Juvenile sea lamprey (Petromyzon marinus) have a wide window of elevated salinity tolerance that is eventually limited during springtime warming

The present study examined changes in biometric characteristics, osmoregulatory capacity, and seawater (SW) tolerance of juvenile sea lamprey (Petromyzon marinus) throughout the varying thermal changes from late autumn to late spring. Body length, mass, and condition factor were maintained until April, when significant declines in mass and condition factor were observed to correspond with increases in temperature. Nearly 100% survival in SW was maintained through April. In May, after river and estuarine temperatures had increased, significant mortality in SW (up to 50%) was observed. After SW acclimation, plasma chloride was maintained at an elevated set point, and gill Na⁺/K⁺-ATPase activity was elevated. Neither parameter appeared to be affected during springtime warming. Together, these results provide a first characterization of the sustained osmoregulatory performance of juvenile sea lamprey after metamorphosis and show that the window of increased hypo-osmoregulatory performance for SW entry lasts for at least 5 months but may ultimately be limited by increases in river water temperatures in late spring.