Salvage using electrofishing methods caused minimal mortality of burrowed and emerged larval lampreys in dewatered habitats

Abstract

Objective Human-induced dewatering of freshwater habitats causes mortality of larval lampreys (family Petromyzontidae). Salvage by electrofishing at dewatering events is assumed to reduce this mortality, but to our knowledge this assumption remains unassessed. Methods We estimated mortality of salvaged larval lampreys ( Lampetra spp. and Pacific Lamprey Entosphenus tridentatus ) within 24 h following collection at field dewatering events in July and October. To assess when salvage may reduce mortality, we compared mortality of salvaged individuals from field dewatering events to mortality of burrowed and emerged individuals in dewatered habitats in the laboratory. Salvage protocols included electrofishing and foot pressure from walking in test enclosures before and after dewatering. Electrofishing after dewatering (“dry shocking”) involves positioning probes on moist sediment to entice burrowed larval lampreys to emerge. Result During the July salvage, air temperature averaged 36°C, bottom water temperature averaged 20°C, and many emerged larval lampreys were dead on the sediment surface. During two October events, air temperatures averaged 18 and 11°C, bottom water temperatures averaged 12 and 7°C, and only one dead emerged larval lamprey was observed. Estimated mortality of salvaged larval lampreys was 0.20 (90% credible interval = 0.09–0.37) in July and 0.00 (90% credible interval = 0.00–0.06) and 0.06 (90% credible interval = 0.01–0.18) in October. All larval lampreys that remained burrowed and were excavated from enclosures after salvage were dead in July but alive in October. Logistic regression suggested that mortality declined with increasing larval length. Mortality of salvaged 80-mm larval lampreys in October was lower than that of 80-mm individuals emerged for 1 h or burrowed for 8 h at similar water temperatures (8–10°C) in the laboratory. Conclusion In this study, electrofishing for salvage caused minimal mortality of burrowed and emerged larval lampreys in dewatered habitats. Thus, salvage using electrofishing methods could aid conservation of native lampreys by reducing mortality associated with human-induced dewatering events, especially when temperatures are elevated.