In the Caribbean, changing rainfall patterns are causing more extreme droughts and floods, but the consequences for tropical aquatic ecosystems and native freshwater fish are poorly understood. Understanding how extreme flow events impact native fish is complicated by the increasing presence of non-native species and dams, which limit the connectivity and flow along streams and rivers. 

In a new National CASC-supported study, CASC researchers and co-authors analyzed data of fish assemblages from 119 sites in the Caribbean Island of Puerto Rico, to assess the effects of extreme flows and dam heights on native and non-native species. They found that native fish can cope with prolonged droughts and low water flow better than expected – but only when non-native species are not present in the streams. In fact, when alone, native fish not only survive droughts but also experience population increases. Both low water levels and competition with non-native species can hinder native fish movement upstream which disrupts the role native fish play in moving important nutrients between the sea and freshwater environments. The researchers also found that higher dams harm native species while benefiting non-native ones. Floods and long periods of high flows, on the other hand, can help control non-native fish to lower their impact on native fishes. The researchers think this is because many native fish have adapted flood-resilient traits –for example, flooding can initiate reproduction and an instinctual behavior to shelter in safe areas.   

This Southeast CASC study highlights how extreme flow events in tropical regions can interact with non-native species and dams to affect native fish populations, providing important management considerations for protecting native species.  

This work was supported by the National Climate Adaptation Science Center Project “Impacts of Global Change on Biotic Resistance, Resilience, and Ecosystem Services in Caribbean Fish Assemblages, Fisheries, and Aquatic Ecosystems.”