Methods that can be used to check the health of fish, determine their physiological condition and detect abnormalities at an early stage, have been attracting recent attention. The health of fish can be determined by the assessment of stress responses, prediction of spawning time, and highly sensitive detection of bacteria that cause diseases in fish, etc. However, it can be difficult to accurately measure these parameters due to factors such as sampling errors, uncontrollable aquaculture conditions and unsound methodological approaches. Also, the methods currently used are time-consuming and may involve complicated processes. Thus, there is a high demand for novel approaches for the assessment of fish health. We have thus focused on the development of a biosensor to measure target factors of fish health by combining biocatalytics and electronics technology. We believe that this technology can greatly improve the assessment of the health of fish produced for consumption. In this article, we described various biosensors that can both rapidly and easily measure indicators of fish health. Future applications of these techniques are also discussed.

We have developed a wireless biosensor system to continuously monitor L-lactic acid concentrations in fish. The blood L-lactic acid level of fish is a barometer of stress. The biosensor comprised Pt-Ir wire (φ0.178 mm) as the working electrode and Ag/AgCl paste as the reference electrode. Lactate oxidase was immobilized on the working electrode using glutaraldehyde. The sensor calibration was linear and good correlated with L-lactic acid levels (R = 0.9959) in the range of 0.04 to 6.0 mg·dL⁻¹. We used the eyeball interstitial sclera fluid (EISF) as the site of sensor implantation. The blood L-lactic acid levels correlated closely with the EISF L-lactic acid levels in the range of 3 to 13 mg·dL⁻¹ (R = 0.8173, n = 26). Wireless monitoring of L-lactic acid was performed using the sensor system in free-swimming fish in an aquarium. The sensor response was stable for over 60 h. Thus, our biosensor provided a rapid and convenient method for real-time monitoring of L-lactic acid levels in fish.