Effects of canals and levees on fish movement in a seasonal wetland

Background/Question/Methods

Fish movement can influence community structure in a spatially and temporally heterogeneous landscape such as seasonally flooded wetlands of the Everglades, Florida, USA.  Water levels rise in the Everglades beginning in the rainy season, June through November, and slowly declined throughout the dry season, December through May. This seasonal pattern of hydrologic variation is closely tied to several biological processes, including fish movement. Under historical conditions it is assumed that fishes were found throughout the landscape in the wet season and forced to deepwater refuges as the dry season progressed, though with much of the ecosystem remaining flooded in all but very dry years.  In recent decades, the Everglades have been divided by an extensive network of canals and levees, disrupting this flow pattern, and drained such that much of the ecosystem now dries in all but relatively wet years.  We used drift fences that intersect in an X pattern with traps facing into the four cardinal directions to estimate catch per unit effort (CPUE) and movement directionality, 1-m² throw traps to estimate density, and the ratio of CPUE and density to measure activity.  Nine sampling locations were established ranging from 0 to 8 km away from canals.  

Results/Conclusions

We found that canal and levee proximity and hydroperiod play a role in fish density, directionality of movement, and activity, and the effects of these variables were species-specific.  For example, eastern mosquitofish tended to move towards the canal in the late wet season, while bluefin killifish showed movement away from the canal at that time.  Dollar sunfish showed similar activity throughout the landscape and no directionality, regardless of canal or levee proximity, but showed greater activity during the late wet season.  Blue-spotted sunfish displayed more activity in late wet season when water level was high compared to samples collected during the transition between wet and dry seasons.  These and other species-specific results indicate that the presence of manmade structures can affect community structure and ecological processes in ecosystems that are strongly affected by a hydrologic driver.