Cage size affects dissolved oxygen distribution in salmon aquaculture

Abstract

Atlantic salmon aquaculture is shifting toward larger cages, but the water quality implications of this shift are unknown. While larger cages could improve profitability through economiesof scale, they may increase the risk of low dissolved O2 (DO) conditions due to reduced water exchange.Low DO conditions reduce feed intake, meaning that the benefits of shifting to larger cagesmust be weighed against potential negative impacts on fish growth. To test the impact of cage size onDO distribution, we recorded DO saturation in several circular cages of 2 different sizes on a commercialsalmon farm: 6 with 168 m and 4 with 240 m circumference. Static strings of DO loggers at 1,4.5, 8, 12 and 16 m depths recorded DO saturation once every 60 s throughout a 10 d period in midsummer.Overall, DO levels in standard 168 m circumference cages were suitable for salmon feedingand growth. DO levels were highly variable (57 to 134% saturation), and were lower in cages than atthe reference site. On average, DO saturation decreased with depth, and was lowest during the earlymorning hours. Lowest DO measurements occurred in the large 240 m circumference cages, where 1in 20 of all recordings were at levels known to reduce salmon feeding and growth. DO levels in largercages can suit salmon production, but site-specific environmental conditions throughout the yearmust be considered to ensure there is sufficient capacity to tolerate reduced water exchange