Landscape factors affecting movement decisions by large predators in an aquatic ecosystem

Background/Question/Methods

Predators can influence communities in spatially and temporally heterogeneous landscapes like seasonally flooded wetlands such as the Everglades.  Seasonal fluctuation in water levels drive population control of small fishes.  Large piscivorous fishes may also affect the population levels of small fishes and historically these piscivores were limited by the onset of the dry season.  Now, canals serve as dry-season refuges for piscivores that move into the marsh during the wet season. Increased availability of dry-season habitat likely removes a substantial control that had previously limited piscivore populations.  We studied a marsh constrained by two parallel levees 1.7 kms apart, with a canal adjacent to the eastern levee; all areas of the marsh were potentially accessible to fish dispersing from the canal.  To investigate the effect of landscape on predator movement, we surgically implanted radio tags in 20 largemouth bass (Micropterus salmoides) and 20 bowfin (Amia calva) during the dry season of 2011.  These fishes were restricted to the canal until water levels rose high enough to inundate the marsh surface during the following wet season.

Results/Conclusions

Distance moved between sightings (step length) of fishes varied both intra and interspecifically.  Bowfin displayed greater variance in step length than bass.  Step length also varied between habitat types.  Intraspecific variation in movements was most evident in the structurally simple habitat of the canal.  Individuals from both species exhibited large variability in distances travelled, with some individuals consistently moving thousands of meters a week and others moving less than ten.  Both species made some large steps in the structurally complex habitat of the marsh, up to several hundred meters, but most were much shorter than steps in the canal.  Initial steps crossing between habitat types were usually long, especially for bowfin, which also moved further into the marsh than bass.  These results indicate that behavioral decisions or movement ability of large fishes are constrained by the more complex habitat of the marsh, but enhanced by the more open canal.  The gradient of predation pressure on small fishes from the marsh edge to its interior changes between the wet and dry season and may be steepened by the presence of continuous open habitat like canals.