Relationship between intensity of livestock grazing and trout biomass in headwaters of east front Rocky Mountain streams, Montana
Gilham, Andrew Terrence
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Livestock grazing is the most common land-use practice in the western United States. Riparian and stream habitats are particularly susceptible to effects of poorly-managed livestock grazing. About 80% of stream and riparian habitats in the western United States are thought to have been damaged by livestock grazing, but because grazing usually pre-dated assessments of fish populations and stream habitats, before and after comparisons are impossible. The spatial and temporal complexity of livestock grazing make it difficult to isolate its effects on instream habitat and channel morphology characteristics. Moreover, instream habitat and channel morphology are also influenced by inherent watershed characteristics (i.e., basin area, gradient, discharge). I assessed the effects of livestock grazing on 25 separate 150-m long sample sites (1400 to 1585 m in elevation) within ten headwater basins along the northeastern Rocky Mountain Front in north-central Montana. I used scat counts as an index of relative grazing intensity to assess the effects of livestock grazing on channel morphology characteristics, stream substrate, instream cover, and trout biomass. To my knowledge, this effort is the first to quantify livestock grazing intensity using scat counts to assess grazing effects on trout biomass. I assessed potential effects that grazing intensity had on habitat condition and fish biomass using linear mixed models, which also accounted for watershed and sample site effects. I found that the proportion of fine sediment in the streambed increased as the number of scats increased (P < 0.001), but the area of undercut banks declined as scat counts increased (P < 0.001). Estimated trout biomass declined as number of scats increased, even when I accounted random effects of stream and year in a linear mixed-effect model (P = 0.009). My results corroborate previous findings that livestock grazing along stream channels may reduce trout biomass, but unlike previous studies I actually quantified grazing intensity using scat counts. Since increased livestock grazing intensities were related to increased levels of fine sediments in streambeds and smaller areas of undercut streambanks, I suggest that these factors may be related to why increased livestock grazing reduced trout biomass.