Estimating the predatory impact of gelatinous zooplankton

We propose a new approach to assess the predatory impact of gelatinous zooplankton on their prey, in which information from field samples and laboratory experiments allows us to estimate predator ingestion rates from analyses of predator gut contents. The feasibility of this approach was tested in laboratory experiments with the ctenophore Pleurobrachia bachei feeding on adult stages of the copepods Pseudocalanus newmani and Acartia longiremis. We developed a simple model of predator ingestion‐egestion dynamics. The model assumes that predator clearance rates, F, and instantaneous egestion rates, ε, are constant over the range of prey concentrations appropriate to the field study. A series of experiments was designed to test the validity of these assumptions and to estimate values of the parameters F and ε for Pleurobrachia feeding on Pseudocalanus and Acartia. Results from these experiments indicate that the above assumptions are reasonable for these predator‐prey pairs until prey concentrations exceed 60,000 prey m −3 . Ingestion rates are shown to be proportional to predator gut contents, with the slope of the relationship providing an estimate of the instantaneous egestion rate. Provided that the model assumptions are met, this approach can be used to estimate the predatory impact of other planktivorous predators on more complex prey assemblages.