Improve Atlantic salmon robustness in freshwater phase through training and feeding to a successfully timed sea transfer

Smoltification is a crucial developmental process in Atlantic salmon that involves morphological, physiological, and behavioral changes, enabling the fish to transition successfully from freshwater to marine environments. Current commercial practices aimed at producing large smolts in recirculating aquaculture systems have revealed reduced smoltification quality, leading to lower performance, compromised welfare, and elevated mortality rates after transfer to seawater. This trend appears to stem in part from the limited accuracy of existing smoltification markers, which do not always reliably predict the optimal point for seawater transfer and risk exposing fish to stressful and unsuitable conditions. The Time4Success project was established to address these challenges by evaluating and refining freshwater rearing protocols, identifying more robust molecular markers in the gills and intestine, exploring the potential benefits of moderate exercise and tailored feeding regimens, devising an improved conceptual framework for smoltification, and validating these approaches at commercial scale. Through comparative trials, an eight-week freshwater phase after the winter signal was shown to enhance post-transfer performance relative to a six-week regime. These experiments demonstrated that fish from the eight-week protocol grew more effectively and exhibited better feed efficiency once in seawater. Detailed gene expression analysis in both gills and intestine further revealed that specific markers, including cftr-I in the gills and a cluster of genes in the mid-intestine, reliably indicate when salmon are fully prepared for the osmotic challenges of seawater. The mid-intestine, in particular, emerged as a critical site for regulating ion and water balance, underscoring the organ’s central role during smoltification. Although interventions involving different swimming speeds and feeding schedules did not yield major effects on smoltification timing or growth, they may hold other health benefits that warrant continued investigation. A conceptual framework was proposed to integrate environmental cues and hormonal signals linked to smoltification, offering a tool for improving rearing strategies. The large-scale commercial trial experienced an unforeseen delay but has now resumed under UiB own funding, with samplings in progress. Despite some ongoing studies, initial results indicate that extending the freshwater phase and systematically monitoring key molecular markers can enhance fish welfare, survival, and growth in seawater. These outcomes provide a promising route toward reduced mortality, greater productivity, and better overall fish performance in the salmon farming industry. The project’s findings are being disseminated through conference presentations, peer-reviewed publications, and direct engagement with stakeholders, aiming to translate scientific insights into more effective commercial practices.

The project has contributed to improve our understanding on the molecular and physiological pathways involved in smoltification, highlighting the importance of the intestine in seawater adaptation. It revealed the complexity of the intestine and how different regions of this organ contribute to a coordinated response hormonal response coupled with changes in the gene expression where the gut have an important role during smoltification. Furthermore, the findings suggest that the “smolt windows” extended beyond the 6 weeks (the common practice in rearing conditions in the aquaculture industry) could be beneficial for the salmon. The RNA-seq data provided deeper knowledge in the molecular changes occurring across several tissues directly and indirectly involved in smoltification. A conceptual framework was proposed to integrate environmental cues and hormonal signals linked to smoltification, offering a tool for improving rearing strategies. In addition, the results suggest that introducing training in fish during smoltification does not affect the seawater adaptation, although it may offer benefits in fish health and welfare. The results have been presented in scientific papers, conferences and as part of a range of talks at national meetings including directly to stakeholders. Details of this dissemination is available in the Cristin database. The work developed during this project was also part of the academic education of on post doctor and three successful bachelor students.

The high mortality rates observed in Atlantic salmon farms after seawater transfer are a major concern for farmers and consumers, compromising the fish welfare and impairing the production of the Norwegian salmon farms. Major reasons for the high mortality are that fish produced in freshwater are not sufficiently robust at the time for seawater transfer. Currently, fish farms produce large smolts to reduce the time in open sea cages that comes at the expense of their robustness, resulting in production of weak and low performing salmon, with a high risk of diseases and also early onset of maturation. This problem is exacerbated by commonly used but insufficient smoltification markers, particularly for larger smolts. TIME4SUCCESS aims to resolve these challenges through a combined approach: 1. improve our understanding of the processes underlying smoltification and seawater preadaptation and identify a new toolbox with smoltification markers that will allow farmers to determine the best timing for the sea transfer; 2. enhance robustness and adaptation potential through synergistic benefits of training and feeding acquired during the freshwater phase. Training combined with a suitable feeding strategy ensures optimal nutritional status of the fish, enhances the energy allocation towards muscle growth, reduces early maturation and stress, and improves cardiac health and resilience. Producing more robust smolts in the freshwater phase while optimizing the timing for seawater transfer, will allow the fish to cope better with the transition and life in their new environment, make them more resistant to diseases and stress, and, hence, increase the output and sustainability of the production. The novel insights gained in TIME4SUCCESS will contribute to a sustainable growth of the Norwegian Atlantic salmon aquaculture industry, while strengthening the sustainable development goals by contributing to food security and reducing impacts on the marine environment.