Three members of AquaCircle, has developed the smallest oxygen sensor for biotelemetry that is commercially available.
LoligoSystems of Denmark now offer a new miniature acoustic dissolved oxygen transmitter for fish telemetry or any other application with a need for limited size, low maintenance and long battery life.

It is the smallest oxygen sensor (Ø13 x 42 mm) for biotelemetry that is commercially available, and has been developed in cooperation with Norwegian Thelma, Danish Oxyguard International and the Danish Institute for Fisheries Research.
The acoustic O2 transmitter measures 0% to 200% dissolved oxygen saturation in fresh and sea water. The tag may be delivered with an external oxygen sensor for independent use or with a build-in oxygen sensor in a more compact design.
More details at the homepage of LoligoSystem and in the scientific paper: Svendsen, J., K. Aarestrup, J.F. Steffensen & J. Herskin (2006). Use of a Novel Acoustic Dissolved Oxygen Transmitter for Fish Telemetry. Marine Technology Society Journal, 40 (1), 103-108.