Dr Frode Oppedal
Effects and treatment of hypoxia in cage environments
Job Title: Senior Researcher
Employer: Institute of Marine Research, Matre
Biography
Dr. Frode Oppedal, senior scientist at Institute of Marine Research, Norway has been working on salmon growth, sexual maturation, behaviour and performance in commercial sea cages during fluctuating environments and variable management practises for more than a decade. He has participated in several EU projects (e.g. WEALTH, FASTFISH, MORECARE), national research and industrial projects. Present focus is on salmon welfare and farm conditions. A major research angle is development of farm technology and management based on biological premises in order to optimise production and ensure adequate welfare of the fish.
Abstract
Within salmon production, volumes of modern sea cages range from 20 000 to 80 000 m3 and may contain 200 000 to 400 000 individuals. In practice, the largest sites produce more than 10 000 tons of salmon biomass per generation involving more than 2 million individual salmon per site. Within these cages complex spatial and temporal variations in oxygen levels are observed (Johansson et al., 2006; 2007; Vigen, 2008). Strong vertical gradients in oxygen typically coincide with the pycnocline, while fluctuating patterns occur over days to weeks. Severely hypoxic conditions (30% saturation at 12 °C) have been recorded over periods of up to 1 hour in the centre of a commercial cage and were correlated with periods of low water flow. Seasonal variations are frequently observed at commercial salmon farms. Adequate oxygen levels are a key requirement to ensure welfare and development of fish.
Recent results show that this fluctuating hypoxia (low oxygen) may have detrimental effects on fish oxygen consumption, physiology, feed intake, growth and well-being. A study with full-feeding Atlantic salmon held in seawater at 16 °C and given fluctuating hypoxic saturation levels of 70% led to reduced appetite; 60% additionally initiated acute anaerobic metabolism and increased skin lesions; 50% additionally initiated acute stress responses, reduced feed conversion and growth; and 40% additionally caused impaired osmoregulation and mortalities. Growth rates and condition factors gradually decreased and proportions of fish with skin infections gradually increased in severity as hypoxia levels rose. Lack of energy from aerobic metabolism for fish within the hypoxic groups may have led to down-regulation of energy-demanding processes such as feed uptake, growth and immune function (e.g. Wu, 2002).
Thresholds levels for the ability to maintain oxygen uptake rates in full-feeding Atlantic salmon found at 60, 40 and 28 % oxygen saturation at 18, 12 and 6 °C, respectively. Within the Mediterranean sparid fish common dentex (Dentex dentex) optimal oxygen levels ranged >67% saturation with dangerous levels at 32 and 44% at 13 and 28 °C, respectively (Valverde et al., 2006).
Modelling of oxygen consumption, fish biomass and densities provide basal knowledge for cage sizes and water flows needed for successful oxygen management within farming of present and future species. Model examples from Atlantic salmon will be presented and discussed.
