Abstract Changes in land use, warming climate and increased drought have amplified wildfire frequency and magnitude globally. Ash mixing into aquatic systems after wildfires rapidly increases water pH, creating an additional threat to wildlife, especially species that are already threatened, endangered and/or migratory. Here, Chinook salmon ( Oncorhynchus tshawytscha ) yearlings acclimated to 15 or 20°C were exposed to an environmentally relevant concentration of ash (0.25% w/v) which caused water pH to rapidly rise from ∼8.1 to ∼9.2. Mortalities occurred within the first 12 hours, and was higher at the higher temperature (33 versus 20 %). The greatest differences in blood chemistry between the two temperatures were dramatically greater (∼7.5-fold) and very rapid (within 1 hour) spikes in both plasma total ammonia (to ∼1200 µM) and lactate (to ∼6 mM) in warm-acclimated salmon, whereas cold-acclimated salmon experienced a much smaller and gradual rise in plasma total ammonia. Salmon at both temperatures experienced extracellular and intracellular alkalosis within 1 hour that recovered within 24 hours, but the alkalosis was smaller in magnitude in fish at warmer temperature. Impacts on plasma ion concentrations were relatively mild and plasma glucose increased by 2- to 4-fold at both temperatures. Notably, the increase in plasma total ammonia in fish at the warmer temperature was far faster and much greater than those reported in previous studies exposing fish despite higher water pH (9.4-10.5) induced without using ash. This suggests that ash has physiological impacts that cannot be explained by high water pH alone which may relate to the complex mixture of metals and organic compounds also released from ash. This demonstrates post-wildfire ash input can induce lethal yet previously unexplored physiological disturbances in fish and highlights the complex interaction with warmer temperatures typical of wildfire-scarred landscapes.

Fish Blood Response to Ash-Induced Environmental Alkalinization, and their Implications to Wildfire-Scarred Watersheds