Recirculating aquaculture systems: The future of fish farming?

 

 

Recirculating aquaculture systems (RAS) cut the pollution and disease that occur in current fish farming operations, and many see it as the future of the industry.

 

RAS are closed-loop production systems that continuously filter and recycle water, enabling large-scale fish farming that requires a small amount of water and releases little or no pollution.

 

About 99.75% of the water in each unit is continuously cleaned and returned to the fish tanks. Manure filtered from the water during the recycling process is used as fertiliser on nearby farm fields. The nutrient-rich water can also be used to feed vegetables and herbs in large-scale aquaponics systems, which in turn filter the water for reuse.

 

One of RAS's biggest benefits is its small "water footprint," which opens the door to commercial fish production in areas with limited water resources. The technology is proven for both fresh- and saltwater species.

 

The closed-loop RAS design also addresses several major environmental concerns associated with traditional aquaculture, particularly ocean-based net pen fish farms where saltwater species are corralled in offshore enclosures.

 

At these densely stocked net pen farms, concentrated fish waste flows directly into the surrounding ocean. Industry critics often cite a European study that concluded that the pollution from an average salmon farm is equivalent to discharging raw human sewage from a town of several thousand people.

 

In addition to fish waste, antibiotics and other chemicals administered to the farmed fish can flow directly into the surrounding water. Disease is another environmental concern associated with net pen farming, as the crowded fish are susceptible to outbreaks that can spread to wild populations.

 

Coinciding with the development of cost-competitive, large-scale RAS technology has been the development of alternative fish feeds, which use plant-based proteins to largely or entirely replace the fish meal and fish oil from wild-caught fish that are used in conventional fish feeds for carnivorous farmed fish such as salmon and trout.

 

Several factors have driven the development of alternative feeds, said Rick Barrows, a fish physiologist with the USDA Agricultural Research Service. One is supply and demand. Global production of fishmeal has remained relatively stable for decades, while demand from aquaculture has grown immensely over the past 20 years. With many of the world's oceanic fisheries harvested at or beyond sustainable levels, there's little capacity to expand fish meal production. By some estimates, raising one pound of farmed salmon on a fish meal-based diet requires three to four pounds of wild-caught fish, which produces an adverse ecological impact.

 

Alternative feeds, said Barrows, also offer improved food security, because conventional feeds can contaminate farmed fish with toxics like PCBs that sometimes accumulate in wild fish populations harvested for fishmeal.