Field and laboratory evaluation of habitat use by rainwater killifish (Lucania parva) in the St. Johns River estuary, Florida

I examined the relative importance of beds of tapegrass (Vallisneria americana) and adjacent unvegetated habitats to juvenile and adult (6–35 mm standard length) rainwater killifish (Lucania parva) over a large spatial scale within the St. Johns River estuary, Florida. Abundance of rainwater killifish did not differ between oligohaline and tidal freshwater portions of the estuary and this species was relatively rare at opposite ends of the St. Johns River estuary. The presence of rainwater killifish at a given site was determined in part by large-scale variation in environmental factors such as habitat complexity and salinity. When present at a site, rainwater killifish were found almost exclusively in structurally complex beds of tapegrass. Behavioral observations in the laboratory indicated that rainwater killifish preferred vegetated over unvegetated habitats in the absence of both potential prey and predators and that use of vegetated habitats increased further upon addition of predatory largemouth bass (Micropterus salmoides). A laboratory predation experiment indicated that survival of rainwater killifish exposed to largemouth bass was significantly higher in vegetation than over open sand. Strong preferences for structurally complex vegetation likely reflect an evolved or learned behavioral response to risk of predation and help explain habitat use of rainwater killifish in the St. Johns River estuary.