Seasonal Variation of Off‐Flavours in a Full‐Scale Recirculating Aquaculture System Rearing Rainbow Trout <i>Oncorhynchus mykiss</i> —A Case Study

ABSTRACT Recirculating aquaculture system (RAS) is a promising strategy for economically and environmentally sustainable fish farming. Unfortunately, microorganisms in an RAS may produce off‐flavours that accumulate in fish flesh and reduce consumer attraction for aquaculture‐produced fish. Traditionally, geosmin (GSM) and 2‐methylisoborneol (MIB), the compounds causing musty and earthy flavour, have been the most studied off‐flavour compounds, but lately other compounds have also been considered important subjects of study. So far, only a little is known about the formation of different compounds at an RAS farm and their concentrations’ fluctuations during the seasons. This case study aimed at monitoring the changes in off‐flavour concentrations in different locations of a full‐scale (1 M kg a −1 ) RAS farm rearing rainbow trout ( Oncorhynchus mykiss ). Off‐flavours were measured in fish, in recirculating water and in the inlet water throughout a year. Some of the compounds were introduced to the RAS via inlet water, whereas others were formed at the farm, mostly ranging from 0 to 30 ng L −1 . The concentrations of GSM and MIB were below 20 ng L −1 and in most cases below 10 ng L −1 , whereas methional peaked up to 70 ng L −1 in the fall and winter. In fish, the concentrations mainly remained below 600 ng kg −1 but occasionally MIB peaked up to 1900 ng kg −1 . The results highlight the need for sufficient treatment of inlet water even in the winter to maintain suitable conditions to produce fish of high quality.