Vulnerability of Larval Lampreys to Dewatering: Effects on Movements and Survival

Larval lampreys (ammocoetes) live for several years, burrowed in sediments, feeding on detritus and organic matter.  Ammocoetes that inhabit regulated environments such as the Columbia River experience dewatering events as part of reservoir management, but little is known of the effects of dewatering.  Such work is critical because Columbia River reservoirs experience faster and more frequent dewatering events compared to unregulated environments. We evaluated larval lamprey responses to dewatering using three size groups (30-49 mm, 50-100 mm, and 101-150 mm TL) of fish in laboratory trials.  Vertical distribution experiments were conducted in a cylinder (38 cm deep) exposed to a dewatering event. We found that 40% of larval lamprey emerged, 58% burrowed between 0 and 7.6 cm, and 2% burrowed between 7.7 and 15.2 cm.  No fish burrowed deeper than 15.2 cm.  To evaluate fish survival relative to duration of exposure to dewatering, ammocoetes were dewatered for 4, 8, 12, 24, 36, and 48 h periods during three replicate trials.  Limited (<5%) mortality occurred for exposures up to 12 h.  At 24 h mortality increased to 15-30% and at 48 h mortality was high (~63-92%).  Further work is needed to evaluate the effects of temperature and humidity during dewatering.