The Gauntlet: Downstream Migration in Juvenile Pacific Lamprey

Anadromous lampreys move downstream as both metamorphosed juveniles and larvae.  For Pacific lamprey that spawn in the upper reaches of the Columbia/Snake River drainage, offspring travel hundreds of kilometers to reach the ocean.  We used 8.4 mm passive integrated transponder (PIT) tags to study lamprey movements.  Juveniles (> 110 mm) were tagged during outmigration through both main stem (Lower Granite Dam) and tributary (Umatilla River) sites.  Tagged lamprey were detected as they were diverted into both irrigation canal and hydropower dam bypass channels.  Average rate of movement between main stem dams was rapid (> 20 km d^-1) when compared to rates exhibited in the tributary (< 1 km d^-1).  The high variation in outmigration timing and prolonged periods of passage we documented (up to six months) suggest that river impoundment and/or aggregation of predators may contribute to losses of emigrating lamprey.  These effects may also impact larvae.  Laboratory studies indicated that the use of 8.4 mm PIT tags was feasible for larvae as small as 80 mm; they exhibited high PIT retention and survival.  Swimming, burrowing and growth were also unaffected by tagging.  Hence, future use of this technology may illuminate the heretofore unknown movements of these very small fish.