Handling operations are a significant cause of mortality in salmon farming. Mortality i ncidents related to gas supersaturation (gas bubble disease) and heavy metal accumulation, have been suspected. During some well boat operations, fish are exposed to a lower than ambient pressure when they are transported by suc tion from the sea surface to the highest point in the loading line. As pressure decreases , total gas saturation increases proportionally, potentially leading to significant gas supersaturation. Gas bubble disease is caused by total gas supersaturation and subsequent bubble formation in the fish’s blood and/or tissue fluids.
Previous studies of mortality incidents associated with freshwater treatments and closed transport of salmon in well boats have shown accumulation of Zinc (Zn2+ ) and Copper (Cu2+ ) in the water, with concentrations reaching high levels. These heavy metals could come from corrosion-protective equipment such as anodes or surface treatment in well boats in addition to other sources .
In controlled experiments using pressure chambers to simulate well boat operations, we demonstrated that gas bubble disease could be induced in salmon under conditions relevant to fish loading (0.4 ata and > 60 minutes duration). Behavioral changes were the first indication of the development of gas bubble disease. U ltrasound examinations and visual observations after exposure revealed bubbles in the bloodstream and fins of some fish. The proportion of fish displaying abnormal behavior and bubbles in the bloodstream (gills and heart) and fins increased with exposure time . Extensive bubble formation in systemic circulation was consistently associated with mortality .
Controlled laboratory experiments were also conducted to investigate the effects of elevated concentrations of Zn2+ (low, moderate and high doses) and Cu2+ in freshwater and seawater on fish physiology over eight hours of simulated transport and freshwater treatment. Before and after exposure, the post-smolts (~500 g) were kept in seawater to simulate a closed transport using seawater and two freshwater treatments in a well boat.
No fish died during the simulated seawater transport , but mortalities occurred during the freshwater treatments, with higher mortality rates in the treated raw water compared to untreated water. Mortalities in the untreated freshwater were observed after returning to seawater, illustrating that the combination of zinc exposure and osmotic stress can be fatal. For freshwater treatments, lower mortality was seen in untreated freshwater compared to treated freshwater with silicate lye. This is likely because the untreated water contained more humus, which binds Zn2+ and reduces its toxicity.
We have demonstrated that salmon can develop gas bubble disease under pressure conditions (loading) relevant to the salmon farming industry. Elevated levels of Zn2+ can lead to increased mortality, highlighting the importance of choosing an appropriate freshwater source during such operations.
Project NYBRØK (901788) – “Ny Brønnbåt Kunnskap ”, New well boat knowledge – biological risk factors when using well boats for transport and treatment of salmon. Funded by the Norwegian Seafood Research Fund (FHF).
1 The Industrial and Aquatic Laboratory (ILAB), Norway
2 University of Bergen, Norway
3 Aqua Kompetanse AS, Norway
4 NUI AS, Norway
5 The Norwegian Institute for water research (NIVA)