Tolerance of juvenile Atlantic salmon (Salmo salar L.) to dissolved gas supersaturation in freshwater

Tolerance of juvenile Atlantic salmon (Salmo salar L.) to dissolved gas supersaturation in freshwater Introduction: Gas bubble disease (GBD) among aquatic animals is a major concern in river systems with high levels of dissolved gas supersaturated (DGS) caused by hydroelectric plants. Etiology and pathogenesis of the disease is well known from studies of pacific salmonids (Oncorhynchus spp.), but the effect of supersaturation on juvenile Atlantic salmonids (Salmo spp.) is poorly documented and has often been overlooked. Hence, an experimental trial was carried out to examine the tolerance of Atlantic salmon parr (Salmo salar L.) to different levels of dissolved gas supersaturation in freshwater. Methodology: Experimental trial was set-up using Atlantic salmon parr in shallow tanks (30 cm) assigned to different DGS levels ranging from 100% to 130% total dissolved gas (TDG). Supersaturated water was generated by mixing atmospheric air with cooled water combined with increasing pressure inside a cone-shaped column. Fish were sampled and examined (diagnostic; gross pathology) within a period of two weeks of exposure. Results: Minor gas bubble formation (emphysema) in fins occurred among some fish in groups exposed to supersaturation ≤ 105% TDG. Among fish exposed to 110% TDG there was no significant mortality (n=1), but an increasing tendency of subcutaneous emphysema, hemorrhages and exophthalmia during days of exposure. Acute GBD and lethal levels occured among fish exposed to TDG levels above 110%. Fish became moribund after 24 hours exposure time in 115% TDG, after 3 hours in 120% TDG and within less than one hour in 130% TDG. Clinical signs of acute GBD included severe emphysema in fins, gas emboli in gills and hemorrhages. Conclusions: Gas supersaturation ≥ 115% TDG was highly lethal and caused acute GBD within hours among Atlantic salmon parr. Although gas bubble formations was observed among fish exposed to low TDG levels at shallow depth, the present study suggests that depth compensated level of ≤ 105 % TDG might be sufficient to avoid severe GBD among juvenile Atlantic salmon if the exposure time is less than two weeks. Compared to similar trials, our observations indicate that juvenile Atlantic salmon may be more sensitive to gas supersaturation than juvenile Pacific salmon species.