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Fish passage at selected culverts on the Hoonah Ranger District, Tongrass National Forest

Abstract

In the month of July 1997, 38 culverts suspected of blocking upstream passage of juvenile salmonids were inventoried on the Hoonah Ranger District. Attributes measured included species/numbers of fish upstream and downstream of each culvert, in addition to physical characteristics such as outlet barrier height, culvert gradient, and upstream habitat. Thirty culverts exhibited some form of physical impediment (excessive barrier height and/or gradient) to the upstream migration of juvenile salmonids. Of the 30 barrier culverts, the height of the lower lip of the culvert outlet above the streambed ranged from 0cm to 205cm and averaged 36.5cm. The gradient of these structures ranged from –0.5 percent to 14.5 percent and averaged 5.0 percent. Thirteen Class I (anadromous) culverts were sampled, of which nine lacked juvenile coho upstream of the culvert (no juvenile steelhead trout were trapped during the study). Boxplots of number of juvenile salmon trapped upstream of culverts relate a considerable reduction in distribution, median, and mean as compared to downstream. All 13 culverts exhibited an outlet perched above the streambed, with barrier heights ranging from 10cm to 99cm, averaging 38.8cm. Class I culvert gradient ranged from 0.5 percent to 9.5 percent and averaged 3.4 percent. Nineteen culverts were identified as Class II culverts (i.e., culverts in streams providing cutthroat trout and Dolly Varden charr habitat occupied upstream of anadromous habitat) during the survey, of which 17 exhibited some physical form of barrier to juvenile passage. Outlet barrier heights ranged from 0cm to 205cm, averaging 37.2cm. Culvert gradient ranged from 2.0 percent to 9.0 percent and averaged 4.3 percent. Eight of the 19 Class II culverts had resident fish species trapped upstream of the culvert, six of which occurred above culverts exhibiting barrier characteristics such as outlet perch or excessive gradient. Boxplot distribution, median, and mean of height of outlet barrier and culvert gradient tended to be greater at Class I and Class II structures without fish trapped upstream as compared to culverts where fish were trapped upstream. This pattern was repeated for culvert length at Class I crossings, but was reversed for Class II structures. Overall, barrier culverts resulted in a loss of 8.11km (16,534m2) of fish habitat, comprised of 2.68km (6,408m2) of Class I and 5.42km (10,126m2) Class II habitat. Habitat lost per culvert at Class I crossings was 206m (493m2 by area), and 319m (596m2 by area) for Class II culverts. Roughly 37% of Class I fish habitat lost (determined by length) was of high-quality Floodplain process group reaches with an additional 47 percent comprised of moderate-quality Moderate Gradient–Mixed Control reaches. Class II habitat lost comprised about 41 percent Mixed-Moderate reaches, followed by 32 percent High Gradient-Contained and 17 percent Alluvial Fan process group reaches, both providing relatively low-quality fish habitat.

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