EFFECTS OF QUALITY, ROBUSTNESS AND WELFARE OF SMOLTS IN THE RESISTANCE TO THE CHALLENGE WITH PISCIRICKETTSIA SALMONIS

In this project we performed the necessary analyzes to clarify the effects of smolt robustness and quality upon the resistance to an infectious disease with a high sanitary and productive impact in the Chilean salmon industry, such as Piscirickettsiosis (SRS). In salmon farming in Chile, within the Atlantic salmon sector, mortalities are around 1 to 2% per month of the total fish population after transfer to the sea. Of this, and during the first half of 2018, 19,6%, 24,8% and 7,1% was due to infectious diseases in Atlantic salmon, rainbow trout and Coho salmon; respectively. Within infectious diseases, 67,9% of mortalities are caused by SRS in Atlantic salmon, while the value goes up to 92,9% in rainbow trout.

In this scenario, producing good quality and robust smolts has become a clear challenge for the salmon farming industry worldwide. Achieving high robustness in a batch of smolts is a task  that covers the entire production process in fresh water, from the fish strain that is decided to farm, through the management, the growing environment, the feed used and the smoltification strategies. Lately, numerous scientific studies have emerged in which it is concluded that farmed salmon with a good cardiac capacity would have greater robustness. This has been achieved, experimentally, by subjecting the fish to exercising conditions by swimming in the tanks at water speeds considered optimal. For this it is necessary to adjust the water inlet to the tanks achieving a higher speed of it; the fish will exercise when trying to maintain their position inside the tank by active swimming. Sustained swimming at optimal speeds has been shown to increase the size of the heart, improve the performance of the cardiac muscle, increase the aerobic capacity of the skeletal muscle and the capacity of transport and oxygen extraction in the muscles; all of the above also associated with greater resistance to infectious diseases, lower levels of stress, lower levels of aggressiveness among fish, improves body growth through increased appetite and increased feed efficiency, promotes RAM-like ventilation (passive branchial ventilation that saves energy), increases the secretion of growth hormone and improves bone quality among other benefits.

In this Final Report we show all the activities performed during the Project. First activities involved the adaptation of the swimming system and speeds in the experimental tanks. At this time, we did the first fish sampling (S1) in order to evaluate the starting conditions of the experimental fish. Once the system was fine-tuned, experimental groups were formed and stocked into their tanks: Control group and Exercise group, swimming at constant speeds of 0,4 and 1,0 bodylengths (BL) per second. This experimental phase went on for 11 weeks. Both groups were monitored daily and raised under strict productive standards and length and weight were controlled, as well as the smoltification process (sampling 2). At the end of the exercise period, and before see water transfer and disease challenge, a sampling (S3) was performed to evaluate the effects of exercise on health and welfare, as well as plasmatic stress markers, tissue level effects by histological analyses and molecular transcriptomic studies via RT-PCR and microarrays. The Exercise group showed a 4% higher weight compared to the Control. While the health status was similarly good in both groups, fin condition which is associated to good welfare was better in the Exercise group.

RT-PCR analyses of a set of selected immune response genes showed that the exercise regime promoted an over-expression of genes involved in the cellular arm of the immune system. In agreement with that, microarray results showed a massive effect of exercise on the cardiac muscle transcriptome.

A total of 522 genes were differentially expressed in the Exercise group compared to the Control. Interestingly, these could be grouped into functional mechanisms such as cellular processes, metabolism, immune pathways and others. In contrast to exhaustion maladaptive stress responses, low to moderate stress is regarded as evidence of increased activity of tissue and acclimation to disturbance. On the other hand, exercise promoted the differential expression of fewer genes in head kidney, but these did not group into any specific functional mechanisms. Exercise also promoted healthier liver and gills as shown by histopathological analyses. These results show that we managed to promote a higher robustness and welfare by subjecting Atlantic salmon pre-smolts to an exercise training regime.

Fish from both groups were marked by maxillae-clipping and stocked together after sea water transfer and P. salmonis challenge by cohabitation. Shedder fish belonged to the same batch as experimental fish but were not part of the fresh water experimental phase. Cohabitation challenge lasted for 62 days, until mortality curves of both groups had flattened out. Overall mortality reached a 73% on average and no differences in mortality were found between Control and Exercise groups. From this we conclude that under this studies’ characteristics, the delivered exercise regime did not increased robustness enough as to promote a higher survival against P. salmonis.

Three samplings took place throughout the challenge period: S4 during the early response phase; S5 during the late response phase; and S6 on survivor fish at termination of the challenge. On s4 the only relevant finding was that the health index was poorer on exercised fish compared to control. Microarray showed that nearly no genes were being differentially expressed in head kidney. Nevertheless, massive gene expression changes (>5500 genes) were seen between this and the previous sampling (S3, un-infected fish) due to P. salmonis infection among all fish (not by groups). Such huge transcriptome changes might override and mask any potential differences between the experimental groups. On the two final samplings, S5 and S6, no relevant differences were found, which is in line with the challenge results.

To conclude, the environmental modification of the rearing environment to promote sustained exercise training in Atlantic salmon pre-smolts promoted a series of structural and genetic changes, from growth to tissue and molecular features. While previous reports have shown that this promotes a higher resistance to some industry-relevant infectious diseases, in this study we did not found such an effect. As suggested by the large exercise-driven cardiac transcriptomic changes, it may be suggested that the intensity of the cohabitation challenge model used was enough as to override any potential positive effects of exercise upon disease resistance.