Ashes in Freshwater Ecosystems: A Systematic Review of their Impacts on Fish

Ash runoff into freshwater systems from fire events, coal combustion, and volcanic activities threatens to biodiversity conservation, given its toxicity to various aquatic organisms. However, despite many studies reporting a negative relationship between ash exposure and the biological traits of freshwater fish, non-significant and even positive associations can also be found in the literature. Here, a systematized review was conducted to explore patterns and biases in studies examining the impacts of ash on freshwater fish. Out of the 43 studies included in this review, 38 reported harmful effects of ash exposure on fish traits. The median lowest concentration of ashes capable of inducing negative effects on fish traits was found to be 2 g/L. Diet restrictions (84.6%), behavioral changes (83.3%), cytological and genetic damages (64.7%), and bioaccumulation of toxic compounds (57.1%) exhibited the highest proportions of negative results in statistical evaluations testing ash effects on fish traits. In contrast, biomarkers related to reproduction (94.7%) and metabolism (66.4%) had the highest proportion of non-significant associations between fish traits and ash exposure. Statistical models indicate that the ash source does not determine the direction (positive or negative) of its effect on fish traits. Furthermore, no evidence supports the hypothesis of similar sensitivity levels to ash exposure among phylogenetically closely related fish species. Together, the evidence provides a cohesive perspective on the detrimental effects of ash on fish, highlighting the effective necessity to manage the influx of ash into watercourses.

Graphical Abstract