Stephen M. Bollens and Heidi Franklin
Department of Biology and Romberg Tiburon Center for Environmental Studies,
San Francisco State University, San Francisco, CA 94132

Larry P. Madin and Erich F. Horgan
Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA
02543

Barbara K. Sullivan and Grace Klein-MacPhee
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, 02882
 

The Georges Bank GLOBEC (Global Ocean Ecosystem Dynamics) program is a multi-investigator, interdisciplinary research program investigating the physical and biological processes affecting the population biology of four target species - two copepods (Calanus finmarchicus and Pseudocalanus spp.) and two larval fishes (cod and haddock) - on Georges Bank/Northwest Atlantic.  Our project has focused on the role of predation on these target species.  Our approach combines direct measures of the co-distribution of predators and prey from plankton nets (1m^2 and 10m^2 MOCNESS), SCUBA observations and fishery research trawls made during broad-scale surveys and site-specific process cruises between 1994-1997.  Feeding rates and prey selection of the main predator species are estimated by several methods, depending on the predator, including analysis of gut contents, feeding experiments in shipboard or laboratory incubations, and energetic calculations.  More typical and expected invertebrate predators of concern include hyperiid amphipods, decapod shrimp, chaetognaths, medusae and ctenophores; vertebrate predators of concern are primarily herring and mackerel.  One unexpected but nevertheless very important predator is planktonic hydroids (principally Clytia gracilis), which we have found in huge numbers suspended in the plankton on Georges Bank, especially over the central, shoal region.  Laboratory studies indicated that the hydroids were capable of consuming cod larvae and young copepods; in the latter case at rates comparable to 50% to over 100% of the daily production in the central region of the Bank.  Additional laboratory investigations on the feeding, growth and life history characteristics of planktonic hydroids indicate an important role for mixing in addition to food availability.  This species of hydroid seems to be peculiarly adapted to thrive in the plankton.