Contribution of woody debris to trout habitat modification in small streams in secondary deciduous forest, northern Japan

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Abstract
Field studies to examine the influence of woody debris on rainbow trout ( Oncorhynchus mykiss ) abundance through habitat modification were conducted in two small streams, the Horonai and Uenae streams, running through secondary deciduous forest in south‐western Hokkaido, northern Japan. Reach‐based woody debris volume (total woody debris volume per 100 m 2 of study reach) was significantly correlated with the total basal area of riparian stands along the margins of the Horonai stream, but no significant relationship was evident between them for the Uenae stream. This inconsistency between the streams was considered to be a result of the difference in stream size (width, depth and discharge). Woody debris was the principal agent for pool formation, although it had a far smaller volume than that found in streams draining old‐growth coniferous forest in North America, where most of the previous studies have been carried out. Untransported debris pieces of larger volume more effectively contributed to pool formation than smaller transported pieces. The volume of individual debris scour pools was positively correlated with the volume of woody debris associated with each. Similarly, reach‐based pool volume increased with total woody debris volume, but the relationship was less clear in the Uenae stream, having more abundant transported woody debris than the Horonai stream. The biomass of rainbow trout in individual pools, which were regarded as the most preferred habitat type for stream salmonids, was correlated with pool volume. A positive relationship also existed between reach‐based standing crop and pool volume. These results revealed that secondary deciduous forest, like old‐growth coniferous forest, plays an important role in enhancing the carrying capacity for rainbow trout by supplying woody debris which promoted preferred habitat formation.

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