Migration Behavior and Habitat Use of Adult Pacific Lamprey from Two Rivers in Western Oregon, USA: Complex Behavior in Complex Systems

Pacific lamprey populations have been decreasing rapidly throughout the Pacific Northwest of North America.  We compare tracking data from 2009-2011 on the migration characteristics of radio-tagged adult Pacific lamprey from the North Umpqua (~4,000 km² drainage area) and Willamette (~30,000 km² drainage area) basins.  The North Umpqua has plentiful snowpack melt in the upper watersheds with strong, cool summer flows.  The Willamette Basin is divided into the lower elevation, rain-fed watersheds in the west and the relatively pristine, snowmelt-fed watersheds on the east.   

In the North Umpqua upstream movements were positively correlated with increases in temperature variables (amplitude and absolute values), and a mixed linear model that contains the best predictors will be presented.  Habitat analysis (n=70 fish) indicates that run and glide habitats were used in significant proportions in comparison to available habitat for holding and overwintering.  In particular, the interface of habitats were used in the highest proportions, especially for run and riffle habitats.  Using the results from the combination of radio tracking, habitat analyses, and thermal infrared surveys, we hypothesize that hyporheic exchange flow may be a potential driver for movements and holding location for this population. 

In the Willamette (n = 334 fish), mobile tracking data suggests that rates of movement and holding locations of lamprey may vary with flow regimes and with the predominant habitat features in the mainstem.  In 2009, most of the fish were detected in the slow-moving, deep lower river.  Of the detected fish, 61% held in the same location for extended periods, the majority holding in revetments and the main channel.  During the comparatively higher flow year of 2010, the fish distributed relatively quickly and evenly throughout the Willamette.  Of the detected fish, 90% held in the same location for extended periods, the majority holding in the main channel, followed by revetments.   In addition to the mainstem Willamette, fish were found mainly in two relatively pristine, meandering tributaries:  the Molalla-Pudding and the Santiam river systems.  

Preliminary analyses from the North Umpqua and the Willamette Basin suggest that a trio of habitat characteristics, including:  1) stream complexity, 2) presence of coarse substrate and 3) temperature variables are associated with the presence and behavior modes of Pacific lamprey in these systems.  We focus on the hypothesis that stream complexity, as represented by stream sinuosity, is an essential habitat feature for thriving Pacific lamprey populations.