Predation Risk and Use of Intertidal Habitats by Young Fishes and Shrimp

Salt—marsh populations of the killifish Fundulus heteroclitus and the grass shrimp Palaemonetes pugio exhibit similar age—specific habitat—use patterns. At low tide, the young of both species are found in shallow aquatic microhabitats on the surface of the intertidal marsh, separated from the adults, which reside in adjacent subtidal creek habitats. As a test of the hypothesis that predation is responsible for the segregation of age classes by habitat, I used 5—m 2 enclosures containing artificial subtidal habitats (water—filled pits) to maintain two size classes (45—60 mm and 70—90 mm total length) of adult killifish on the intertidal marsh surface both in the resence and absence of adult grass shrimp. During the 10—wk experiment, larval killifishes and postlarval grass shrimp recruited into the enclosures through the 1.2 mm square mesh walls. The numbers of young killifishes (F. heteroclitus and Fundulus luciae) in enclosures that contained adult fish of either size were <5% of those in plots that excluded both adult fish and shrimp. The overall effect of fish on the abundance of shrimp postlarvae was not as great because large adults had less effect on young shrimp than did small adults. However, adult shrimp mortality was higher in enclosures that contained large fish than in those that contained small fish. Enclosures that received only adult grass shrimp additions contained °50% fewer young killifishes than those from which both adult fish and shrimp were excluded. Adult shrimp had no effect on the abundance of shrimp postlarvae. Treatment effects were enhanced with increasing elevation in the intertidal zone. The experimental results suggest that young killifishes and shrimp remain in the intertidal zone at low tide as an adaptation to avoid a high risk of mortality from concentrations of predatory adults in subtidal marsh habitats.