Sexual maturation in fish is a major challenge in aquaculture that can negatively impact fish growth and health . In farmed cod (Gadus morhua ), early maturation can result in discharge of fertilized eggs in the sea cages from spawning individuals and therefore to possible genetic interactions with wild cod . To avoid any additional pressure on already depleted local coastal cod stocks , a rapid and easily applied solution to control early maturation in this species is needed. Indeed , Norwegian cod aquaculture is currently developing, and its future growth depends on the capacity to solve these issues. While continuous light (L L) treatment is known to prevent or delay maturation in cod in indoor tanks or in sea cages, respectively, it does not seem to completely arrest maturation in the cages. This because the reproductive cycle in cod is controlled by photoperiod, and the suppression of melatonin secretion is tied to previously experienced daytime intensity limits. Therefore, reducing the relative intensity difference between day and night will enhance the effectiveness of additional artificial light in suppressing sexual maturation.
To develop a better protocol for controlling sexual maturation in farmed cod by light treatment we combined either net/ tarpaulin shading at the surface with a strong LL (2500W) or shading using submerged sea cages to 30 m depth with LL of 600W. W e tested triplicate cages of shaded surface or submerged cages with natural light (NL) or LL . For each group, 1200 cod of 200 g, reared at LL over summer , were placed in the cages in September and sampled bimonthly for growth, blood, tissue collection and ultrasound to analyze fish sexual maturation at macroscopic, microscopic, hormonal and gene expression levels . In addition, the presence of parasites was examined to document any potential effects of different environments.
Fish growth did not significantly differ between environments during the initial phase of the experiment, although individuals in submerged cages tended to be larger. However, sexual maturation was observed in individuals of all NL cages, whereas very few individuals initiated maturation under L L conditions, and none in submerged LL cages. The proportion of maturing fish varied between the sexes, with males exhibiting higher rates compared to females. The treatments influenced the presence of parasites, specifically Caligus , with submerged cages exhibiting a higher prevalence of parasites, particularly under L L conditions. Overall, the obtained results are promising, and we anticipate that these revised methods can promptly benefit the aquaculture industry. With standardized and optimized technology for light control and shading, aligned with biological principles , we can potentially effectively prevent fish from becoming sexually mature before reaching slaughter size.