Variation in movement strategies of small fishes in a dynamic wetland

Background/Question/Methods

Animal movement strategies need to adapt to changing conditions in dynamic environments. Wetlands present fish with extreme challenges to cope with seasonally expanding and contracting habitat. Furthermore, wetlands commonly experience anthropogenic modifications that further challenge the ability of fish to respond adaptively to changing availability of aquatic refuges.  We studied wetland sites constrained by two parallel levees 1.7 kms apart, with a canal adjacent to one levee to document how small fishes responded to season water level fluctuation; all areas of the marsh were potentially accessible to fishes in the wet season, but not the dry season.  We used drift fences that intersect in an X pattern with traps facing the four cardinal directions to estimate catch per unit effort and movement directionality and 1-m² throw traps to estimate density. An equation modeled after the catch rate of a sit-and-wait predator was used to combine these sampling techniques to assess movement speed.  Nine sampling locations were established ranging from 0 to 8 km away from canals.  Fishes were restricted to the canal until water levels were high enough to inundate the marsh surface during the wet season, though variability in monthly water depth was great between years. Samples were collected across four years.

Results/Conclusions

The seven species of fishes examined displayed movement patterns that were highly variable spatially, temporally, and interspecifically. For example, higher marsh depths were positively correlated with speed for bluefin killifish (Lucania goodei), but Everglades pygmy sunfish (Elassoma evergladei) displayed low speed throughout the year.  Blue-spotted sunfish (Enneacanthus gloriosus) moved in a similar direction throughout 2011-2012 at a site 150 m from the canal, regardless of marsh water depth, but exhibited non-directed movements across the same time period at a site adjacent to the canal. Dollar sunfish (Lepomis marginatus) exhibited high speeds throughout the year, but directed movement was only seen at certain sites, such as in January 2012, where dollar sunfish made movements away from the canal at sites 150-300 m from canal, but exhibited non-directed movement patterns at sites immediately adjacent to the canal. These variable responses to water depth and landscape structure can have large impacts on freshwater metacommunities, including possible alterations in the behavior of piscivores that prey on these fishes. These results will also help clarify effects of novel habitats in remote systems.