US GLOBEC Progress Report. Lynch et al.

US GLOBEC Progress Report

Importance of Physical and Biological Processes to Population Regulation of Cod and Haddock on Georges Bank: A Model-Based Study

Principal Investigators

D.R.Lynch, F.E. Werner, J.W. Loder, M.M. Sinclair, R.G. Lough, R.I. Perry, F.H. Page, D.A. Greenberg, P.C. Smith, W.C. Smith

Co-Investigators

C.E. Naimie, C.G. Hannah, J.F. Manning, B.O. Blanton
(more details about PI's)

Grant Period

July 1, 1993 - June 30, 1996

Project Summary

This is a modeling study of cod and haddock early-life stages on Georges Bank, within the framework of the GLOBEC NW Atlantic Implementation Plan. We employ a) a detailed hydrodynamic circulation model which is shelf-wide in coverage and includes all major shelf and bank physical processes with high local resolution on Georges Bank; b) particle-tracking and advective-diffusive models driven by the circulation; and c) behavioural and trophodynamic Individual-Based Models to incorporate swimming and interaction between larval fish and their principal prey, Calanus finmarchicus and Pseudocalanus sp.

Computational experiments, historical data analysis, and a moored field program will support increasingly detailed simulations focused on 4 major scientific issues:

Lagrangian circulation and larval retention on the Bank

Turbulence distribution and its impact on predator - prey encounter rates

Frontal circulation and the cross-frontal exchanges of zooplankton

Interannual variability in key physical variables and its role in cod and haddock population variability.

(Click here for more detail.) These investigations will provide understanding and methodology required for analysis of Georges Bank's response to hypothesized climate change scenarios.

Study Elements

The elementary tools of our work are models and data. Here we discuss each of the major elements.

Models

Computer models are a principal tool of analysis and integration in this project. A suite of 3-dimensional models has been assembled. All use state-of-the-art finite element methods. The geographic scope includes the entire Gulf of Maine and the Scotian Shelf (figure 1); and extends seaward to the 1200m isobath, terminating in an idealized ocean (figure 3a). Realistic topography with resolution as fine as 1 km on Georges Bank has been achieved (figure 2). There are several models:

Prognostic Model with Turbulence Closure: This is the principal hydrodynamic tool. It is a 3D free-surface model with full nonlinearity, operating in the time domain. The Mellor-Yamada level 2.5 turbulence closure provides stratification- and shear-dependent vertical mixing. Heat, salt, and two turbulent variables are transported in tidal time.

Diagnostic Model: This model was developed in earlier studies and brought to maturity in the present study. It provides faster solutions in cases where the density field can be prescribed a priori. It is currently used for generating initial conditions for the Prognostic Model, and in large-scale diagnostic studies of decadal-scale variability.

Advective-Diffusive-Reactive Model:This is a 3-D time-domain model with the prognostic model's algorithms for transport calculations. It is driven by stored flow fields taken from either of the hydrodynamic models. It is useful in studying the transport and interactions of species which can be represented at the population or concentration level of abstraction (e.g. NPZ models.)

Lagrangian Particle-Tracking:This model follows the path of an individual particle in the flow field, through tidal time. There are two basic options relating to vertical velocity: passive (i.e. a fluid parcel) and fixed-depth (simulating the motion of a drifting instrument or an organism with perfect depth regulation).

Individual-Based Models: These models follow the trajectories of individuals, as they move through the variable environment. The base state of the IBM is the passive particle, which involves only Lagrangian particle-tracking in the computational flow field. To this is added behavioural simulation, including buoyancy and swimming; growth and feeding relationships; and hydrodynamic dispersion via random walk. All of these features are in general sensitive to both the current state of the individual (e.g. age, weight), to the ambient fluid state (temperature, salinity, stratification, turbulence), and to the biological environment (e.g. prey concentrations, predation).

Historical Data

At present we are relying on the large amount of historical data for both model construction and testing, and as an object of study in themselves. Fresh data from the GLOBEC Georges Bank field program is incorporated as it becomes available.

Temperature/Salinity Database and Optimal Estimation. The Atlantic Fisheries Adjustment Project (AFAP) historical database, assembled and maintained at BIO, is our principal information source for historical temperature, salinity and density fields. Optimal linear interpolation procedures are used, including refinements for anisotropy in the correlation scales.

Currents. We have assembled a database of monthly-mean currents on Georges Bank from previous moored measurements by various U.S. and Canadian agencies (DFO, EG&G, NMFS, USGS, WHOI). This database is used in the validation of our circulation models and, for selected areas, further analyzed to improve our understanding of key processes.

Collation of Biological Data. There is considerable historical biological data available for Georges Bank. Published data relative to egg, larval, and juvenile distributions are available e.g. in Smith and Morse (1985); Morse et al, (1987); Lough and Bolz (1989); Polacheck el al (1992); Sherman et al. 1988). Unpublished historical data includes the 1978 Larval Herring Patch Study (Lough and Trites 1989); and an 11-year MARMAP time series (1977-87) of monthly/bimonthly surveys of cod and haddock eggs and larvae.

Field Program

Moored Measurement Program. In order to monitor the two primary inflows to the Gulf of Maine-Georges Bank region during the GLOBEC field years, we have deployed three long-term current meter moorings off southwest Nova Scotia (C2), on the eastern and western sides of Northeast Channel (NECE, NECW), and the Bank's Northeast Peak (NEP). Smith (1983) has demonstrated that C2 measurements provide a reasonable proxy for the coastal input form the Scotian Shelf, while Ramp et. al.'s (1985) results indicate that the net inflow of Slope Water may be monitored by the deep moorings in Northeast Channel. The primary goal of the mooring program is to monitor annual and interannual variability in the inputs to the Gulf circulation, in order to interpret other observations from Georges Bank and to validate the model results.

Progress to Date

Last update: 14 September 1995

Daniel Lynch: d.r.lynch@dartmouth.edu