Coastal cutthroat trout in headwater stream networks: Distribution and abundance in space and time
Walter, Jason Kirk
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Understanding the linkages among physical and biological processes across scales is vital to understanding the diversity and heterogeneity of headwater streams. Most research investigating fish–habitat relationships in headwater catchments has been conducted over a limited geographic extent and at small spatial scales. We conducted spatially continuous single-pass electrofishing and physical stream habitat surveys, sampling all habitat units possible, within the fish–bearing portions of 16 catchments where coastal cutthroat trout were the only salmonid species present. Each catchment was surveyed twice, first in the autumn and then again the following spring, prior to the emergence of young-of-the-year cutthroat trout. Spatial and temporal (seasonal) variability of cutthroat trout abundance within each study catchment was assessed at coarse (catchment) and intermediate (stream segment) spatial scales. Catchment-scale cutthroat trout density in both the autumn and spring was positively correlated with catchment area, and significantly associated with catchment shape, with highest densities in heart-shaped catchments. These high densities may be associated with increased habitat heterogeneity imparted by tributary confluence effects, which tend to be greater in dendritic stream networks in heart-shaped catchments. Stream segments were identified and grouped by analyzing intermediate–scale stream channel characteristics (i.e., channel gradient and the location of significant tributary junctions) derived from high-resolution (≤2 m) LiDAR digital elevation models. Segment-scale fish density in the autumn was positively correlated with segment contributing catchment area, negatively correlated with channel gradient, and significantly associated with the interaction of the two variables (P < 0.05). No segment-scale variables were significant predictors of fish density in the spring. The change in predictive ability for models of cutthroat density in the spring relative to the autumn was coupled with a significant decline in fish density between the seasons (P > 0.05). Improved understanding of the relationships between fish abundance and stream habitat is needed to help land managers identify areas of high biological potential where specific habitat protection or restoration efforts may be warranted or most productive.
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