Off-Flavor in RAS: Geosmin, MIB and Depuration

Earthy and musty off-flavor in recirculating aquaculture: geosmin and MIB, the microbes that make them, what prevention studies found, and how pre-harvest depuration is run.

What off-flavor is

Off-flavor is an earthy or musty taint in fish flesh. In recirculating aquaculture systems (RAS) it is caused mainly by two microbial compounds: geosmin, which gives an earthy taint, and 2-methylisoborneol (MIB), which gives a musty one. SRAC Publication 452 describes off-flavor in recirculating systems as a common and persistent problem, and a 2025 study in Applied and Environmental Microbiology calls it one of the industry's most economically significant challenges; Schrader and co-authors note that producers lose money because tainted fish cannot be marketed.

The compounds

Geosmin is a bicyclic sesquiterpenoid alcohol (C12H22O). Wikipedia gives an odour detection threshold in water of 0.006–0.01 µg/L and notes that it causes the muddy taste of carp, catfish and tilapia.
MIB is an irregular monoterpene with an odour detection threshold in water of 0.002–0.02 µg/L, according to Wikipedia. Together, geosmin and MIB account for most biologically caused taste and odour episodes in drinking water worldwide.
Both compounds concentrate in fatty skin and dark muscle (Wikipedia). A study of RAS-farmed European whitefish adds that the sensory threshold for these lipophilic compounds rises with the fat content of the fish.
For market-size Atlantic salmon (Salmo salar) from land-based systems, a consumer taste panel of 85 people placed the best-estimate geosmin detection threshold at 381 ± 153 ng/kg of fillet; only 44 percent of panellists detected geosmin within the range tested, but 13 percent could distinguish fillets with 87 ng/kg from those with 181 ng/kg.

Where the compounds come from

Geosmin and MIB are secondary metabolites of several microbial groups. Wikipedia names cyanobacteria (for geosmin mainly Anabaena, Phormidium and Planktothrix; for MIB Oscillatoria, Phormidium and Planktothrix) and filamentous actinomycetes, chiefly Streptomyces. The whitefish study notes that in indoor recirculating systems the producers are typically streptomycetes, myxobacteria and other actinomycetes, and that the water phase and the biofilters have been suggested as the main sources, without consensus on which RAS compartment dominates. In 2025, researchers isolated two geosmin-producing myxobacteria from RAS for the first time, Myxococcus virescens and Corallococcus exiguus; their geosmin production per cell was highest in a low-nutrient medium and in water taken from RAS.

Fish take up the compounds from the water: the whitefish study describes exchange between water and fish by passive diffusion across the gills. Uptake is reversible, which is the basis of depuration.

Prevention in the grow-out system

SRAC 452 links off-flavor to organic solids: high levels of solids stimulate the microorganisms that produce off-flavor compounds. The concentration at which this happens is not known, but the fact sheet advises that the water should never develop a foul or faecal smell, and that if it does, the operator should increase water exchange, reduce feeding, improve solids removal and/or enlarge the biofilters. Solids capture is covered in the separate RAS solids removal guide.

Several controlled studies with rainbow trout (Oncorhynchus mykiss) in replicated RAS tested other factors. Ozone dosed to hold an oxidation-reduction potential of 248 mV (below 1 µg/L residual ozone) improved water quality but did not significantly reduce geosmin or MIB in water or fish flesh. Nitrate-nitrogen at 80–100 mg/L versus 20–40 mg/L made no significant difference to geosmin in water or fish. Adding membrane bioreactors to cut water use also left geosmin and MIB levels unchanged.

Depuration (purging) before harvest

Before sale, fish with off-flavor are usually purged. SRAC 452 describes moving fish to a clean system with uncontaminated water, where purging can take from a few days to many weeks depending on the type and severity of off-flavor. Davidson and co-authors describe the standard procedure as relocating fish to separate finishing systems, withholding feed and rapidly exchanging makeup water that has very low or undetectable geosmin and MIB.

European whitefish (Coregonus lavaretus), about 600 g, in a 127 m3 commercial tank switched to flow-through with temperature-unadjusted incoming water near 0 °C in winter: geosmin fell mostly below detection limits after 13 days, MIB required up to 16 days, and low residues remained in some fish even after 16 days.
Atlantic salmon at about 11.5 °C or 14.5 °C, depurated for 144 hours: a water exchange rate of about 1,200 L per kg fish per day significantly increased geosmin elimination compared with stagnant water, whereas the higher temperature did not enhance elimination.
Atlantic salmon of 5–7 kg: depuration systems with hydraulic retention times of 2.4 and 4.6 hours left less geosmin in water and fillets than systems at 11.3 hours.
Atlantic salmon of 5–8 kg: swimming speeds of 0.3 or 0.6 body lengths per second and dissolved oxygen at 90 or 100 percent saturation did not significantly improve geosmin removal at the usual slaughter points.

Depuration has costs. Fasting salmon lose weight: in one trial with 3–4 kg Atlantic salmon, fasted fish lost 1.1 percent of initial body weight and fish on a low ration lost 0.3 percent, while both groups eliminated geosmin and MIB rapidly, with slightly higher residues in the fed fish. Another trial found that salmon stocked at 100 kg/m3 in a depuration tank still released waste, with total ammonia nitrogen levelling off as fish began to catabolise body tissue. SRAC 452 adds that if fish stay in purging tanks for long, feeding may need to be reduced or off-flavor may develop within the purging system itself.

The durations above come from specific studies and conditions; the whitefish authors note that depuration time depends on initial concentration in the fish, water temperature, fish density, water renewal rate and the off-flavor content of the supply water.