The GLOBEC Georges Bank program is seeking to understand the physical and biological processes which determine the population dynamics of four target species -- larval cod and haddock, and their main forage prey, the copepods Calanusfinmarchicus and Pseudocalanus spp. Central among these processes is predation by invertebrate and fish predators on these four species, and the objective of this study is the description and quantification of this source of mortality. The approach combines information on the abundance and co-occurrence of predators and target prey species on Georges Bank, derived from the continuing Broad Scale Survey, with new data on feeding specificity and rates of the principal predator species. During Phases I and II, Drs. Madin, Sullivan and Bollens have used both gut-contents analysis (by microscopy and with immuno-assays) and feeding experiments (shipboard and laboratory, in small containers and mesocosms) to obtain feeding data. The field sampling during the first two Phases focused on the themes of stratification effects (1995) and source, retention and loss of water and organisms (1997). In Phase III they will direct field efforts (in 1999) to questions concerning effects of frontal regions on the transport, aggregation and interaction of target species and their predators. Conventional sampling will be supplemented with a video system to detect large gelatinous predators. Significant new contributions to the study of plankton community dynamics will be identification of the role of omnivorous copepods in limiting production of herbivores, and experimental studies of the role of turbulence in controlling predation rates of gelatinous predators. The physical and hydrographic environment of Georges Bank sets the stage for the biological interplay of target species and their predators. The bathymetry of the Bank and the pattern of circulation on and around it create frontal regions on its north and south boundaries, and along its 60 m isobath, between the well-mixed central Crest and the seasonally stratified southern Flank. This last region, the tidal mixing front, divides two areas which are very different in terms of hydrography and biology. These researchers will work at this front on two cruises in 1999, to determine the distribution and abundance of prey and predators along and across the front, the effect of frontal dynamics on aggregation and interaction, and the exchange of organisms from one side to the other. The Crest and Flank regions are where they have concentrated efforts in previous process studies, and where there appears to be a critical overlap in occurrence between target species and their main predators. Turbulence, convergence and exchange associated with the front may have different effects on predators with different feeding biologies. The key to long-term understanding of the Georges Bank system is development of comprehensive physical-biological models. This study should make a major contribution to these models with data on predation mortality which is specific to season, location, species, and for principal predators, stage and density of the prey.
