Project Summary

Francisco Werner and Jeff Runge

Productivity of Calanus finnmarchicus and fluctuations in larval cod and haddock abundance on Georges Bank: A synthesis of observations and modeling

This proposal addresses a hypothesis linking spatial/temporal variability in copepod productivity and the growth and survival of cod and haddock larvae on Georges Bank. It represents a synthesis of survey results and process observations conducted prior to and during the U.S. GLOBEC Georges Bank Program, and uses the recently-developed biophysical, individual-based model (IBM) of cod and haddock larvae to investigate the importance of variation in copepod prey field production relative to other factors, including circulation, temperature, turbulence and light, that potentially contribute to the success or failure of recruitment into Georges Bank fish stocks.

Cod and haddock larvae on Georges Bank feed primarily on the eggs and nauplii of the dominant copepod species (Kane, 1984; Lough and Mountain, 1996). The vertically-integrated copepod egg production (VIP, in terms of eggs m^{-2}d^{-1} or mg-C m^{-2}d^{-1}) is therefore an index of the daily production of larval fish prey. Research during earlier phases of the Georges Bank program showed that, in March and April, 1995, the total copepod VIP varied by 1-2 orders of magnitude on the southern flank of Georges Bank, where the fish larvae are typically found in highest concentrations as they drift in a southwesterly direction from spawning grounds on the Northeast Peak. The production of Calanus finmarchicus was a dominant influence on the pattern of total copepod VIP. This dominance derives in part from the tremendous capacity of C. finmarchicus to produce and sustain egg production, given sufficient nutrition, relative to other copepod species (Runge, 1988).

These data suggest that variability in Calanus and total copepod egg production is potentially a major influence on the availability of prey to fish larvae on the southern flank. However, there is very little knowledge about interannual variation in both the magnitude and spatial structure of total and Calanus egg production. Are there indeed large interannual differences in mean depth-integrated production? Are the patterns of production observed in 1995 persistent interannual features in Mar-June on Georges Bank? We suggest here that the pattern and magnitude of total egg production on Georges Bank is highly variable from year to year. If a high abundance and sufficient nutrition for Calanus females is set up in some years on the northeast peak and southern flank, food conditions for fish larvae could increase by an order of magnitude or more, with consequent dramatic improvement in their growth and survival.

Based on these considerations, we put forward the following, three-part hypothesis: (1) the spatial, seasonal and interannual variation in the total VIP (all dominant copepods) is substantial (i.e., orders of magnitude), (2) there is a significant relationship between the VIP and growth and survival of fish larvae on the southern flank and 3) variability in the productivity of Calanus finmarchicus is a primary determinant of the fluctuations in the total copepod VIP during the larval growth period (March-June).

The hypothesis would be tested by a combined retrospective and modeling study. Variability in the total and species-specific egg production rates (part 1) would be estimated and spatial productivity maps generated from analysis of preserved plankton samples taken during broadscale surveys conducted between 1996-1999 and during finer-scale bongo surveys conducted on the southern flank in April/May 1993-95 and 1997 by G. Lough and co-workers. Vertically integrated production is calculated for each dominant copepod species from the product of the female abundance and the egg laying rate, estimated either from an empirically-calibrated reproductive index measured from preserved females or from directly-observed rates obtained during process cruises. The prediction of an empirical link between the VIP and larval fish growth rates (part 2) would be tested by investigation of relationships between the VIP and growth rates of fish larvae, estimated from RNA/DNA ratios by L. Buckley and co-workers during the finer-scale southern flank surveys. The role of variation in Calanus production in the Georges Bank ecosystem (part 3) would be investigated by sensitivity analysis using the IBM model and the range of VIP values obtained from the retrospective analysis.

Calanus finmarchicus and pelagic stages of cod and haddock are target species within the U.S. GLOBEC Georges Bank program. A linkage between Calanus and recruitment success of fish has been proposed in other North Atlantic systems (e.g. Cushing, 1985; Runge, 1988; Ellertson et al. 1989; Skreslet, 1989; Brander and Hurley, 1992; Anderson, 1994; Runge et al. submitted) but it is not clear whether such a connection exists on Georges Bank. Our proposed study addresses this issue, and has the longer-term objective of identifying key biological and physical variables controlling fluctuations in copepod productivity and abundance of fish larvae on the Bank.