Predator-prey relations between age-1+ summer flounder (Paralichthys dentatus, Linnaeus) and age-0 winter flounder (Pseudopleuronectes americanus, Walbaum): predator diets, prey selection, and effects of sediments and macrophytes

Laboratory experiments and weekly trammel net surveys in the Navesink River, New Jersey (USA) were used to examine the predator-prey interaction between age-1+ summer flounder (Paralichthys dentatus) and age-0 winter flounder (Pseudopleuronectes americanus). Winter flounder (24-67 mm TL) were the dominant piscine prey of summer flounder (n=95, 252-648 mm TL) collected in trammel nets. We observed a temporal shift in summer flounder diets from sand shrimp (Crangon septemspinosa) and winter flounder, dominant during June and early July, to blue crabs (Callinectes sapidus) and other fishes (primarily Atlantic silversides, Menidia menidia and Atlantic menhaden, Brevortia tyrannus) later in the summer. Variations in prey selection appeared to be related to changes in the spatial distribution of predators and spatio-temporal variation in prey availability. In laboratory experiments, summer flounder (271-345 mm total length, TL) preferred demersal winter flounder to a pelagic fish (Atlantic silversides) and a benthic invertebrate (sand shrimp) prey, and the vulnerability of winter flounder increased with increasing prey body size from 20 to 90 mm TL. Experiments testing habitat effects showed that mortality of winter flounder in three different size classes (20-29, 40-49, 60-69 mm TL) was not influenced by sediment grain sizes permitting differential burial of the prey. However, vegetation enhanced survival, with fish suffering lower mortality in eelgrass (Zostera marina, 15+/-0.04%) than in sea lettuce (Ulva lactuca, 38+/-0.04%) or bare sand (70+/-0.07%) when the macrophytes were planted to produce similar leaf surface areas (5000 cm(2) m(-2)). Prey vulnerability appeared to be related to the role of vision in the predator's attack strategy and prey activity levels.