Seasonal variation in metal concentrations in the Gualaxo do Norte River following the Fundão dam collapse is associated with behavioural and cholinergic alterations in Astyanax lacustris

The Fundão dam collapse in Mariana (Southeast Brazil) released metal-rich tailings into the Doce River basin and its tributaries, including the Gualaxo do Norte River. Metal concentrations in aquatic environments vary seasonally, generally increasing during the rainy season. Exposure to metals may lead to behavioural alterations and changes in acetylcholinesterase (AChE) activity in fish. This study investigated the effects of water from the Gualaxo do Norte River (collected during the dry and rainy seasons) on the behaviour and cholinergic activity of Astyanax lacustris. One hundred fish were assigned to four treatments: dechlorinated water during the dry (CDS) and rainy (CRS) seasons, and river water from the dry (DS) and rainy (RS) seasons. Fish were exposed to the treatments for 15 days, and behaviours were recorded for 10 days. After that, brain and muscle AChE activity was analysed in the fish. Metals such as Al, Cu, Fe, Hg, Mn, Pb, and Zn showed higher concentrations in the water collected during the rainy season. Fish from the DS group showed increased activity, whereas those from the RS group showed reduced activity, reduced sociability, and lower aggressiveness. Muscular AChE activity increased in DS and RS groups relative to CDS and CRS. In contrast, cerebral AChE activity was higher in DS (compared to CDS and RS) and lower in RS (compared to CRS). Our results indicate that water collected during the rainy season may pose a greater threat to fish health because of increased metal runoff. Even a decade after the dam rupture, residual contamination continues to affect fish physiology and behaviour, emphasising the persistent ecological risks and the urgent need for long-term monitoring and management of impacted aquatic ecosystems.