THE EFFECTS OF SPECIES MANIPULATION ON GROWTH AND SURVIVAL OF AN ASSEMBLAGE OF JUVENILE ESTUARINE FISH

Growth and survival of Cyprinodon variegatus, Fundulus heteroclitus, M enirlia beryllina, and Lucania parva, held in combination with an omni vorous grass shrimp-Palaemonetes pugio, and a predatory diving beetle- Tropisternus lateralis, were measured at 12-60 parts per thousand in o utdoor static microcosms that simulate salt marsh ponds. We predicted that the low species richness of this abiotically harsh but highly pro ductive habitat would lead to a high degree of interaction among speci es. 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. va riegatus>F. heteroclitus>M. beryllina. Survival of M. beryllina was gr eater when it was alone at low densities of its own species (333 v. 66 7 fish m(-3)), and less when it was paired with F. heteroclitus. Inter actions with ether species diminished the growth of M. beryllina. Surv ival of F. heteroclitus was greater when it was alone at low density ( 333 m(-3)), than when the same number were placed together with Menidi a and Cyprinodon at a total fish density of 999 m(-3). There were no e ffects of removal of other species on survival or growth of C. variega tus at either 125 or 24 parts per thousand. This study showed that a c omplex array of interactions occurred among six. common members of a s alt marsh food web, but the degree of interaction was less than we pre dicted. (C) 1996 The Fisheries Society of the British Isles