The effects of species manipulation on growth and survival of an assemblage of juvenile estuarine fish

Growth and survival of Cyprinodon variegatus, Fundulus heteroclitus, Menirlia beryllina, and Lucania parva, held in combination with an omnivorous grass shrimp-Palaemonetes pugio, and a predatory diving beetle-Tropisternus lateralis, were measured at 12-60 parts per thousand in outdoor static microcosms that simulate salt marsh ponds. We predicted that the low species richness of this abiotically harsh but highly productive habitat would lead to a high degree of interaction among species. The presence of the beetle had no effect on the fish or the shrimp. Removal of the shrimp similarly had no detectable effect on the fish. There was a trend in proportions of survival among three fish: C. variegatus>F. heteroclitus>M. beryllina. Survival of M. beryllina was greater when it was alone at low densities of its own species (333 v. 667 fish m(-3)), and less when it was paired with F. heteroclitus. Interactions with ether species diminished the growth of M. beryllina. Survival of F. heteroclitus was greater when it was alone at low density (333 m(-3)), than when the same number were placed together with Menidia and Cyprinodon at a total fish density of 999 m(-3). There were no effects of removal of other species on survival or growth of C. variegatus at either 125 or 24 parts per thousand. This study showed that a complex array of interactions occurred among six. common members of a salt marsh food web, but the degree of interaction was less than we predicted. (C) 1996 The Fisheries Society of the British Isles