Introduction
At low temperatures (<4 oC), ulcer formations cause welfare and economic issues in the production of Atlantic salmon. However, this problem also persists in land-based productions at higher temperatures, contrasting with what is established at sea production. Seawater flow-through systems in land-based production of salmon is gaining popularity, which escalates the issues of bacteria-mediated ulcers. This is especially true for smolt; the stage of fish which is more susceptible to bacterial infections. We hypothesize that the ulcer outbreak is related to the relative increase in surface area the fish is exposed to as well as a function of the higher density to water exchange rates in tanks compared to the traditional sea cages. Although susceptibility to marine bacteria such as Moritella viscosa and Tenacibaculum sp. may be increased in tank production, the self-contained nature of the production method may simplify treatment without pumping fish or using well boats or tarps.
This project aimed to establish a link between starting density, skin health, growth rates, and wound formation in the production of Atlantic Salmon in tanks using seawater. When wounds were observed in the experiment, we explored the potential of freshwater treatment to combat the pathogenic marine bacteria.
Material and Methods
The trial was conducted at LetSea’s R&D Land facility in Northern Norway between January 2023 and May 2023. Smolts used for the trial were obtained from Kvarøy Smolt, Norway. At the start of the trial and with a mean weight of 135 g, the fish were randomly distributed amongst eight trial tanks (2 m3) at an abundance of fish per tank that reflected 15 kg/m3 (four tanks) and 20 kg/m3 (four tanks). All tanks were given a continuous light regime and a 20-hour feed regimen aimed at 12% overfeeding daily. The tanks were all equipped with cyclone feed collecting systems. The feed chosen was a commercially available feed designed for strengthened skin health and minimizing wound formation. Water for the trial was derived from the facility’s saltwater intake, located 140 m below the surface and 700 from the shore. The water was treated with UV before usage.
Five days after the start of the experiment, the first ulcer outbreak was observed, and a freshwater treatment was performed. All fish were starved 48 hours pre-treatment. On the day of treatment, 10 fish from each of the eight tanks were taken out and scored for ulcers on the body and fins and for the loss of scales (all scores 0-3 based on Fishwell welfare indicators)1. Samples were taken for bacteriology, and PCR on starting and open wounds were performed. After sampling, four tanks were randomly chosen for treatment (two high-density and two low-density tanks). 8 ml Aqui-S Vet. sedative (to a final concentration of 4 ml/m3) was added to all four tanks. This was done to minimize the impact of the handling itself. Earlier trials also show Aqui-S reduce loss of blood chloride during freshwater treatments in Atlantic Salmon as well as reduces both cortisol and lactate formation during handling (results not published yet). The water column was reduced to 20 cm (20 % of the original volume), and 2 m3 of fresh water was added. New salinity was measured, and the fish was kept under observation with continuous measurements of CO2 and oxygen. The treatment lasted three hours. After the completed, treatment saltwater supply was turned back on, and new samples were taken from the handled fish.
Results and conclusions
Our results show that there were no clear differences in wound formation on the body or fins in tanks between low and high-density groups. It should be noted that body wound scores of 2 and 3 were only presented in high-density tanks. The freshwater treatment operations were successful, with little to no observable stress on the fish and only a drop in salinity to 2 ppt. PCRs taken from wounds before and after freshwater treatment showed little difference in CT values between pre-and post-treatment samples. Bacteriology showed positive results pre-treatment but no growth of either M. viscosa or Tenacibaculum sp. after treatment. No new wound formations were observed 30 days after treatment. The data on growth and FCR will be available in week 24, 2023, and will be presented at the conference. The current results suggest that freshwater treatments can potentially reduce marine bacteria-mediated ulcer formations and should be explored further.
References
1. Noble C, Gismervik S, Iversen M, et al. Welfare Indicators for farmed Atlantic Salmon: tools for assessing fish welfare. 2018.