Abstract
Organisms that undergo metamorphosis can be particularly vulnerable to environmental changes. However, if these transitions are ignored as an explicit life stage in evaluations and models, such threats can be overlooked. In lamprey, larvae that achieve enough energy storage appear to metamorphose when exposed to cold winter water temperatures followed by a rise in spring temperature. Winters of the Pacific Northwest (USA) are experiencing climatic warming that may alter the conditions necessary for larval metamorphosis. Therefore, we conducted a laboratory study that compared the incidence of metamorphosis between larval Pacific lamprey ( Entosphenus tridentatus ) reared at natural winter river temperatures and those reared at water temperatures prevented from falling below 9 °C over 2 years. Repeating this experiment with two cohorts of larval lamprey resulted in no difference in the incidence of metamorphosis between temperature treatments in either rearing year. The group growth rate of cohort 1 larvae in the first year of laboratory rearing was greater than growth rates reported for stream-reared larvae, and more of these larvae metamorphosed in their first year compared to cohort 2. However, the incidence of metamorphosis was similar (about 78%) between both larval cohorts over 2 years, despite experiencing differences in collection years, temperature regimes, body sizes, and initial metamorphosis. We found no evidence that warming winter stream temperatures driven by climate change would limit Pacific lamprey metamorphosis the following summer. Subsequent work exploring whether individual growth and water temperature interact to influence near-term metamorphosis may allow managers to apply these results to other lamprey populations.