Seasonal Fluctuations in Geosmin and Terpenes in Rainbow Trout (Oncorhynchus mykiss) in an Outdoor Commercial Recirculated Aquaculture System Facility

Seasonal changes in geosmin and selected terpenes in rainbow trout (Oncorhynchus mykiss) in an outdoor recirculated aquaculture system (RAS) in Denmark were followed over three months. In the water, geosmin varied from 4.1 to 25.7 ng/L, but no seasonal trends were obvious. Variable trends occurred for geosmin relative to concentrations of inorganic N (ammonium, nitrite, and nitrate) and phosphate, and did not indicate a general relationship. The geosmin content of the fish ranged from 669 to 2264 ng/kg and covaried with geosmin in the water. The average number of the geoA gene (encoding geosmin synthase) was 2.41 × 10⁵ copies/L in the water and 3.29 × 10⁷ copies on each bio-element in the biofilter. In only one of the three examined RAS units, geosmin in the water and on the bio-elements covaried with the number of geoA copies. Among the identified four terpenes in the fish flesh, α-pinene was dominant and made up 55.9–62.8% of the terpenes. The presence of aroma-active terpenes in the fish suggests that not only geosmin concentrations should be considered when characterizing taste profiles of farmed rainbow trout.