Effects of wildfire on growth and demographics of coastal cutthroat trout in headwater streams

Graduation date: 2007Wildfire is a largely terrestrial perturbation broadly recognized as an agent of disturbance and ecological change in forested biomes. Effects of post-fire conditions on biotic components of aquatic systems have been less well-documented, although hypothetically, the two are strongly connected. In fact, the influence of wildfire may be most profound in headwater streams because of the tight linkage between aquatic and terrestrial ecosystems. We sought to understand the influence of post-fire conditions on fish in headwater streams by observing growth and demographics of coastal cutthroat trout (Oncorhynchus clarkii clarkii) following wildfire. During the summer of 2002, wildfire burned portions of three headwater watersheds in the North Umpqua basin of western Oregon. Burn severities ranged from moderate to severe. A reference stream was selected from a nearby fourth watershed. All fish-bearing portions of the stream network in the four watersheds were surveyed for 3 years following the wildfire, and 1,295 scale samples were collected from coastal cutthroat trout for age and growth analysis. Stream temperatures in the burned watersheds changed significantly following the wildfire (p < 0.016), with increases of up to 23% in the most severely burned watershed. During summer electrofishing surveys in the burned watersheds, both young-of-the-year and fish ≥ age 1 were captured; in the unburned watershed, the vast majority of fish captured were ≥ age 1. Age-frequency analyses revealed that fish in older age classes (age ≥ 3) were more prevalent in the unburned watershed than in the burned watersheds. Relative growth rates and length at last annulus formation of coastal cutthroat trout were positively related to the number of degree days during the growing season, and Pearson correlation coefficients ranged from 0.60 (p < 0.07) to 0.90 (p < 0.01). Growth rates and size of fish were not significantly correlated with relative density of fish (p ≥ 0.14). Contrary to the commonly held assumption that wildfire is detrimental to biota in aquatic systems, these results suggest that in the short term (≤ 3 years), fish in the study streams adjusted to environmental change caused by wildfire