How Are Man-Made Changes in Freshwater Flow Related to the Abundance of Juvenile Estuarine Fishes?

Abstract

Changes in freshwater flow to estuaries can cause a suite of ecosystem impacts including eutrophication and alterations to plant communities, zooplankton populations, and other biota. In southwest Florida, historical manipulation of freshwater flow due to development, canals, and drainage ditches is pervasive. Because there are estuaries with reduced, increased, and relatively natural freshwater flow, this region presents an ideal system to study how these changes relate to downstream fish abundances. We used a 20-year trawl dataset focused on juvenile and small-bodied fish from three mangrove-lined sub-estuaries with contrasting flow conditions in southwest Florida’s Ten Thousand Islands to identify important environmental and temporal variables influencing fish populations. We used generalized additive models to investigate total fish abundance, species richness, diversity, and the abundances of 23 ecologically, recreationally, or commercially important species and describe their relationships with important environmental and temporal variables. While salinity and temperature had species-specific relationships with fish abundances, seasonality, interannual variability, and sub-estuary were more closely related to fish than salinity and temperature in most cases. Only 8 of 23 species responded most strongly to temperature, and only 1 species responded most strongly to salinity. This suggests that for most species in our study, temporal factors such as timing of spawning and recruitment variability had stronger relationships with the structure of fish populations than changes in freshwater flow. This work quantified how changes in freshwater flow, using salinity as a proxy, may relate to downstream fish abundances and therefore the potential implications of planned watershed restoration that is part of the Comprehensive Everglades Restoration Plan.