Meeting the global demand for marine resources requires high efficiency in the aquaculture industry. This makes it necessary to monitor the stress level of cultured fish and control their health and breeding environment. Blood glucose level has been reported to increase with increasing stress level in cultured fish, and wearable glucose sensors have been used for fish, which can continuously monitor their blood glucose level. Here, we demonstrate the construction of a self-powered biosensor without a power supply. This biosensor operates through the generation of electricity during the glucose oxidation reaction at the anode and the simultaneous O2 reduction reaction at the cathode, with the connection of the anode and cathode in the same circuit. The prepared anode exhibits a response that depends on the glucose concentration, and the cathode detects O2 reduction reaction. The developed biosensor detected current responses when 3 mM glucose was added to black sea bream. Because the self-powered biosensor does not require a power supply, the overall system is smaller than the conventional biosensor system. The developed biosensor is expected to serve as a continuous and wearable monitoring device for cultured fish.