A study on endangered giant catfish has shown that transmitter tags must be embedded in the body to prevent them being dislodged.
Giant pangasius, Pangasius (formerly Pangasianodon) gigas, are endemic to the Mekong basin and are on the brink of extinction due to decades of overfishing.
In order to monitor the wild stocks and the new fish being introduced to rivers through captive breeding programmes, it's necessary for biologists to be able to track their movements accurately using biotelemetry.
But large catfish have a tendency to dislogde the acoustic tags that transmit and receive signals, so tracking the endangered fish is somewhat problematic.
To determine the best method of attaching the tags, a team of scientists from Kyoto University studied externally attached tags and surgically implanted tags on farmed Pangasius gigas kept in earth ponds.
The methods of attachment didn't cause serious harm to the fish and they didn't contract fungal infections, but the catfish managed to dislodge all of the tags that were attached to their skin.
Those tags that were deeply embedded inside the peritoneal cavity of the fish couldn't be dislodged by the fish, so they continued to work throughout the trial period.
For more details on the experiment see the paper: Mitamura, H Mitsunaga Y, Arai N and T Viputhanumas (2006) - Comparison of two methods of attaching telemetry transmitters to the Mekong catfish, Pangasianodon gigas. Zoological Science, 2006 Mar; 23(3):235-8.