Digestion times and predation potentials of Pelagia noctiluca eating fish larvae and copepods in the NW Mediterranean Sea

Abstract

Predation is the principal direct cause of mortality of fish eggs and larvae (ichthyoplankton). Pelagic cnidarians and ctenophores are consumers of ichthyoplankton and zooplankton foods of fish, yet few estimates exist of predation effects in situ. Microscopic analyses of the gastric 'gut' contents of gelatinous predators reveal the types and amounts of prey eaten and can be used with digestion time (DT) to estimate feeding rates (prey consumed predator?1 time?1). We measured the DT and recognition time (RT) of prey for Pelagia noctiluca, an abundant jellyfish with increasing blooms in the Mediterranean Sea. DT of fish larvae averaged 2.5 to 3.0 h for P. noctiluca (4?110 mm diameter) and was significantly related to jellyfish and larval sizes. In contrast, DT of fish eggs ranged from 1.2 to 44.8 h for jellyfish ?22 mm diameter ('ephyrae'), but DT was not significantly related to ephyra or egg diameter. Approximately half of the eggs ingested were not digested. DT of copepods averaged 4 h. We also measured DT and RT of salps, euphausiids, and miscellaneous zooplankton. Temperature (20?25C) generally did not significantly affect DT of any prey. Estimated potential predation effects of ephyrae on fish larvae in the Catalan Sea in 1995 showed great variability among 9 stations (0?3.7% consumed h?1). We discuss how sampling methods contributed to variation in predation estimates and offer recommendations to improve accuracy. Our results enable estimation of predation on ichthyoplankton and competition for zooplankton prey, which can have extremely important effects on fish populations globally.