Today, some 45% of fish consumed by humans, 48 millions tonnes in all, is raised on fish farms. The actual relating European market, of which Norway is the leader, produces 1.3 millions of tonnes of fish farming products every year, which represents an approximate value of 3 billion Euros. Due to the highly competitive market the aquaculture business is confronted with the challenge to increase its productivity. The accurate measurement and control of CO2 can contribute to sustain the health of fish and in consequence guarantee the productivity of fish farms and the quality of human food. There are actually no on-line measurement systems available (titration tests and electro-chemical sensors), which are able to determine the CO2 concentration accurately enough over time for an efficient control of the CO2 concentration in intensive cultures. Furthermore there are still no reliable sensors on the market, which can be used for the measurement of dissolved CO2 in oceanography, but they are strongly requested by researchers for stationary and mobile applications. The objective of the project OptoCO2Fish is to develop a CO2 sensor to meet these demands. We intend to develop a novel solid state opto-chemical CO2 sensor system based on the principle of Resonance Energy Transfer. This principle offers the possibility to generate a compact and cost effective sensor system and furthermore is able to meet the essential requirements of fish farming staff, which is high accuracy and stability combined with low maintenance and low re-calibration effort. Titration tests and electro-chemical sensors together dominates the market. This one is currently led by American companies. This project can enhance the competitiveness of European companies, strengthen the position of aquaculture in Europe against the worldwide competition and can contribute a reliable sensor system for the research on effects of global warming due to CO2 emission.