Wet season flood magnitude drives resilience to dry season drought of a euryhaline elasmobranch in a dry-land river

The increase in severity and occurrence of drought from environmental change poses a significant threat to freshwater ecosystems. However, many of the mechanisms by which periodic drought affects aquatic animals are poorly understood. Here we integrated physical, physiological, and behavioural measurements made in the field over a twelve-year period to provide a comprehensive understanding of the factors affecting the loss of body condition of fish in arid rivers, using the Critically Endangered freshwater sawfish (Pristis pristis) in the dryland Fitzroy River, Western Australia, as a model species. Sawfish lost condition throughout the long dry season in all years and had significantly poorer body condition throughout years characterized by low volumes of wet season flooding and little occurrence of overbank flooding. A mechanistic examination of factors leading to this loss of condition using measurements of body temperature, field energetics, and habitat use from telemetry techniques showed that the loss of condition throughout the season was likely due to substantial habitat compression and low productivity in drier years, while high rates of competition were more likely to drive this pattern in wetter years. This information can be used to forecast how climate change and water abstraction will affect aquatic fauna experiencing intermittent drought and can inform management decisions to help mitigate these threats.