SIGNIFICANCE OF BODY SIZE TO THE INTERACTION BETWEEN A LARVAL FISH (MALLOTUS-VILLOSUS) AND A VERTEBRATE PREDATOR (GASTEROSTEUS-ACULEATUS)

We investigated the influence of variations in the size of prey (Mallotus villosus) and a vertebrate predator (Gasterosteus aculeatus) on larval fish mortality rates during the period of yolk absorption using mid-size mesocosms (2.7 m3). Increasing predator size increased mortality rates of capelin larvae. Variations in larval capelin size resulted in 2 distinct patterns. Between experimental trials, greater mean size of larval capelin in the mesocosm reduced mortality due to predation. Within an experimental mesocosm, larger larvae suffered higher mortality than smaller individuals. Contrasting patterns of size-dependent vulnerability to predation reflect the hierarchy of processes that determine the probability that a larval fish will be preyed upon. The broad scale response of the predator was determined by the mean relative sizes of prey and predator which govern the average probabilities of encounter, attack and capture. Within the search ambit of a predator (e.g. area or volume searched within a complete diurnal foraging cycle) active prey selection for larger prey due to either greater visibility or higher energy reward was an important factor. A comparison of our results with estimated predation rates by the jellyfish Aurelia aurita indicates that at a similar size a gelatinous zooplankter consumes fewer larvae than a stickleback and is a less efficient predator as measured by the energy ingested relative to energy demands. For both vertebrate and invertebrate predators, the ratio of prey to predator lengths was a strong predictor of the daily mortality rate due to predation. Relative prey-predator sizes may provide a useful perspective to assess changes in larval fish vulnerability as they grow through a predator field.