TROPHODYNAMIC AND ADVECTIVE INFLUENCES ON GEORGES BANK LARVAL COD ANDHADDOCK

Using a model-based approach, the relative effects of advective and tr ophodynamic (feeding and growth) processes are considered on populatio ns of larval cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank. Building on previous studies that describe the role of advection, this study incorporates trophodynamic relationships to e xamine starvation mortality and growth rates at the level of individua l larvae on the Bank. Estimates of prey concentrations and flow fields appropriate for late winter/early spring are used. Both trophodynamic processes and advection influence larval losses from the Bank where, in the absence of starvation, advective losses are on the order of one -fifth of the eggs and larvae spawned on the Bank. Starvation is most important in the first feeding larvae and is much reduced for older la rvae. The contact rates between larval fish and zooplankton prey when turbulence is included are 2-5 times greater than the contact rates wi th no turbulence, and allow the model cod larvae to achieve growth rat es similar to those observed on the Bank, although mean rates for larv al haddock are still lower than observed. Turbulence-enhanced contact rates are thus determined to be a necessary component in our descripti on of the growth of cod and haddock larvae on Georges Bank. Model cod larvae with growth rates comparable to those observed in the held are located below the surface layer (deeper than 25 m) and inside the 60 m isobath. The region of highest retention due to circulation processes (Werner et al., 1993; Fisheries Oceanography, 2, 43-64) coincides wit h the region of highest growth rates and highest larval survival. Ther efore, there is a complementary interaction between trophodynamic and circulation processes, with those larvae most likely to remain on the Bank also being those in the most favorable feeding regions. Haddock l arvae require higher prey densities than cod larvae to survive. Copyri ght (C) 1996 Elsevier Science Ltd