Determination of peracetic acid exposure concentration without effect on Atlantic salmon parr health, welfare or growth cultured in RAS.

Abstract

Recirculating aquaculture systems (RAS) are replacing land-based flow through systems in Norway and other producing countries. RAS has many advances but also bring some biological challenges as water disinfection that may compromise biofilters performance and fish health and welfare. For example, Denmark is using peracetic acid (PAA) as a water disinfection strategy to control pathogens for rainbow trout production. PAA is a highly reactive peroxygen compound; 100 times more effective than hydrogen peroxide and has the potential to control bacterial, and parasitic infections and has a wide range of antimicrobial effects. PAA can be applied continuously or as a pulse to the water and degrades into oxygen and water. In Norway, PAA has been used to disinfect surfaces and different types of equipment in RAS facilities for salmonids, but not for water treatment.

The effects of PAA on Atlantic salmon (Salmo salar) smolt and rainbow trout (Oncorhynchus mykiss) have already been studied, but no studies on Atlantic salmon parr have been performed to date. In this study, two trials (9 identical RAS in each trial) have been performed to identify the PAA concentration threshold that bears no effect on welfare and growth of Atlantic salmon parr.

In trial 1, which included looking at fish external welfare, histology of gills and skin, swimming behaviour, and hand feeding was used to identify threshold. In the trial, the parr were exposed to PAA concentrations of 0, 0.05, 0.1,0.2 0.4, 0.8, 1.6, 3.2, and 6.4 mg/L for 1h and re-exposed two days later with the same treatment for 1h. The two highest treatments resulted in significant histological effects at skin and gills’ level. The feeding and swimming behaviour was significantly reduced with treatments of 1.6 - 6.4 mg/L PAA, respectively. Overall welfare indexes indicated that a treatment over 0.8 mg/L had a negative impact on fish wellbeing.

In trial 2, DNA standard brakes in gills was analysed, growth and external fish welfare was used to identify threshold to Atlantic salmon. Three PAA treatments in triplicates, control (no PAA), low (0.1 mg/L) and high (1.0 mg/L) were performed. The PAA was added continuously for 4 weeks after the first sampling. There was no significant difference in fish growth, external welfare score, overall welfare index, or cellular DNA damage in the gills as measured by comet assay. The latter showed that 95% of the gill cells were intact and not affected by PAA.

On the conditions tested in the present work, PAA exposure equal or below 1 mg/L seems to be safe for Atlantic salmon parr welfare and it can be considered a potential disinfectant in salmon production.