Background

Production of fish in aquacultures is the largest growing source of protein globally, but environmental concerns may hamper the fish breeding. Fish producers face environmental restrictions with respect to release of pollutants and availability of water for the ponds, but growth of microorganisms that produce taste-and-odour compound (TOC) may also threaten the production. The most common TOCs are geosmin and 2-methylisoborneol (MIB) that both has a characteristic earthy taste and odour. The compounds are produced by cyanobacteria and streptomycete bacteria that appear to encounter good conditions in aquacultures due to the ambient concentrations of organic and inorganic nutrients. 

Geosmin and MIB can be detected by humans at very low concentrations (few ng per liter). Fish take up TOCs via gills and may accumulate the compounds 200- to 400-fold in their lipids. This means that even low concentrations in the water may lead to earthy-smelling fish that are unattractive to consumers. TOCs constitute a serious problems to many of the 9,000 fish farms in Europe, and future water scarcity may worsen the situation.

(click to enlarge)

The reduction of water consumption due to environmental concerns and the need for larger and more profitable production facilities imply that fish farmers replace traditional small fish ponds with large concrete bassins. The new systems are named raceway aquacultures because of a high water flow. TOC problems appear to be more severe in the raceway systems than in smaller and traditional fish ponds. Even though there is an efficient water treatment to remove nutrients in the raceway systems, the recirculation means that nutrient concentrations typically are higher in the raceways, see figure at left.

A general introduction to raceway aquaculture systems, exemplified by breeding in Korean and American trout farms, can be found here.


Present project

In  this project we study raceway systems in Denmark and aim at characterising dominant TOC-producing organisms and their biology, abundance of TOCs in water and fish, attempt to apply a photocatalytic approach for removal of the TOCs. Specific research activities in the aquaculture project are:

• Identification of major TOC-producing organisms
• Biological mechanisms controlling the TOC production
• Seasonal changes in TOC in different types of raceway systems
• Relations between content of geosmin in fish and the ambient water
• Sensory panel tests to characterize human acceptable TOC levels in fish
• Degradation of TOC by photocatalytic methods