Movement and True Survival of a Stream Amphibian in Relation to Sediment and Culvert Design

Abstract/Summary

Habitat disturbance from stream culverts can affect aquatic organisms by increasing sedimentation or forming barriers to movement. Land managers are replacing many culverts to reduce these negative effects, primarily for stream fishes. However, these management actions are likely to have broad implications for many organisms, including amphibians in small streams. To assess the effects of culverts on movement and survival of Dicamptodon aterrimus, the Idaho giant salamander, we used capture-mark-recapture surveys and measured sediment in 9 streams with 3 culvert types: unimproved culverts, improved culverts, and no culverts. We predicted culverts would increase stream sediment levels, limit movement, and reduce survival of D. aterrimus. We also determined the effect of sediment levels on survival of salamanders, because although sediment is often associated with distribution and abundance of stream amphibians, links with vital rates remain unclear. To estimate survival, we used a spatial Cormack-Jolly-Seber (CJS) model that explicitly incorporated information on movement, eliminating bias in apparent survival estimated from traditional (i.e., non-spatial) CJS models caused by permanent emigration beyond the study area. To demonstrate the importance of using spatial data in studies of wildlife populations, we compared estimates from the spatial CJS to estimates of apparent survival from a traditional CJS model. Although high levels of sediment reduced survival of salamanders, culvert type was unrelated to sediment levels or true survival of salamanders. Across all streams, we documented only 15 movement events between study reaches. All movement events were downstream, and they occurred disproportionately in 1 stream, which precluded measuring the effect of culvert design on movement. Interestingly, although movement was low overall, the variance among streams was high enough to bias estimates of apparent survival compared to true survival. Our results suggest that where sedimentation occurs from roads and culverts, survival of D. aterrimus could be reduced. Though culverts clearly do not completely block downstream movements of D. aterrimus, the degree to which culvert improvements affect movements under roads in comparison to unimproved culverts remains unclear, especially for rare, but potentially important, upstream movements.

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