Introduction
At low temperatures (<4 o C), 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. Seawater flow-through systems in l and-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. 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.
Material and Methods
The trial was conducted at LetSea’s Land facility in Norway. At the start of the trial , the fish were randomly distributed amongst eight trial tanks (2 m3 ) at an abundance of fish per tank that reflected 15 kg/m3 and 20 kg/m 3 (four tanks per group). The tanks were equipped with cyclone feed collecting systems. Water for the trial was derived from the facility’s saltwater intake and was treated with UV.
When the first ulcer outbreak was observed, a freshwater treatment was performed . On the day of treatment, 10 fish from each tank were scored for ulcers on the body and fins and for the loss of scales (all scores 0-3 based on Fishwell)
. Samples were taken for bacteriology, and PCR on starting and open wounds. After sampling, four tanks were randomly chosen for treatment for three hours (two high- and two low-density). Aqui-S Vet. sedative (final concentration of 4 ml/m3 ) was added to all four tanks. The water column was reduced to 20 cm, and 2 m3 fresh water was added. Salinity was measured, and the fish was kept under observation with continuous measurements of CO2 and oxygen. After treatment, saltwater supply was turned 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. 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 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.