Project Summary

Larry P. Madin, Barbara K. Sullivan, Grace Klein-MacPhee, Steve M. Bollens.

U.S. GLOBEC: Predation Impacts on Target Species: Roles of Frontal Processes and Small Predator Species

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 Calanus finmarchicus and Pseudocalanus spp. Central among these processes is predation by invertebrate and fish predators on these four species. The overall objective of our proposed research is to make a quantitative assessment of time- and site-specific predation mortality on populations of the target species. Our 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.

We have two major foci for our Phase III research. The first is to determine the role of frontal processes in aggregating prey and predators, and in modulating predator-prey interactions. We will examine different functional groups of predators, each with a different predicted response. For instance, non-motile predators may be concentrated in the fronts to the same degree as their prey, in which case the impact of these predators may depend on their functional response. Predators with a linear response (e.g. lobate ctenophores) would inflict higher per capita mortality on the prey, because these predators are not saturated at high prey densities. Other predators (e.g. omnivorous copepods and hydroids) which do experience saturation in their feeding response might impose reduced per capita prey mortality rate due to "swamping" of their feeding mechanisms. Under these conditions, non-saturating predators may become disproportionately important. On the other hand, motile or strong-swimming predators, such as juvenile fish and large crustaceans, might also have an important behavioral response, actively locating and exploiting frontal regions, and thereby having a larger predatory impact than in non-frontal regions. Physical transport processes at fronts may move predators and prey from one side to another, and seasonal or interannual variations in these exchanges may affect predation mortality.

Our second major focus is to examine the role of omnivorous copepods (e.g. Centropages spp.) as predators on target copepods. We suspect an important role for these species, based on recently observed abundances and the feeding behavior of congeners, but they have not yet been thoroughly studied by GLOBEC or other investigators. This work will rely heavily on laboratory experiments to determine feeding rates, and functional responses of common ominvorous copepods on eggs and nauplii of target species.

Specific tasks proposed here for Phase III are:

1. Determination of Bank-wide distribution and abundance of vertebrate and invertebrate predators of target species, from Broad Scale Survey samples (continued from Phases I and II).

2. Determination of distribution and abundance of predator populations within frontal regions, particularly at the tidal mixing front between the well-mixed and stratified waters on the southern flank, and determination of transport within and across such fronts. The work would be done on two cruises to the S Flank coincident with high concentrations of Calanus and predators. The first is in collaboration with Durbin on the south flank in late April-early May. A more extensive analyis will be conducted in June in collaboration with the linked proposals of Wishner; Gifford, Sieracki, Belkin; and Hebert, Barth.

3. Comparison of predation impact by invertebrate predators on Calanus larvae with demographic estimates of mortality developed from simulataneous sampling by Ohman and Durbin. This would be done on the late April-early May cruise to the south flank with Durbin, Campbell, Runge, Ohman.

4. Determination of specificity and rates of predation on copepod nauplii and eggs by omnivorous copepods and hydroids, using both ship- and shore-based manipulation experiments.

5. Estimation of rates of predation on all target species by larger invertebrate predators not yet studied, e.g., euphausiids, mysids, gammarids, based on gut evacuation rates and stomach content analysis using a combination of microscopic, immunological, and DNA-probe methods.

6. Estimation of total predation mortality on target species as a function of season and location on the Bank, and incorporation of these estimates into collaborative modeling efforts with other GLOBEC investigators.