Impact of CO2 and salinity in aquaculture on physiology, growth and health of coho salmon

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BBSRC : William Davison : BB/M009122/1 (1921484)As of 2017 salmonid aquaculture was worth $22 billion USD per year with the UK contributing $1.4 billion USD and Canada responsible for $988 million USD. However, despite UN directives stating a need to double production by 2050, growth is hampered by negative public perception. Typically salmonid aquaculture combines land-based freshwater hatcheries with sea-pen rearing systems. While requiring lower maintenance costs the use of sea-pens increases risk of disease in farmed fish and has been linked with parasite overspill into wild populations of salmon causing serious declines in native populations. As such there is a demand to reduce the duration of the marine grow out phase, or transition entirely to land based farm systems (referred to as recirculating aquaculture systems - RAS) which largely avoid many of these problems. However, thus far development of RAS farms has been limited due to reduced growth observed in RAS compared to pens, and the scale of RAS required to maintain fish up to harvest size. Reduced growth and adverse health outcomes have largely been attributed to various issues relating to water chemistry (e.g. CO2, salinity, pH etc.). Previous research from Prof. Richards and Prof Brauner's labs identified an optimal salinity for growing Coho salmon within RAS. Salmon grown at a salinity approximately isosmotic to blood were found to have the fastest growth rate and lowest food conversion ratio compare to fish grown at other salinities ranging from freshwater to full strength seawater. This has been hypothesised to be reduced energy expenditure for osmoregulation. However, that study was conducted at relatively low pHs indicative of a build-up of respiratory CO2 in the water, a problem that has been characterised extensively in RAS. Due to the link between osmoregulation and acid-base balance in fish, any reduction in environmental CO2 may therefore benefit fish health and growth by reducing energy expenditure on acid-base balance and therefore allow increased growth compared to fish grown at high CO2.Here we plan to acclimate fish to either freshwater (1 ppt) or isosmotic water (10 ppt) and then expose them to either atmospheric levels of CO2 or to the elevated levels of CO2 found within fish farms. We then hope to measure a variety of physiological parameters such as growth, acid-base balance and immune function. This information will then be used to determine optimal water chemistry conditions to maximise growth of salmon in aquaculture.