Prespawning Holding Behavior of Pacific Lampreys and Implications for Management and Conservation

A few species of anadromous lampreys spend a prolonged period in freshwater prior to spawning.  The potential for long spawning migrations characterizes these species, but apart from that there is little detailed information about the freshwater life history of maturing lampreys.  Recent and ongoing radio telemetry studies of Pacific lampreys in the Columbia River basin and in coastal basins in Oregon have identified prespawning holding as an important phase of their year-long freshwater residency.  We investigated the freshwater residence of maturing lampreys, with a focus on the prespawning holding phase, in the Smith River (Umpqua River basin).  We implanted 91 transmitters (400 d battery life) in migrants caught moving into a fish ladder trap at Smith River falls (RK 48) from April to July in 2006 and 2009.  We tracked 56 fish to holding locations distributed 51 to 120 km from the Pacific Ocean.  Movement of tagged fish had mostly ceased by 7 July, which coincided with peak water temperatures and the onset of base flows.  In base flow conditions, holding lampreys were strongly associated with glides at the habitat unit scale and boulders as cover.  They generally were found at moderate depths and experienced mean summer temperatures of 17-20 C and maximum temperatures of 29 C.  We identified two distinct periods within the holding phase.  From July to November, 80% did not move from their holding location.  In winter (November-March), 85% moved at least once.  These movements were upstream, ranged from 0.1-22 km, and coincided with winter freshets.  Holding fish were relatively invulnerable to predation:  97% of tag loss occurred March through June, at least 51% of which was associated with predation.  Lampreys began moving from winter holding locations between 9 March and 28 April (median, 10 April).  Most spring movement (89%) was in a downstream direction and spawners were seen on redds between 0.4-16.8 km downstream of their holding locations.  This case study occurred in a relatively small coastal basin (957 km²) with no impoundments, which provides an opportunity to compare lamprey behavior regionally and in relation to basin size and passage constraints.  This comparison is needed to make informed decisions about management and conservation of this species.