Effect of environmental factors on swimming depth preferences of Atlantic salmon ( Salmo salar L.) and temporal and spatial variations in oxygen levels in sea cages at a fjord site

The welfare of intensively farmed fish is highly dependent on water quality. Our knowledge of the variable physical environment in sea cages is limited. In particular, problems may result from high biomass and high temperature, which reduces oxygen solubility when consumption rates of the fish are high. In this study, Atlantic salmon ( Salmo salar L.) were farmed at normal (7–11 kg m − 3 ) and high stocking densities (18–27 kg m − 3 ) in production cages. Swimming depth and relative fish density were observed in relation to conditions of oxygen, temperature, salinity, light intensity, feeding and water currents during three periods of 3 days during autumn 2002. A novel application of a statistical method, the regression tree, was introduced and used in the analyses. The analyses showed differences in behaviour between normal and high stocking densities, caused by the limited space in the preferred environment. The fish appeared to distinguish between two different environments separated by the pycnocline. In the upper part, the salmon crowded at high relative densities both during the day and at night. Temperature was one key environmental factor associated with the density. The preferred temperature seemed to be between 16 and 18 °C. Avoidance of light during the day was another important factor, resulting in the highest observed relative densities at night. Together, this exposed some of the fish in high-density treatment to sub-optimal environmental conditions. In general, a considerable temporal and spatial variation in salinity, temperature and oxygen were observed. As day length got shorter in autumn, there was a general trend of lowered total oxygen content, as well as an increased difference in oxygen level between the two stocking densities. There was a negative correlation between stocking density and oxygen conditions. The results demonstrate the complex spatial and temporal variations in environmental factors vs. the behavioural response of salmon in a stratified cage environment at a fjord site.