Over the past few years, scientists from NIWA and the University of Tokyo have tried tracking New Zealand longfin eels to their spawning grounds, using a system of electronic tags. The tags record and store environmental information along the journey. Data recorded includes water temperature and day-length figures, and the times of sunrise and sunset; the latter can be used to calculate the latitude and longitude of an eel each day and map the track of the migrating fish.

Tags are attached to the eels using a tether. At a specified date and time, the tag actively corrodes the pin to which the tether is attached, releasing the tag from the animal. The tag then floats to the surface and transmits summarised information to an Argos satellite. The Argos system also uses the transmitted messages to provide the position of the tag at the time of release. The transmitted data are forwarded on to the researcher.

Each tag has a small, pressure sensitive guillotine, so if an eel dies and begins to sink, the tag will be automatically cut adrift at the 1,800m mark. As the tags are comparatively large (75g, 175mm long), they are attached only to large longfin females (8-11kg). The eels used in the study were caught by commercial fishers in Te Waihora (Lake Ellesmere) near Christchurch and acclimatised in sea water before being released.