Nowadays salmon lice (Lepeophtheirus salmonis) constitutes one of the biggest threats to the welfare of farmed Atlantic salmon (Salmo salar). Lumpfish (Cyclopterus lumpus) are currently deployed in sea cages with salmon as an effective biological treatment against salmon lice. However most of the lumpfish used nowadays come from a wild caught broodstock, therefore there is a need to establish a captive broodstock to supply the demand for lumpfish in farms all year around and to alleviate the pressure on the wild lumpfish population. This thesis was part of a pilot experiment running from September 2017 to March 2018 which aimed at investigating the biological mechanisms of sexual maturation in lumpfish. Lumpfish were reared under continuous daylight (CDL) for 18 months prior to the experiment and then split in four groups. Two groups were kept at CDL for the whole duration of the pilot experiment while the other two were exposed to short daylight (SDL) for 4 months before switching back to CDL. In addition, the water temperature in one group for each treatment (SDL0T and CDL0T) was kept ambient while the others had ambient +3°C (SDL3T and CDL3T). Temperature manipulation was done in late January 2018 when the photoperiod in all groups was changed to CDL.Sexual maturation in males was investigated by estimating both the gonadosomatic index (GSI) and the spermatogenic maturity index (SMI). Sexual maturation in lumpfish did not appear to be significantly affected by photoperiod or temperature manipulation within this experiment. Water temperature was found to have an effect on the last stages of spermatogenesis in lumpfish.