DRIFTING MESOCOSMS - THE INFLUENCE OF GELATINOUS ZOOPLANKTON ON MORTALITY OF BAY ANCHOVY, ANCHOA-MITCHILLI, EGGS AND YOLK-SAC LARVAE

A mesocosm system was developed to be deployed from a research vessel and set adrift with its enclosed plankton, including fish eggs, larvae, and gelatinous predators. The system consists of an array of mesocosms that are 1 m diameter, 5 m long, and 3.2 m(3) capacity, and which are constructed of 20-mu m porosity dacron. Deployment and harvesting procedures are described. The mesocosms ''capture'' a sample of the water column and provide an assay method to examine planktonic processes in experiments of a few hours' to a few days' duration. Mortality rates of bay anchovy, Anchoa mitchilli, eggs and yolk-sac larvae were estimated from drifting mesocosm experiments in 1989 and 1991. Overall mean instantaneous rates were 0.073 h(-1) for eggs and 0.051 h(-1) for larvae, indicating that 95% of a cohort had died within 2 days of hatching. Egg and larval mortality rates were variable due to the variability in the initial numbers of captured eggs and larvae and to the complex mix of predator sizes and biovolumes that were enclosed in different deployments. An attempt (only partially successful) was made to relate observed mortalities to the abundance of two enclosed gelatinous predators, a ctenophore Mnemiopsis leidyi and a scyphomedusan Chrysaora quinquecirrha. The mesocosm results are compared with those from an encounter model that predicts predation mortality. The results are also compared with synoptic ichthyoplankton surveys and an analysis of predator gut contents, which provide independent estimates of egg/larvae mortality and their consumption by gelatinous predators. The drifting mesocosm method holds promise and, with modifications of experimental design, can be a valuable tool to study zooplankton population processes, including those of early life stages of marine and estuarine fishes.