Exotic fishes alter nutrient dynamics in tropical streams

Background/Question/Methods

The rates at which biotic invasions are occurring are unprecedented and this is especially true with fish species. Though many investigations have suggested animals may be important drivers of nutrient cycling, few studies have demonstrated how the effects of fish invasions alter these processes in tropical systems. Over the past decade, armored catfish (Siluriformes: Loricariidae: Pterygoplichthys), have been introduced into the watersheds of southern Mexico, and have attained high population densities. Previous studies have demonstrated that armored catfish have high phosphorus demands associated with extensive amounts of body armor. Thus, we hypothesized that aggregations of armored catfish would sequester nutrients that potentially limit primary production. In Chiapas, Mexico, we used nutrient-diffusing substrata to estimate nutrient limitation of primary productivity in invaded systems. Moreover, we compared remineralization and sequestration of nitrogen and phosphorus by invasive armored catfish and abundant native fish species. We estimated fish densities, biomass, excretion rates (NH₄⁺, TDP), and body nutrient content (TN, TP) for armored catfish and native fishes. We used these data to estimate fish-derived nutrient remineralization and sequestration, per fish biomass and per unit area of streambed, in order to explore whether exotic fish species potentially alter nutrient dynamics in invaded stream systems.

Results/Conclusions

Introduced armored catfish attained high biomass relative to native species and created large, novel nutrient sinks in invaded streams. They also sequestered the majority of the nitrogen and phosphorus retained in fishes within invaded systems. Armored catfish represented large pools of phosphorus relative to other ecosystem compartments including, macroinvertebrates, epilithon, and coarse particulate organic matter. These exotic fish have few predators in invaded systems; thus, nutrients stored in the bodies of armored catfish may not be biologically available for several years. Due to their high phosphorus requirements, introduced armored catfish selectively retained phosphorus, resulting in high nitrogen to phosphorus ratios of excretion compared to native fishes. Nutrient diffusing substrates indicated that primary productivity in invaded streams was limited by phosphorus. Therefore, exotic armored catfishes may be exacerbating the nutrient limitation of primary productivity in these streams. These results suggest that invasive fishes attaining high biomass can be important drivers of nutrient dynamics in invaded systems. Furthermore, in streams where primary productivity is limited by nitrogen or phosphorus, invasive fishes have the potential to alter native communities and ecosystem function through the remineralization and sequestration of nutrients.