Predation Rates of Zooplankton by Fish Quantified with a Novel Acoustic and Optic System

Zooplanktivory by fish is a major trophic link in most marine food webs. Quantifying the rates of that predation remains a major challenge which has rarely been measured in situ. Here we used a novel high-frequency sonar and camera system, called ZOOPS-O, to measure the rate of zooplankton feeding and prey selectivity by coral-reef fishes in the Red Sea. The system’s high resolution camera was used as a “ground truth”, relating the density of photographed zooplankters to that obtained with the sonar in different ranges of target strengths. Copepods were dominant (>75%). Two ZOOPS_O systems were deployed in the coral reef, one up-current and the other down-current of a site-attached group consisting of hundreds of fishes, dominated by the Reef Anthias (Pseuanthiass squamipinnis). Predation rate by the whole group was most intense with a removal rate of up to 250 zooplankters s^-1 per cross section of 1 m² along the space occupied by the group. Predation rates dependent mostly on ambient prey density, with no apparent effect of current speed. The feeding was highly selective for larger targets. The use of acoustics combined with optical imaging for target validation is a very effective, non-intrusive method for quantifying zooplankton predation in the sea. The high abundance of zooplanktivorous fish in the coral reef together with the fact that most of the prey originated in the off-shore waters, indicate that the trophic link created by the fish is a major pathway for the import of allochthonous nutrients to the reef community.