Assessing relative predation risk for small fish in dry-season refuges of a freshwater oligotrophic wetland

Background/Question/Methods

Detecting the underlying mechanisms involved in density dependence is essential for understanding population dynamics.  Empirical and theoretical work has pointed to predation as a regulatory mechanism for controlling prey populations via density-dependent mortality.  Theoretical work suggests that in wetlands, the role of predation should decrease as a function of water permanence.  We examined the role of large, piscivorous fish within the freshwater system of the Everglades, Florida, USA, which is characterized by wet and dry seasons yielding a water-permanence gradient.  As water levels recede with the onset of the dry season, fish emigrate to hydrological­­­­ refuges (i.e., ponds and canals) where they reside until connectivity to the marsh surface is restored in the wet season.  During this critical period, biotic interactions may play an important role in structuring the fish community.  We documented the relative risk of predation in deep-water refuges for small fish within the Everglades and asked if predation by large, piscivorous fish contributes to the regulation of prey populations at a local scale.  We used a dual-frequency identification sonar (DIDSON) in high frequency mode operating at 1.8MHz to measure the density of large fish (>10 cm) and observe fish behavior in two canals, one within a short-hydroperiod wetland (L31W) and the other within a long-hydroperiod wetland (L67C).  The DIDSON is a hydroacoustic sonar capable of recording high-resolution, video-like images underwater.  Sonar videos were taken during the transition period from the wet to dry season.

Results/Conclusions

Fish densities were greater in the L31W than L67C resulting from an earlier onset of dry conditions and lack of connectivity to the marsh surface, characteristic of this short-hydroperiod wetland.  Densities within sites changed little during the seasonal transition period.  Foraging and territorial behaviors, as well as changes in movement, speed, and direction of fish, were observed and enumerated, indicating the DIDSON is a useful tool for monitoring fish behavior and can be used to study predator-prey dynamics.  We observed more foraging and territorial behaviors at the high density site than in the low density one.  Predation events were observed exclusively in vegetation of the littoral zone of the canals, with no events observed in the adjacent open water.  Ongoing experimental work examines the potential of these predation events in regulating small-fish population dynamics.