Movement and demography of larval coastal giant salamanders (Dicamptodon tenebrosus) in streams with culverts in the Oregon Coast Range

Graduation date: 2005Barriers to the movement of aquatic organisms can increase the genetic and\ud spatial isolation of populations and may decrease the viability of these regional\ud populations. Focus on culvert passage issues has increased as federal agencies\ud attempt to inventory and replace road-crossing stream culverts that are barriers to the\ud movement of anadromous fish. However, the effect of road-crossing culverts on the\ud movement of other aquatic organisms is not known. The coastal giant salamander\ud (Dicamptodon tenebrosus) is an important component of headwater stream\ud communities and their movement may be affected by culverts in the larval aquatic\ud stage. The objectives of this study were to determine the effect of road-crossing\ud culverts on the movement and survival of larval coastal giant salamanders in the\ud Oregon Coast Range. We conducted a mark-recapture study on larval D. tenebrosus\ud 3rd and 4th order streams to: i) determine culvert use and passage by design type\ud (pipe and arch types) relative to reference stream reaches without culverts; and ii)\ud model larval survival and growth by season, age and culvert presence. We assessed the movement of over 2,000 larval D. tenebrosus in 14 streams\ud and found that larvae were highly sedentary. Mean movement distances did not\ud differ with culvert presence. However, a small portion of larvae (20%) moved\ud sufficient distances to assess culvert passage. Larvae moved less frequently through\ud stream reaches with culverts than stream reaches without culverts, suggesting a\ud barrier effect. There was less upstream movement through pipe culverts than arch\ud culverts. Also, there were lower larval densities in pipe culverts, indicating arch\ud culverts provided more larval habitat. Larval density both inside culverts and in the\ud adjacent stream reaches was associated with the presence of large substrates, which\ud may be important in facilitating larval D. tenebrosus movement through culverts.\ud Stream reaches with culverts were associated with higher levels of fine sediments,\ud however, which may reduce the suitability of near-culvert habitats.\ud Survival estimates indicated high selection pressure early in the larval period.\ud Apparent survival was lower in summer, and for first-year larvae in comparison to\ud second/third-year larvae. Larval survival for both age groups was lower in reaches\ud of stream with culverts although this effect was weak.\ud Culvert effects on movement of coastal giant salamander aquatic larvae\ud indicate they can operate as barriers but their effect on survival remains unclear.\ud Culvert replacements that simulate both the natural stream bed and hydraulic\ud conditions would help provide both habitat and passage opportunities for larval D.\ud tenebrosus