The Effects of Ozone on Atlantic Salmon Post-Smolt in Brackish Water—Establishing Welfare Indicators and Thresholds

Abstract

Ozone is a strong oxidant, and its use in aquaculture has been shown to improve water quality and fish health. At present, it is predominantly used in freshwater systems due to the high risk of toxic residual oxidant exposure in brackish water and seawater. Here, we report the effects of ozone on Atlantic salmon (Salmo salar) post-smolts (~100 g), in a brackish water (12 ppt) flow-through system. Salmon were exposed to oxidation reduction potential concentrations of 250 mV (control), 280 mV (low), 350 mV (medium), 425 mV (high) and 500 mV (very high). The physiological impacts of ozone were characterized by blood biochemical profiling, histopathologic examination and gene expression analysis in skin and gills. Fish exposed to 425 mV and higher showed ≥33% cumulative mortality in less than 10 days. No significant mortalities were recorded in the remaining groups. The skin surface quality and the thickness of the dermal and epidermal layers were not significantly affected by the treatments. On the other hand, gill histopathology showed the adverse effects of increasing ozone doses and the changes were more pronounced in the group exposed to 350 mV and higher. Cases of gill damages such as necrosis, lamellar fusion and hypertrophy were prevalent in the high and very high groups. Expression profiling of key biomarkers for mucosal health supported the histology results, showing that gills were significantly more affected by higher ozone doses compared to the skin. Increasing ozone doses triggered anti-oxidative stress and inflammatory responses in the gills, where transcript levels of glutathione reductase, copper/zinc superoxide dismutase, interleukin 1β and interleukin were significantly elevated. Heat shock protein 70 was significantly upregulated in the skin of fish exposed to 350 mV and higher. Bcl-2 associated x protein was the only gene marker that was significantly upregulated by increasing ozone doses in both mucosal tissues. In conclusion, the study revealed that short-term exposure to ozone at concentrations higher than 350 mV in salmon in brackish water resulted in significant health and welfare consequences, including mortality and gill damages. The results of the study will be valuable in developing water treatment protocols for salmon farming.

Oxidative Chemical Stressors Alter the Physiological State of the Nasal Olfactory Mucosa of Atlantic Salmon

“…Gr, as an antioxidant, is responsible for the regeneration of reduced glutathione during detoxification of peroxides and free radicals formed in mitochondria, thus maintaining the redox potential of the cell [53]. The upregulation of gr in both instances, as well as in previous oxidant studies in salmon [7,10], provides strong support for its canonical function in mucosal antioxidant defence, that is likely ubiquitously regulated by the oxidative chemical stressor. There was no apparent tendency for AA-induced changes in the antioxidant repertoire in the nasal olfactory leukocytes, though both gpx and cu/zn sod were responsive.…”

“…Exogenous ROS may come from various sources, and their impacts on redox status have consequences on cell viability, activation, proliferation, and organ function. Farmed fish encounter an increased flow of exogenous ROS several times during a lifetime, as many husbandry practices employ ROS-generating compounds either as a form of disinfectant or water treatment, or as a chemotherapeutant [ 7 , 8 , 9 , 10 , 11 ]. The antimicrobial activity of ROS towards opportunistic and pathogenic microorganisms underlines their use in providing fish with a favourable rearing environment [ 12 ].…”

Crowding reshapes the mucosal but not the systemic response repertoires of Atlantic salmon to peracetic acid

“…The section was scored by an impartial evaluator (no prior knowledge of sample treatment) using a 0-to 3-point system, with 0 indicating healthy skin with intact epithelial surfaces and 3 indicating severely damaged conditions characterised by a rough surface and the complete disappearance of the outer epidermal layer. For the gill sections, case scoring was performed following a previously published strategy (Reiser, Schroeder, Wuertz, Kloas, Hanel, 2010), with modifications (Stiller, Kolarevic, Lazado, Gerwins, Good, Summerfelt, Mota, Espmark, 2020). The evaluation was carried out by randomly selecting five gill filaments (i.e., two upper half, two lower half, and one middle of the whole gill arch section).…”

The pathology associated with putative algal toxicosis in red snapper, Lutjanus species (Bloch 1790)

“…Heavy toxic algal blooms or direct exposure to toxins may be expected to cause more diffuse gill lesions, while milder algal blooms may cause multifocal gill lesions and low-grade mortality as observed in this study. The replacement of normally squamous gill epithelium by plump hypertrophied epithelial cells is likely a general indication of exposure to toxic compounds, as is also reported in prolonged exposure to ammonia or ozone (Roberts & Rodger, 2012;Stiller et al, 2020 Multifocal areas of depletion of lipid and glycogen stores in Fish 9, 10, 11 and 14…”