Introduction
Interest in cod farming is once again growing in Norway, driven by a new generation of domesticated cod selected for improved growth through breeding programs. Despite these advancements, several challenges remain—one of the most pressing being the control of sexual maturation, which can compromise growth performance, fish health, and pose environmental risks. Indeed, in cod (Gadus morhua) farming, fish are typically reared in open sea cages. In case of individuals reaching maturity, they may release fertilized eggs either during a spawning event occurring in the cages or after escaping. This could result in a possible interaction with wild cod and genetic pollution of the local cod stocks in Norway. One proposed mitigation strategy is the use of continuous light (LL), or LL and shading of natural light (at surface or using submergence) which is known to delay maturation of cod in cages. Additionally, the use of standard feeds aimed at maximizing somatic growth may influence reproductive output in maturing fish when compared to broodstock diets. However, the extent to which these effects persist in the current domesticated cod strains—now in their 6th to 8th generation—remains unclear.
Methods and setup
To improve control of sexual maturation in farmed cod through light and submergence strategies, we employed surface cages exposed to strong continuous light (LL, 2500 W) and submerged sea cages positioned at 30 m depth (away from strong natural daylight), also under LL conditions (600 W). Additionally, two dietary treatments - broodstock (Vitalis Prima, Skretting) and standard (Amber Neptun, Skretting) diets - were tested under natural light (NL) conditions. For each treatment group, 240 to 480 cod with an average weight of 1.5 to 2 kg were stocked in cages in September 2024. LL-fish were held under continuous light in indoor tanks before transfer while NL-fish were kept under natural photoperiod. Fish from each group were sampled every six weeks for growth measurements, blood and tissue collection, and assessment of sexual maturation through various biological indicators. Parasite prevalence was also monitored, and a welfare score was developed to assess overall fish health. At the onset of the spawning season, mature fish were transferred to indoor tanks equipped with egg collectors, using a male-to-female ratio of 2:1. As a reference, wild cod captured near the cages were maintained in a separate tank under similar conditions. Egg volume and fertilization rates were recorded daily. Additionally, newly fertilized eggs were sorted and incubated in NUNC plates at 6 °C to monitor survival until hatching, determine hatching success and evaluate larval morphometry at hatch.
Results and discussion
At 2.5 years old, fish reached an average weight of 3.7 kg with no significant differences observed between dietary treatments. Preliminary welfare assessments - based on indicators established for salmon - suggest that the welfare of farmed cod was generally acceptable, despite the occurrence of some skeletal deformities, particularly in the backbone and head regions. Consistent with previous trials, parasite prevalence, specifically Caligus, was higher in LL-fish held in submerged cages. Sexual maturation was first detected in NL-individuals, while those in LL treatments entered early stages of maturation 5 months later. Both males and females underwent maturation across treatments.
Egg production during the spring spawning season (NL treatment) varied, with females fed the broodstock diet producing a greater egg volume compared to those on standard growth diets. Fertilization rates were lower and more variable in farmed cod than in the wild reference group. Survival and hatching rates were also treatment-dependent. Overall, reproductive output and developmental success were superior in wild cod compared to farmed groups. Within farmed treatments, eggs from broodstock-fed fish outperformed those from standard-fed fish, which showed poorer fertilization, survival, and developmental outcomes.
Our results indicate that both light regime and diet significantly influence the reproductive performance and developmental success of farmed cod. Continuous light effectively delayed sexual maturation, while standard diets had a negative impact on egg quality and larval outcomes compared to broodstock diets. Nonetheless, a substantial proportion of larvae survived post-hatching across treatments, suggesting that accidental spawning in cages—or following escape events—could contribute to the presence of farmed-origin individuals in the wild.