By Asbjørn Bergheim*
Of the global production of fish meal some 65% is utilized for aquaculture, while even 83% of the global fish oil goes to aquaculture (www.fisheries.no/aquaculture). Production of salmon and trout is a significant consumer of fish protein and oil, but the consumption has not increased concurrently with the growth of the salmonid industry. Optimized composition and digestibility of the feed, and improved feeding systems have contributed to a 15 – 20% reduced feed conversion ratio (FCR, kg feed/kg fish produced). Importantly, reduced FCR also means lower waste load per unit biomass produced. Plant proteins and vegetable oils are increasingly replacing marine fish in feed for salmonids in order to improve the sustainability of the industry (Table 1).
Land-based farms for production of cold-water fish species are generally consuming large volumes of water. However, introduction of efficient technology for injection of oxygen and removal of carbon dioxide has significantly reduced the required flow over the last decade (Fig. 1). Present day farm systems based on flow-through or partial recirculation of water typically use 0.2 – 0.5 liters per minute per kg of fish, only 10 – 20% of the volume needed before 1990. Anyway, an average-sized farm producing 1 – 2 million salmon or trout smolt per year based on oxygen injection/carbon dioxide stripping is consuming a water flow at peak biomass which is corresponding to the municipal water supply of 50,000 – 70,000 persons. During dry periods with high water temperatures, many farms are facing water supply problems.
Over the years, development of efficient recirculating systems (RAS) for salmonid production in countries such as the USA and Denmark has minimized the required water flow. RAS farms combine several advantageous properties compared to flow-through systems – among these are higher growth rate of the stock and better sustainability in terms of lower waste load and water demand. According to up-dated figures, half of the production in Chile, and some 30% in Norway, now takes place in fully recirculating farms and such intensively run facilities will probably dominate the production in the future. In North America, RAS is the pre-dominating technology in land-based farms producing cold-water fish.
Recently, introduction of a prolonged production stage in closed or semi-closed smolt farms from 100 g to 500 – 1,000 g (individual size) before stocking in open cages has become a hot topic. The traditional way, with direct stocking of newly smoltified salmon of 80 – 150 g in seawater is a well introduced procedure, but the entire production cycle from hatching to harvest may be reduced by more than half a year if this third, intermediate cycle producing post-smolt is incorporated. The increased growth during this cycle is due to higher and more stable temperatures, especially during winter (0-year old smolt), better water quality control (e.g. DO concentration kept above 80% of saturation), strongly reduced parasite and disease risk, etc. in closed systems. Not least, such systems contribute to improved sustainability of the entire production cycle characterized by waste solids removal and less potentially harmful effects on local stocks of wild salmon and sea trout (reduced risk of fish escapes and spreading of sea lice). A sketch of a newly developed closed cage is presented here. During several tests, this system based on water intake at 25 m depth has demonstrated no sea lice infestation, unlike the surrounding open cages where the salmon stock was frequently affected by a high number of lice (Arve Nilsen, personal communication).
More than 95% of the total biomass produced along the coast takes place in seawater cages. All attempts to prevent episodic fish escapes (e.g. use of external coarse-meshed nets), frequent sea lice and disease control, imposed fallowing of cage sites, etc. are of vital importance to the fish stock in the cages and to the environment. As previously mentioned, public regulations and frequent control of farming sites are decisive in order to minimize the harmful effects of the industry. Today’s cage farms are large systems (3000 – 5000 MT produced/year) and thorough inspection of the fish’s health is not only important to the welfare of the stock in the cages, but also to prevent spreading of diseases and parasites to wild stocks. Another useful approach is the introduction of movable cameras in the cages that allow running control of the fish and, importantly, of the net wall where simple damage might result in massive escapes over a few days.
The land-based Danish farm, Langsand Laks, claims to be the salmon farm that produces in the most environmentally friendly way (www.biomar.com). At this facility, Atlantic salmon is grown from hatching to harvest size in an onshore, indoor RAS. Entirely indoor production excludes (in theory) the mutual exchange of pathogens and parasites, and there is no risk of fish escapes. The outside climate has no impact on water temperature and lighting (insulated buildings and controlled photoperiod). Langsand Laks has also diminished the energy consumption by geothermal cooling and heating, and not least, by utilization of wind power. Such systems are, however, more vulnerable to technical failures, such as loss of recirculation. And when compared to on-growing in open cages, full-cycle onshore farming is more labour-intensive.
Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.