Trophodynamics of the Fish Valenciennellus tripunctulatus. II. Selectivity, Grazlng Rates and Resource Utilization

Abstract

Various patterns of resource utilization in the feeding strategy of V. tripunctulatus, a small mesopelagic fish of tropical-subtropical oceans, were explored. Ivlev electivity indices suggest that copepods in general and the genus Pleuromamma in particular are being actively selected. Prey-size selection was apparent and strong negative indices were calculated for all plankton smaller than 1 mm. Non-crustacean prey in all size classes gave negative indices. Strong positive indices were indicated for prey size classes > 2 mm. In order to estimate grazing rates and efficiencies, a model which is linearly influenced by swimming speed and plankton abundance but exponentially affected by predator perception distances was developed. Grazing efficiencies (i.e., prey ingested/prey encountered) for individual fish based on the model are relatively high (> 45%) for copepods in the 2-5 mm size classes. Daily grazing pressure by the V. tripunctulatus population on copepod resources was low (< 9%) for prey size classes < 3 mm, but were as high as 43% for copepods in the 3-5 mm size range. Predation impact on members of the genus Pleuromamma, > 4 mm, approached 100% in zones of highest predator density. The model suggests that V. tripunctulatus must have a sparse and nonaggrgated distribution, and search considerable volumes of water each day to obtain its daily ration. While the impact of V. tripunctatus alone on the prey resource field is not great, it apparently time-space shares common food resources with a broad array of zooplanktivorous predators in the mesopelagic environment.