Distribution and Survival of Cod Larvae in Relation to Hydrography on Georges Bank

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This project will investigate the "distributional histories" of larval cod on Georges Bank during the spring and early summer period of developing thermal stratification. The period of thermal stratification is a time during which the intricacies of larval cod vertical and areal distributions may hold important implications for survival and ultimately recruitment. This project will utilize a new technique of otolith analyses (Sr/Ca ratios), which has recently been developed, to hindcast the temperature histories, and hence distributional histories (that is from the time of hatching to capture), of individual cod larvae collected in various hydrographic regimes in the Georges Bank area. The technique will be advanced by employing a new technology, electron energy loss spectroscopy, which will increase the resolution of the Sr/Ca technique to a level capable of resolving sub-daily temperature histories. These analyses could reveal diel vertical migrations into or through a thermocline. Larval cod are believed to be more closely associated with the stratified waters on the sides of Georges Bank than the tidally mixed waters on top of the Bank, and are transported around Georges Bank and to the southwest with the residual circulation. The abundances of their zooplankton prey are also more abundant on the stratified side of the thermal front in early spring, and the larvae are often associated with the depth of maximum prey abundance in the thermocline. Later in the year the late larvae and early juveniles are on the top of the Bank, where zooplankton densities become greater than on the flanks. The exact nature of the vertical and areal distributions of the larvae in relation to mixed and stratified waters may be very important with regard to horizontal transport processes, temperature effects on growth, and the availability of zooplankton prey items. This project will employ new technology to examine the distributions of cod larvae and will form a collaborative link with an already-funded study (NOAA) that focuses on the physics of the vernal stratification process as it relates to the complete suite of zooplankton, and the distribution, growth and condition of cod and haddock larvae, thus making for a strong interdisciplinary program