Greg Lough, Jim Manning, Elaine Caldarone, Larry Buckley, Lew Incze and Dave Townsend
INTRODUCTION and OBJECTIVES
Phase III process studies of the GLOBEC NW Atlantic Program are intended to measure, model and understand cross-frontal fluxes, the physical processes that drive them, and the biological consequences for communities on Georges Bank with emphasis on target species. Larval fish (in this case cod and haddock) often are concentrated in the region of tidal fronts, which have unique biological and physical conditions and can result in enhanced along-isobath advection as well as exchange and retention in shoal areas. 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 physcial 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 directly. Consequently, we plan to make high-resolution meaurements 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/fuorescence/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/VPR, 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. In GLOBEC Phase II (April and May 1997) we sampled larval fish with 10 m vertical resolution (ca. 90 minute MOCNESS tows) and prey at 2-5 m vertical resolution (ca. 30 mins. per CTD/pump profile) following drifters and x-shelf transects. With dedicated ship time we were able to document diel and environmentally modulated changes in distribution of larval fish and their prey in and across a front, along with physiological sampling of the larvae.
Our project implements a modeling component. As originally proposed, we would have used a three-dimensional circulation model 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 and to test the forecasting we had made with model runs aboard ship. Budget reductions have forced us to eliminate the onboard modeling, although we have kept the hindcasting and are trying to determine how we might do some on-board modeling. Also cut from our program was a field experiment with a direct comparison between model trophodynamic simulations and the deployment of marked larvae in tethered open-mesh chambers. Up to three chambers were to be deployed on either side of the front for a week to provide direct estimates of advection, growth and survival in the absence of predators at ambient prey concentrations. In addition to the two specific cuts above, component budgets of our joint proposal were given various reduced targets, as much as 25%. All of these targets have been met in budgets to NEFC (Lough et al.), URI-GSO (Buckley), Bigelow Lab (Incze) and U. Maine (Townsend).