Effect of Container Size on Estimates of Mortality and Predation Rates in Experiments with Macrozooplankton and Larval Fish

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We investigated the effect of container size on the predation rate of the jellyfish Aurelia aurita on postemergent capelin (Mallotus villosus) larvae in large in situ cylindrical enclosures. Total depth of the enclosures varied between 1 and 9 m and yielded volumes that ranged from 0.26 to 6.35 m3. Instantaneous mortality rates of capelin larvae due to jellyfish predation varied inversely and nonlinearly with the volume of the enclosure. Over the range of container sizes used, mortality rates differed by one order of magnitude. Container volume explained 80% of the variance in mortality rates, while predator size and larval size accounted for 8.3 and 2.3% of the variability, respectively. Mortality rate was independent of the initial larval density within each container size. Differences in mortality rates among container sizes are explained by the observed differences in the vertical patchiness of larvae among various enclosure sizes. These findings indicate that experiments conducted in small containers typical of those used in laboratory studies seriously overestimate rates of larval mortality due to predation. Direct extrapolation of the results of such laboratory studies to field conditions is therefore not warranted. We conclude that the vertical distributions of predator and prey may be the most important factor controlling predator–prey relationships in situ.