Fire increases the productivity of sugarcane, but it also generates ashes that negatively affect native fish species in aquatic systems.

Sugarcane is one of the main crops used around the world as a feedstock for the production of sucrose and biofuel. Prior to harvesting, sugarcane dry leaves are burned to facilitate manual cutting and enhance productivity. This practice generates ashes from sugarcane straw (hereafter referred as SCA), which may be carried to aquatic ecosystems, where its impacts on organisms and ecosystem integrity remain unknown. Here, we experimentally tested the toxicity of five different concentrations of SCA (0, 1000, 1500, 2000 and 2500 mg/L) on three native (Astyanax lacustris, Moenkhausia bonita and M. forestii) and two non-native (Oreochromis niloticus and Poecilia reticulata) fish from the Paraná River Basin, Brazil. The toxicity was estimated by calculating the median lethal concentration (LC50-24h) and the hepatosomatic index (HSI). We hypothesised that native fish are more sensitive to an increase in SCA than non-native fish. We verified that the mortality of native fish sharply increased with the increase in higher SCA concentration (LC50-24h values: A. lacustris = 2525.71 mg/L, M. bonita = 2124.95 mg/L and M. forestii = 1981.74 mg/L). However, no deaths were recorded for non-native fish species in any SCA concentrations. Accordingly, the HSI index values statistically differed with the increase in SCA concentrations for native fish, while for non-native fish we did not observe any difference. Therefore, only native species died or suffered liver damage with an increase in SCA concentrations. Extrapolating our findings to natural environments, we suggest that sugarcane burning, a widely used agricultural technique, has the potential to reduce the population size of native organisms and facilitate the dominance of non-native fish species in aquatic ecosystems.