Introduction
Since 2018, interest in Norwegian cod (Gadus morhua) farming has seen a significant resurgence, driven by the development of 6th to 8th generation domesticated strains selected for enhanced growth and disease resistance. To ensure the long-term sustainability of this renewed industry, it is essential to address a persistent challenge and long-standing biological bottleneck: early sexual maturation and the potential release of fertilized eggs within open-sea cages, as highlighted in recent environmental risk reports. Spawning in cages might results in a possible risk of genetic pollution of wild Atlantic cod populations, while the maturation process itself is energetically demanding, often compromising fish health and growth performance. Continuous light treatment (LL) is a common management strategy used to delay maturation by interrupting the photoperiodic control of the reproductive cycle, a process mediated by melatonin regulation, which is sensitive to prior diurnal light exposure. While LL is widely implemented, its efficacy remains inconsistent. To address this, combining continuous light with cage submergence represents a promising solution to stabilize the light environment and minimize the intensity differential between day and night.
Material and methods
To investigate enhanced maturation control in Atlantic cod, two sequential trials were performed using a single cohort of fish at distinct stages of the production cycle. The first experiment (EP1) was conducted in 2023-2024 with fish ranging from 200 g to 1.5 kg, while the second (EP2) focused on fish between 1.5 and 5 kg. We tested, in triplicate, surface cages exposed to high-intensity continuous light (LL, 2500 W) with or without net/tarpaulin shading, as well as submerged cages positioned at 30 m depth (shielded from strong natural daylight) under either LL (600 W) or natural light (NL) conditions. Stocking densities consisted of 1200 fish per cage in EP1 and 240 to 480 fish in EP2. Prior to transfer to the sea cages, LL-groups were held in indoor tanks under continuous light, while NL-groups followed a natural photoperiod. Bi-monthly sampling was conducted across all experimental groups to monitor growth performance and collect blood and tissue samples. We employed ultrasound imaging alongside hormonal, histological, and molecular analyses (gene expression) to provide a comprehensive assessment of sexual maturation. Additionally, health and welfare were tracked through parasite prevalence monitoring and the implementation of a dedicated welfare scoring system.
Results and discussion
In both EP1 and EP2, the gonadosomatic index (GSI) of the NL-groups rose during winter and spring, culminating in spawning. During EP1, this was dominated by males due to their earlier maturation compared to females. In contrast, LL-groups in EP1 showed a delayed GSI increase whereas those in EP2 maintained GSI levels similar to initial values throughout the experiment, with no spawning observed by the end of the trial despite fish reaching commercial size. Notably, the submerged LL group in EP1 exhibited no significant GSI elevation, with females showing no initiation of vitellogenesis. However, fish kept in NL-submerged cages in EP1 presented an increase in GSI similar to that observed in NL-surface cages. In EP1, plasma levels of sexual hormones, specifically estradiol (E2) and 11-ketotestosterone (11-KT), mirrored the GSI and histological findings; 11-KT increased in males across all NL groups, while an elevation in E2 was noted in females from submerged NL cages. Corresponding analyses for EP2 are currently ongoing.
Regarding fish health, experimental treatments significantly influenced parasite dynamics, with a notably higher prevalence of Caligus spp. observed in submerged cages, particularly in LL-treatments across both trials. Preliminary welfare evaluations, adapted from established Atlantic salmon indicators, suggest that cod welfare was generally maintained within acceptable limits; however, some skeletal anomalies, specifically in the cranial and vertebral regions, were noted.
These preliminary observations suggest that the efficacy of maturation control may be dependent on the timing of implementation in the sea cages within the production cycle. While LL treatment resulted in only partial inhibition for the smaller fish during EP1, initial observations from EP2 indicate a more robust suppression in larger individuals. Although further histological and hormonal analyses are required to confirm these trends for EP2, optimizing light management during the production cycle appears to be a promising path for the industry to prioritize somatic growth over early maturation.
Acknowledgment
This work was funded by the Norwegian Seafood Research Fund (MOTOR, project no 901815).