Predation by juvenile Platichthys flesus (L.) on shelled prey species in a bare sand and a drift algae habitat

Due to increasing eutrophication of the coastal Baltic waters. drifting algae are a common phenomenon. Drifting algal mats accumulate on shallow sandy bottoms in late summer and autumn. and affect the ambient fauna. Juvenile flounder, Platichthys flesus, utilize these habitats during their first few years. They feed on benthic meio- and macrofauna, part of their diet consists of shelled species, such as Ostracods, and juvenile Hydrobia spp. and Macoma balthica. Earlier studies have shown that up to 75% of ostracods and 92% of hydrobiids survive the gut passage of juvenile flounder, while all M. balthica are digested by the fish. We conducted laboratory experiments to study how the shelled prey responded to a drift algal mat, and the predation efficiency of juvenile P. flesus on these prey species on bare sand and with drifting algae (50% coverage). Hydrobia spp. utilized the drift algae as a habitat and, after 1 h, 50% had moved into the algae: ostracods and M. balthica were mon stationary and, after 96 h, only 23 and 12%, respectively, were found in the algae. Fur the predation efficiency of P. flesus, a two-way ANOVA with habitat (algae, bare sand) and predation (fish, no fish) as factors revealed that both algae and predation affected negatively the survival of all three prey species. The algae, thus, affected the predation efficiency of juvenile P. flesus and the consumption of prey was much reduced in the algal treatments compared to the bare sand. This was due probably to increased habitat complexity and the ability of prey, especially hydrobiids. to use the algal mat as a refuge. Altered habitat structure due to drift algae, together with the resultant changes in habitat (refuge) value for different prey species, may profoundly change the structure of benthic communities.