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Key elements of the 2015 national assessment of stream fish habitats follow the 2010 assessment, including: 1) the idea that distributions and numbers fishes reflect the quality of habitat in which they live; and 2) human landscape factors pose a risk to the condition of stream habitat, and indirectly, to fishes. The 2015 inland stream assessments for the contiguous United States, Alaska, and Hawaii all followed five broad steps (Figure 1) that are described in detail below for the inland stream assessment for Alaska. Note that analytical details for the Alaska assessment differed in southeast Alaska as compared to the remainder of the state (referred to as greater Alaska) due to differences in the resolution of...
Accounting for natural variation With the exception of differences in spatial units, assessments for greater Alaska and southeast Alaska were conducted similarly across regions. Because stream fish assemblage data were not available for the state, no steps were taken to account for natural variation in stream habitats for either southeast or greater Alaska. This represents an important need for future work.
Identifying disturbances to fish habitat The approach for identifying disturbances to fish habitat was based on the assumption that greater intensities and types of human landscape disturbances would most likely lead to more disturbed stream fish habitat (e.g., Danz et al. 2007, Esselman et al. 2011). Twenty-two human landscape variables were identified for the Alaska assessment, with 21 variables used in the southeast and 19 in greater Alaska. We grouped variables into six sub-indices representing specific types of disturbances including: urban land use, agricultural land use, stream fragmentation, point source pollution, infrastructure, and active mines. Each sub-index of disturbance was represented by 2 to...
Integrating data into a spatial framework Greater Alaska For most of Alaska excluding the southeast portion of the state, watershed boundaries for individual stream reaches were unavailable, and the highest resolution spatial units available for assessment were 12-digit USGS hydrological units (HUC-12s). Greater Alaska includes 12,824 HUC-12s that partially follow watershed boundaries; however, boundaries are also intended to capture roughly similarly-sized regions vs. entire upstream landscape areas draining to streams (Figure 11). After acquiring data, variables were attributed to HUC-12s for the greater Alaska assessment. Southeast Alaska For the southeast portion of Alaska, watersheds were delineated from...
Creating cumulative habitat condition scores Greater Alaska All six sub-indices of disturbance scores in each HUC-12 were summed together to yield a cumulative habitat condition index (CHCI) score for each HUC-12. The maximum value for the CHCI was 6, indicating that a HUC-12 was in the worst condition class for each sub-index of disturbance, while the minimum value of the CHCI was 0, indicating that a HUC-12 was in the best condition class for each sub-index of disturbance. We followed methods applied for the conterminous US and created condition classes using Jenk’s natural breaks. With the exception of the HUC-12s that received a CHCI score of <0.001, which were given a priory assignment of “very low” risk of...
Assembling data Many different human landscape factors were assembled and used to characterize condition of Alaska stream fish habitat (Table 3). Factors include: urban and agricultural land uses; density of point sources of pollution in catchments; measures of stream network fragmentation, including densities of dams and culverts; density of infrastructure (road length, pipelines, etc.) and locations of mines. The availability of some landscape factors varied between southeast Alaska and greater Alaska (see Table 3). Some important threats to fish and fish habitats could not be incorporated into the assessment due to data availability limitations (one example includes forest harvest information across the entire...
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