Abstract
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°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.
Highlights
The frequency and duration of large wildfires in Western North America has increased by nearly four times over the last two decades (Westerling et al 2006)
During the post-fire summer, the difference in temperature between burned and reference pools indicated an average increase of 0.348C 6 0.098C in the burned region mean daily stream temperatures; there was a significant difference among burned pools (Figs. 2D and 3A, Table 2)
Mean daily stream temperatures in the burned region were best explained by a multiple linear regression model that included the reference temperatures and factors for ‘‘Time,’’ ‘‘Pool ID,’’ and an interaction between ‘‘Time’’ and ‘‘Pool ID’’
Summary
The frequency and duration of large wildfires in Western North America has increased by nearly four times over the last two decades (Westerling et al 2006). This dramatic increase could be driven by altered land-use, changes in patterns of precipitation, and increases in temperature (Westerling et al 2006). Our understanding of the short- and long-term effects of wildfire are based on limited information, especially regarding the conservation and management of thermally-sensitive species (Minshall et al 1989, Dunham et al 2003)
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