Wildfire and the effects of shifting stream temperature on salmonids

The frequency and magnitude of wildfires in North America have increased by four-fold over the last two decades. However, the impacts of wildfires on the thermal environments of freshwaters, and potential effects on coldwater fishes are incompletely understood. We examined the short-term effects of a wildfire on temperatures and Steelhead/Rainbow Trout (Oncorhynchus mykiss) bioenergetics and distribution in a California coastal stream. One year after the wildfire, mean daily stream temperatures were elevated by up to 0.6 degrees C in burned compared to unburned pools. Among burned pools, light flux explained over 85% of the variation in altered stream temperatures, and 76% of the variation in light flux was explained by an index of burn severity based on proximity of the pool to burned streamside. We estimated that salmonids of variable sizes inhabiting burned pools had to consume between 0.3-264.3 mg of additional prey over 48 days to offset the 0.01-6.04 kJ increase in metabolic demand during the first post-fire summer. However, stomach content analysis showed that fish in the burned region were consuming relatively little prey and significantly less than fish in the reference region. Presumably due to starvation, mortality, or emigration, we found a significant negative relationship between the change in total salmonid biomass over the post-fire summer and the average energy costs (kJ.g(-1).day(-1)) within a burned pool. This study demonstrates that wildfire can generate thermal heterogeneity in aquatic ecosystems and drive short-term increases in stream temperature, exacerbating bioenergetically stressful seasons for coldwater fishes.