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 economies of scale, they may increase the risk of low dissolved O 2 (DO) conditions due to reduced water exchange. Low DO conditions reduce feed intake, meaning that the benefits of shifting to larger cages must be weighed against potential negative impacts on fish growth. To test the impact of cage size on DO distribution, we recorded DO saturation in several circular cages of 2 different sizes on a commercial salmon 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 feeding and growth. DO levels were highly variable (57 to 134% saturation), and were lower in cages than at the reference site. On average, DO saturation decreased with depth, and was lowest during the early morning hours. Lowest DO measurements occurred in the large 240 m circumference cages, where 1 in 20 of all recordings were at levels known to reduce salmon feeding and growth. DO levels in larger cages can suit salmon production, but site-specific environmental conditions throughout the year must be considered to ensure there is sufficient capacity to tolerate reduced water exchange.

The Transcriptomic Responses of Atlantic Salmon (Salmo salar) to High Temperature Stress Alone, and in Combination with Moderate Hypoxia

“…Seasonal uctuations in water temperature and oxygen content are unavoidable changes at aquaculture cage-sites and have profound impacts on the growth, survival and welfare of farmed sh [1,3,33,36,40,42]. With global warming, it is predicted that the occurrence of suboptimal temperatures (both high and low) and low water oxygen levels (hypoxia) may become more frequent and severe in coastal areas [7][8][9][10][11], and ultimately more challenging for aquaculture species [80].…”

“…Yet, Atlantic salmon have an optimal growth performance at water temperatures between 10-14 °C [37,38]. In addition, the dissolved oxygen levels within the cages uctuate substantially due to temperature, sh density, feeding and low water exchange [39][40][41], and frequent hypoxic events (~ 60-70% air saturation) occur in late summer [33,34,36]. These suboptimal conditions may negatively affect their physiological and growth performance [1,42], and recently led to mass mortalities at cage-sites in Newfoundland, Canada [33].…”

Cyclic Hypoxia Exposure Accelerates the Progression of Amoebic Gill Disease

“…To accurately gauge the most extreme conditions experienced by the fish, DO should be monitored inside cages at night or during the early morning, when DO levels are lowest due to lack of photosynthetic activity [49]. During high-risk periods for pathogens, such as summer and autumn for AGD and complex gill disease (CGD), gill checks should be performed frequently to facilitate early detection [21,50,51].…”

“…A control group of three systems was maintained at ≥90% DO saturation (normoxic treatment), and three systems were subjected to cyclic hypoxia (hypoxic treatment). Cyclic hypoxia was chosen to mimic conditions in marine cages, where DO typically fluctuates daily due to changes in photosynthetic O2 production [3,49]. Daily at 09:00 the pumps were turned off and circulation discontinued in all six systems.…”

“…Swabs were immediately placed in 500 μL lysis solution (7.8 M Urea, 0.5% SDS, 10 mM Tris and pH 7.5). After sampling, each fish was marked on its dorsal side just above the anal fin with a sub-dermal Cyclic hypoxia was chosen to mimic conditions in marine cages, where DO typically fluctuates daily due to changes in photosynthetic O 2 production [3,49]. Daily at 09:00 the pumps were turned off and circulation discontinued in all six systems.…”