Woods Hole, MA 02543, USA
2 University of Rhode Island/NOAA CMER Program, Graduate School of Oceanography, Narragansett, RI 02882, USA
3 Bioscience Research Institute, U. Southern Maine, Portland, ME 04104 USA
4 Marine Sciences Department, University of North Carolina, Chapel Hill, NC 27599-3300, USA

Cruises were conducted in spring 1999 to describe the interaction between tidal-front processes and the transport, retention and growth of cod and haddock larvae and their prey during the transition to stratified-water along the southern flank of Georges Bank. The project implemented a modeling component, where a three-dimensional circulation model was used in near real-time aboard ship to help guide the field program, as well as in hindcasting mode to integrate all the physical and biological observations in coupled circulation-trophodynamic simulations. Data assimilation methods were used in the circulation model to achieve the cell-like secondary circulation associated with the tidal front; downwelling on the stratified side and weak upwelling on the mixed side of the front. Passive drifter trajectories showed accumulation of particles in the front due to surface convergence, and cross isobath, on-bank, near-bottom flow under the tidal-front jet. A larval cod biophysical model (IBM) was used to consider trophodynamic effects on the growth and survival of larval cod and haddock. Prey fields were specified for mixed and stratified water columns from net and pump profiles and allowed to mix in the circulation model. Encounter and ingestion rates of larvae are functions of prey concentration, larval search patterns, sensitivity to light, swimming speeds of predator and prey, and turbulence. Model outputs provide hourly depth-dependent estimates of growth, prey biomass ingested, larval length and weight. Simulations were conducted to test the relationship among temperature, water-column structure, food availability, and larval growth. Observed larval growth, based on RNA-DNA, had higher mean growth rates from the upper 20m at the front and stratified stations. Simulations indicated that larval growth can be maximized at the tidal-mixing front due to near optimal temperature, turbulence, light, and given a surface convergence zone, high food availability.

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