Juvenile flounder, Platichthys flesus (L.), under hypoxia: effects on tolerance, ventilation rate and predation efficiency

The flounder, Platichthys flesus, is the most abundant flatfish species in the northern Baltic Sea. Juvenile P. flesus are important epibenthic predators on shallow sandy bottoms in this area, and play an important role in structuring benthic infaunal communities by both lethal and sublethal predation. These areas are affected by periodic oxygen deficiency during summer and autumn when drifting algal mats are abundant, which causes changes in both structure and function of the zoobenthic community. Experiments were performed to test for the effects of hypoxia on: (1) tolerance; (2) ventilation rate; (3) lethal predation efficiency; and (4) sublethal predation efficiency (siphon cropping) of P. flesus. In the lethal predation experiment, the amphipod Bathyporeia pilosa was used as prey, and in the siphon cropping experiment, the bivalve Macoma balthica was used. Recorded LT values of P. flesus exposed to hypoxia were 1835687 50 (mean6SE) min at 20%, 14968 min at 10% and 2362 min at 5% oxygen saturation (138C, S 5‰). In hypoxia, ventilation rate of P. flesus increased at 30% and 20%, and decreased at 10% and 5% oxygen saturation compared with normoxia. The predation efficiency of juvenile P. flesus was significantly lower at 20% and 30% compared with 40% and 100% oxygen saturation. In the sublethal predation experiment, the flounders cropped siphons most efficiently at moderate hypoxia (40% oxygen saturation) compared with normoxia. Our results indicate that sublethal oxygen saturations (30–40%) cause changes in physiology and predation behaviour of juvenile P. flesus. Such functional changes in benthic communities, at sublethal oxygen saturations, may influence the energy flow of benthic food webs in areas that are affected by periodic oxygen deficiency.  1999 Elsevier Science B.V. All rights reserved.