Abstract
It is increasingly common to rear Atlantic salmon smolts for longer periods in recirculating aquaculture systems (RAS) before transfer to sea cages. The conditions for this part of the life cycle are currently being optimized, especially the water velocity in the tanks since its impact has been correlated with.growth. Earlier studies indicate that higher water velocities promote growth in salmon, nonetheless, the optimal velocity and its associated health and welfare consequences are yet to be established. In the present study, we determined the effects of different water velocities on growth, muscle development and welfare to estimate the optimum velocity for the rearing of post-smolts in RAS.We divided 2400 salmon post-smolts (average start weight 80 g) into twelve tanks (200 fish per tank) and set the water velocities in four triplicate tanks to low (L) – 0.5 body length per second (BL/s); medium (M) – 1.0 BL/s; high (H) – 1.8 BL/s; and very high (VH) – 2.5 (BL/s). The velocity for the VH group was the highest tested for salmon post-smolts to date. The trial lasted for three months and organ samples were collected at three time points. Time-lapse cameras revealed a relatively even fish distribution in the tank in L and M groups. In contrast, fish in H and VH groups displayed strong schooling behavior at specific regions in the tanks. We observed a close to a linear relationship between water velocity and average growth rate, which resulted in a 5.7% higher average body weight in the VH group in comparison to the L group at termination. The condition factors of fish from the L group was lower than in the other three groups. Muscle cellularity analysis revealed smaller fibers in the L group, while bigger muscle fibers were identified in H and VH groups which contributed to somatic growth. Nonetheless, increased cases of inflammation were observed in H and VH groups. Muscle proteomics revealed decreased translation and carbohydrate activity in the L group. The overall external welfare status of the fish was favorable; however, increased incidence of skin damage (i.e., scale loss, hemorrhaging) and pelvic fin damage (i.e., splitting) in the H and VH groups was documented. Skin histology revealed relatively thinner epidermis at higher velocities. On the other hand, increased cases of aneurysm, lamellar clubbing and fusion were identified at higher velocities too.In conclusion, the increased body weight of fish reared in high water velocities was likely mostly due to enhanced somatic growth of muscle fibers. These findings provide further evidence that elevated water velocities have positive effects on the growth rate of post-smolts even at the highest levels tested to date. On the other hand, the external welfare scores, histological analyses and the molecular data indicate that mucosal health was negatively affected by higher velocities. Thus, based on this study, the optimal water velocity for long term rearing of salmon post-smolts in RAS is most likely located slightly above 1 body length per second.
Highlights
The Atlantic salmon (Salmo salar L.) farming industry is expanding and progressively integrating recirculating aquaculture systems (RAS) into the production cycle
Juvenile salmon are produced in freshwater land-based facilities where they undergo the parr-smolt transformation which prepares them for migration to seawater, after which they are referred to as post-smolts
Faster growth rates are often regarded as the primary indicator of rearing regime success, increased growth cannot come as an expense to fish welfare
Summary
The Atlantic salmon (Salmo salar L.) farming industry is expanding and progressively integrating recirculating aquaculture systems (RAS) into the production cycle. Juvenile salmon are produced in freshwater land-based facilities where they undergo the parr-smolt transformation which prepares them for migration to seawater, after which they are referred to as post-smolts. Salmon smolts are increasingly kept for longer periods in RAS in order to take advantage of the more controlled sea water environment and to produce larger fish prior to open sea cage transfer. The rising demand for prolonged land-based rearing has resulted in research initiatives (e.g. CtrlAQUA, https://ctrlaqua.no/) to optimize the conditions for post-smolt production. Some of the advan tages of RAS-based production compared to flow through systems include minimal exposure to challenging conditions, better control of the rearing environment and less water consumption. The successful adoption of this technology during the parr-smolt transformation introduced some technological challenges. Over and above ethical considerations around fish welfare, compromised early fish welfare may result in losses in production or a lower value product due to longer term associated effects on the resilience, disease resistance and health
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
