In sea trout, the physiological response to salmon louse is stronger in female than in males

Per Gunnar Fjelldal; Sussie Dalvin; Mathias Stølen Ugelvik; Audun Østby Pedersen; Tom J Hansen; Bjørnar Skjold; Lise Dyrhovden; Ann Kathrin Kroken; Ørjan Karlsen

doi:10.1093/conphys/coac078

In sea trout, the physiological response to salmon louse is stronger in female than in males

Conserv Physiol. 2023 Jan 12;11(1):coac078. doi: 10.1093/conphys/coac078. eCollection 2023.

Authors

Per Gunnar Fjelldal¹, Sussie Dalvin², Mathias Stølen Ugelvik², Audun Østby Pedersen¹, Tom J Hansen¹, Bjørnar Skjold², Lise Dyrhovden¹, Ann Kathrin Kroken¹, Ørjan Karlsen³

Affiliations

¹ Reproduction and Developmental Biology, Institute of Marine Research (IMR), Matre Research Station, 5984 Matredal, Norway.
² Pathogens and disease transfer, Institute of Marine Research (IMR), PO Box 1870, Nordnes, 5817 Bergen, Norway.
³ Reproduction and Developmental Biology, Institute of Marine Research (IMR), PO Box 1870, Nordnes, 5817 Bergen, Norway.

Abstract

The aims of this study were to compare male and female sea trout (Salmo trutta) with respect to their hypo-osmoregulatory ability over a simulated migration to seawater and their clinical and physiological response to salmon louse (Lepeophtheirus salmonis) infection in seawater and over a simulated pre-mature return to freshwater. For this purpose, 2-year-old hatchery-reared male and female brown trout (F1 offspring of wild caught anadromous fish) were infected with salmon lice and measured for changes in plasma ions, glucose, lactate and osmolality and relative heart, liver and gonad sizes during a simulated seawater migration and thereafter a premature return to freshwater after 4 weeks in seawater (pre-adult louse). Un-infected trout served as control. Male trout used longer time to develop full hypo-osmoregulatory ability in seawater and showed a stronger response in plasma glucose and lactate following simulated premature return to freshwater, compared to female trout. Response to salmon louse was stronger in female trout, shown by stronger osmotic stress by chalimus (plasma Cl^-) and pre-adult louse (plasma osmolality) and elevated relative liver size (hepatosomatic index) by pre-adult louse in female compared to male trout. Moreover, high plasma cortisol in infected female and low plasma cortisol in infected male trout produced a significant treatment-sex interaction on plasma cortisol. Lice infection intensity was initially higher in male (0.18 lice g^-1) compared to female trout (0.11 lice g^-1) at the chalimus stage, but equal between sexes at the pre-adult stage (male 0.15 and female 0.17 lice g^-1). This study showed that female trout were better adapted for changes in water salinity, while male trout were more robust against salmon louse infection. These results suggests that the elevated salmon louse infection pressure generated by salmon farming have strong and unexplored negative effects on wild sea trout populations. Further research on this topic is vital for the conservation of wild sea trout populations.

Keywords: Anadrome; brown trout; cardiosomatic index; hatchery reared; hepatosomatic index; osmoregulation; salmon louse; sea trout.