Individual and interactive effects of salinity and initial fish density on a salt marsh assemblage

We used static outdoor mesocosms (155 1) to study the effects over 60 d of 3 levels of total fish density and 3 levels of salinity (1, 5, and 32 ppt) on growth and survival of juvenile fish (silversides Menidia beryllina, hereafter referred to as Menidia; and sheepshead minnows Cyprinodon variegatus, hereafter Cyprinodon) that are common to salt marsh pools of the eastern United States. Interactions between these 2 species and rainwater killifish Lucania parva (hereafter Lucania) were also examined. There was a general decline in the rate of growth and the percentage survival with increasing fish density. Salinity and density interacted significantly such that the percentage survival of Cyprinodon was lowest in the high salinity-high density treatment compared to most other treatments. It did not appear that Cyprinodon or Menidia interacted strongly with Lucania, since growth and survival of the former 2 species were independent of abundance of Lucania. Fish simultaneously maintained in permeable enclosures in the field generally had similar or lower rates of growth than those in mesocosms, indicating that conditions in the mesocosms were not inherently more stressful than conditions in natural salt marsh pools. It appears that this fish assemblage is regulated in part by both a biotic (density/competition) and an abiotic fatter (salinity). We conclude that, under circumstances in which conditions are severe due to high total fish density, a level of salinity that appears to be otherwise inconsequential may have important implications for community structure by impacting survival of one species.