Fish use of intermittent watercourses draining agricultural lands in the Upper Willamette River Valley, Oregon

Historically the upper Willamette River Valley, western Oregon, was characterized by seasonal floods and large expansions of its stream network.  During the past century, human activities have altered or eliminated many intermittent stream and floodplain habitats in the valley.  As a result, the remaining intermittent streams and ditches may still provide habitat critical for native fish. Our objectives were to determine: (a) fish presence; (b) spatial gradients of fish distribution; (c) fish use of the intermittent streams as spawning and nursery habitats; and (d) main factors that influence numbers of both fish and fish species in intermittent watercourses that drain grass seed producing fields.  During the winter-spring periods between 2002 and 2006, we collected water samples and sampled fish by trapping and electrofishing, and collected habitat variables at all sites.  Thirteen fish species were found and only three of them were exotic.  The presence of recently hatched and juvenile fish indicated that conditions in these systems were suitable for spawning and juvenile rearing.  The two watershed-scale variables with the most influence on fish species richness were % watershed covered by forest and distance to perennial water.  In turn, fish abundance showed a negative relationship with distance to perennial water.