Fish Movement and Migration in the Seasonally Varying Wetlands of the Florida Everglades

Background/Question/Methods A major challenge of movement ecology is to understand how an organism’s motivation and environment affect its movement behavior. The Everglades encompass a water-permanence gradient with flooding periods from nearly permanent in central sloughs to very short-duration towards the edges. During the dry season, fish must escape the drying front or risk stranding. Short-hydroperiod wetlands are rapidly colonized by small fish when re-flooded. Modeling suggests that wetlands are colonized more rapidly than predicted by reaction-diffusion and contributions from refuges; realistic rates are yielded when movement is treated as biased in the direction of the flooding front. This would be expected if fish followed resource gradients (for example, prey in newly flooded areas) with a dynamic ideal free distribution (IFD). We used encounter samplers to assess fish activity (proportional to speed) and directional bias to distinguish their movement strategies. Our null hypothesis was that speeds were similar in all sites and direction was random, equivalent to reaction-diffusion. Our alternative hypothesis was that speed is greater in short-hydroperiod regions or when depths are shallow, but movement is undirected (“random, active movements”). A second alternative hypothesis was that activity is greater in short-hydroperiod regions/low depths, and fish move non-randomly (directed movement, consistent with dynamic IFD). We tested these hypotheses for the most common species collected over a five-year study. Results/Conclusions For the most abundant species, mosquitofish and marsh killifish, activity and speed were greater in short-hydroperiod sites than in long; directional bias in movement was also greater there, especially when depths were low. These patterns were weaker in long-hydroperiod sites, possibly because depths were rarely shallow enough to trigger increased movement. We interpret this as fish moving in and out of short-hydroperiod sites following fitness gradients, possibly generated by resource availability and stranding risk (directed, active movement, consistent with the dynamic IFD). Flagfish and dollar sunfish showed an increase in speed, but no directional bias in short-hydroperiod sites. These fish may be more likely to reach favorable environments by chance, owing to generally increased activity. Finally, bluefin killifish and sailfin mollies showed no difference in speed or directional bias. These different movement strategies may owe to differences in resource needs, dispersal abilities, or thresholds for escaping shallow water and partially explain differences in the order of appearance of species in newly flooded areas