|Abstract or Summary
- Debris flows, common disturbances in many mountainous areas, initially scour or bury stream habitats; however, debris flows deliver vast amounts of wood, boulders, and gravel that may ultimately form complex stream habitat to potentially support a diverse salmonid assemblage. The materials deposited by debris flows would otherwise be inaccessible to streams, and thus deposits may play an important role in creating and maintaining complex salmonid habitat over time. Despite the potential of deposits for increasing habitat complexity, most fish studies have focused on the destructive effects of debris flows and short-term recovery and re-colonization in scour zones. Debris-flows that occurred during the record-setting winter storms of 1996 in western Oregon, USA, provide an opportunity to study intermediate-term effects of debris-flow deposits on abundances and habitat for juvenile salmonids. In this setting, I surveyed salmonid abundance and habitat in three Oregon Coast Range streams that contained several debris-flow deposits from the 1996 storms. I explained fish abundance using hierarchical models, accounting for heterogeneous detection probabilities with repeated counts from multiple-pass snorkeling. The "best" hierarchical model of detection probability and abundance was selected (QAIC) from pool and snorkel-pass characteristics separately
for juvenile coho salmon (Oncorhynchus kisutch), age 0+ trout, and age 1+ trout (Oncorhynchus spp.) in each stream. Adding distance to the nearest 1996 debris-flow deposit (DDF) produced a significant drop-in-deviance for four of nine "best" models, including at least one in each stream and for each species/age-class. In these four models, salmonid abundance decreased with increasing distance from deposit. As a potential explanation, several pool habitat characteristics were correlated (Spearman's rank) with DDF. Results varied across streams, but generally, percent of substrate as bedrock was lower and boulder density and percent substrate as gravel were higher closer to deposits. Although repeat counts are increasingly used in hierarchical modeling of heterogeneous detection probabilities and abundance for other wildlife species, studies of fish often rely on uncalibrated, single-pass snorkel counts. When exploring the value of repeat counts, I found that juvenile salmonid abundance decreased with increasing distance from debris-flow deposits in more multiple-pass hierarchical models that accounted for heterogeneous detection probabilities than for single-pass models that did not. Thus, modeling heterogeneous detection probabilities with repeated snorkel counts may be beneficial in other situations, addressing limitations of uncalibrated indices without relying on methods such as electrofishing, which may be difficult or impossible for remote study areas, longer surveys, or sensitive species. My findings suggest that debris-flow deposits may influence salmonid abundances after 15 years, and support management of debris flow-prone hillslopes and low-order channels to deliver elements of stream habitat complexity.