Reproductive performance of lumpfish (Cyclopterus lumpus, L. 1758) males: Effects of integrated photoperiod and temperature manipulations on sexual maturation

The use of juvenile lumpfish for sea-lice control in salmon aquaculture faces several challenges, including limited understanding of how environmental factors regulate reproduction. This study investigated the effects of photoperiod and temperature manipulation on sexual maturation in male lumpfish. Fish were exposed to natural (NP) and compressed (CP) annual photoperiods, with temperature elevation applied close to and during the spawning period. Although body weight appeared slightly higher under NP before temperature elevation, the differences were not significant. Following temperature elevation, growth was significantly higher at only two sampling points: in the elevated-NP compared to CP, and later within NP. Gonadosomatic index increased prior to temperature elevation and declined during the spawning period. Based on descriptive temporal patterns, spermatozoa were detected early in development and increased over time, with earlier and higher proportions observed under CP with elevated temperature. Although not all changes were statistically significant, plasma testosterone (T) and 11-ketotestosterone (11-KT) tended to increase towards and decline during the spawning period, particularly in CP. Only a few statistically significant differences between treatments were detected because of substantial individual variability and experimental constraints, limiting firm conclusions regarding treatment effects. Future studies would benefit from considering genetic variation, sex-specific experimental groups, optimized timing of temperature manipulations, improved sampling consistency and larger sample sizes. Nevertheless, the observed temporal patterns and the few significant differences between treatments suggest that photothermal manipulation may influence the timing of sexual maturation in male lumpfish. These findings provide insight into male lumpfish reproductive dynamics under photothermal manipulation and lay a foundation for future research aimed at improving broodstock management strategies.