L. P. Madin, E. Horgan, M. Butler, B. K. Sullivan, G. Klein-MacPhee, and S. M. Bollens
Large zooplankton predators such as pelagic hydroids, hyperiid amphipods, decapod shrimp, chaetognaths, medusae and ctenophores affect survival and recruitment of larval cod and haddock on Georges Bank both through direct predation and through competition with the larvae for copepod prey, the target species Calanus finmarchicus and Pseudocalanus spp. As part of the GLOBEC Georges Bank program, we are estimating the magnitude of these effects, using information on the co-distribution of predators and prey and the specific feeding rates for predator-prey pairs. Distributional data for predators and prey is from 1m2 and 10m2 MOCNESS net hauls made during Process and Broad-scale Survey cruises in 1994-97; these data indicate wide interannual variations in the qualitative and quantitative composition of the predator assemblage. Gelatinous predators are still not adequately sampled with conventional gear. Feeding rates and prey selection are estimated by several methods depending on the predator, including analysis of gut contents, feeding experiments, and energetic calculations. Because prey are sometimes unidentifiable in gut contents, we are developing an immunological assay for the specific detection of Calanus remains in predators. Results for predation impact on copepods are discussed in this presentation.Estimates of predation impact at well-mixed ("crest") and stratified ("flank") sites on Georges Bank in May, 1995 indicate the relative importance of predator and prey abundances. Calanus were over ten times more abundant at the flank site that at the crest. Estimated daily removal by single predator species ranged from <1% to 43%, and total invertebrate predation summed to 110% at the crest site, where predators were almost as abundant as prey. At the flank site, where prey outnumbered predators by one or two orders of magnitude, maximum removal by a single predator type was 7%, and the total of all invertebrate predators was only 10%. Estimates of predation mortality for crest and flank stations in May 1996, based on Broad-scale data for abundances of predators and prey, show large differences depending on relative abundance. At the crest station, where hydroids were abundant, they removed up to 400% of the nauplii per day, but at the flank station, where they were sparse, only 1-2% of the nauplii were consumed. While specific feeding rates may differ by a factor of 2-5 among predator species, predator abundances can vary by several orders of magnitude, making this parameter the most important determinant of predation mortality for target-species populations. We will use time- and site-specific estimates of prey and predator distributions to calculate removal by invertebrates, and compare this mortality with other losses of target species over the entire Bank, throughout the winter-spring sampling season.