Improving Atlantic salmon smolt robustness to reduce losses in sea by development of screening tests, exercise regimes and markers

Poor smolt quality and performance after sea transfer causes substantial losses in Atlantic salmon aquaculture. This project investigated three strategies for improving the robustness of smolts during freshwater production: 1) Grading out the weakest fish based on screening tests targeting robustness traits during early life stages (Wp1). 2) Optimizing exercise regimes for improving robustness traits from juvenile stage (Wp2). 3) Identifying genetic and physiological robustness markers for improved breeding programs (Wp3). Wp1 task 1 evaluated if time of emergence of fry (alevins) relates to smolt robustness characteristics such as growth, cardiac capacity and disease resistance. Groups of early and late emerging fry were sorted and reared under identical conditions from start-feeding to smolt stage. Results show that early emerging fry had faster growth, higher heart rates and optimum temperature for aerobic scope and higher swimming endurance capacity at parr stage. At smolt stage, the late emerging group compensated with respect to growth, cardiac performance were equal but the early group had reduced levels of skin lesions during winter ulcer disease. Wp1 task 2 evaluated if screening for inherent swimming capacity in juvenile salmon could be utilized as a test for selecting smolts with improved growth and robustness. Small parrs were sorted into inferior or superior swimmers according to an optimized swim test, and were further reared under identical conditions until post-smolt stage. After screening, superior swimmers had larger ventricles, thicker compact myocardium and longer gill lamellas than inferior swimmers. At smolt stage, superior swimmers had better growth and the gill and cardiac morphological differences observed at parr stage had persisted for eight months. Thus, early selection for swimming capacity can increase growth and capacities related to oxygen supply, and represent an approach for early grading of smolt quality. Wp2 evaluated the impact of swimming exercise regimes in order to establish optimal water current for improving robustness traits already from fry stage. We tested if early adaption to exercise from fry to parr stage could enhance tolerance for higher exercise intensities during smoltification. Groups of salmon fry were given high (H), medium (M) or low (L) exercise until parr stage. Results show improved growth, enhanced cardiac capacity, higher relative heart weight and swimming endurance for H compared to M and L. The same exercise regimes were maintained from parr to smolt stage. High intensity during both periods resulted in better growth, stronger cardiac capacity, higher upper temperature tolerance, larger heart ventricles, higher swimming endurance and blood hemoglobin levels. Challenge with pancreas disease resulted in significantly higher survival for H vs M and L. This was neither explained by differences in virus load nor ventricle pathology, but epicardium pathology was less severe for H vs L. H also had better growth during disease and stronger and broader clonal expansion of immunoglobulin genes, possibly reflecting a stronger antibody response. These results show that salmon producers can increase tank water velocity to stimulate exercise already from fry stage improving tolerance for higher intensities during smoltification, ultimately giving smolts with superior health and robustness. Wp3 evaluated genetic and physiological markers associated with improved robustness by comparing wild (Lærdal) and farmed (SalmoBreed) salmon strains from juvenile to pre-smolt stage. The hypothesis was that selective breeding focusing on rapid growth have compromised cardiac and physiological robustness of farmed salmon. When reared under identical conditions, wild fish had larger heart ventricles but slower growth. At parr stage, both strains were ranked for swimming capacity and placed in swim tunnels for three weeks with and without intensive exercise, before measured for cardiac and aerobic respirometry capacity. Results showed that wild salmon had higher aerobic scope and better hypoxia tolerance than farmed salmon, with no differences due to swim performance in either strain. Intensive exercise training increased ventricle mass, gill lamellae length, aerobic scope, hypoxia tolerance and aerobic enzymatic activity significantly for wild but not for farmed salmon, indicating reduced plasticity in response to changing environment for farmed salmon. Further support came from transcriptome analysis in heart, showing that significantly higher numbers of genes responded to training in wild compared to farmed salmon. This suggests that decades of selective breeding for rapid growth have compromised the physiological performance and athletic robustness of farmed salmon compared to their wild conspecifics. Knowledge and tools developed in this project will be highly valuable for all areas of the industry focusing on strengthening the health and robustness of salmon smolt.

Despite steady improvements in the efficiency of Atlantic salmon aquaculture production during the last century, endemic and emerging infectious diseases and reduced survival of smolt following seawater transfer is currently hampering further growth. Impr oved robustness in farmed salmon was recently demonstrated in the FitnessFish project (RCN: 190067), where optimized aerobic exercise training was shown for the first time to significantly benefit health and disease resistance traits. Furthermore, it was demonstrated that inherent swimming endurance capacity was associated with improved fitness. Building on these results, this project aims to further explore different robustness characteristics in juvenile Atlantic salmon and their associations to health and disease resistance during early and later life stages. Specifically, this project will: 1) explore the potential of using early life traits, time of emergence (swim-up) and swimming endurance capacity, as novel screening tests for smolt robustness, 2) evaluate the impact of swimming exercise regimes in juvenile salmon in order to establish optimal exercise protocols for improving inherent robustness traits of salmon during the freshwater production cycle, and 3) exploit life history differences of cul tured and wild salmon strains related to cardiophysiological health to identify genes, polymorphisms and functional variations that could potentially be used as markers for improving robustness through marker-assisted or genomic selection. The results fro m this project will provide knowledge and tools for improving the key underlying factors affecting robustness of smolt, and as a result will lead to substantial improvements in welfare, sustainability and cost-efficiency of salmon aquaculture production. The project will strengthen national and international aquaculture sciences and collaborations through interdisciplinary research merging classical fish physiology with exploitation of the salmon genome resources.