Seagrass habitat complexity does not always decrease foraging efficiencies of piscivorous fishes

The foraging efficiencies of predators in relation to seagrass density and microhabitat use patterns of prey during predator presence/absence were examined by laboratory experiments. The foraging efficiency of the permanent resident piscivorous sculpin Pseudoblennius cottoides, which employed ambush and stalk-and-attack tactics, was higher in the presence of seagrass, while that of the transient piscivorous gnomefish Scombrops boops, which employed chase-and-attack tactics, decreased in denser seagrass. When coexisting with gnomefish, juveniles of the prey goby Gymnogobius heptacanthus sometimes entered seagrass areas to avoid attacks by such predators. In the absence of gnomefish, however, goby juveniles almost always stayed outside the seagrass areas (instead inhabiting open areas adjacent to the outer seagrass edge), even when sculpin were absent. These phenomena suggested that dense seagrass may be characterized by a constant potential risk (for prey) resulting from permanent resident predators whose foraging efficiency was improved by the habitat structure. Accordingly, prey species lacking tactics to efficiently decrease risks from such predators may remain outside the seagrass habitat, although close enough to successfully retreat to it in response to acute temporal risks offered by transient chase-and-attack predators. Such differential habitat use patterns corresponding to different predator types may be at least partly responsible for the concentrations of small fishes sometimes observed in open areas adjoining the outer margins of natural seagrass habitats.