Playing tag with the big fish

While the movement of some large animals which surface regularly - many sharks and New Zealand's striped marlin - can be tracked directly through satellite telemetry, most fish do not rise to the surface of the water often enough for this method to keep track of them. A more innovative approach is needed. One alternative is archival tags, particularly suited to large fish species like great white sharks or tuna.

The technology is based on a small computer chip in which a highly accurate clock is combined with external sensors which record temperature, depth and light level every 60 seconds. Thousands of pieces of this information, data, is stored (or archived) for up to several years.

Some archival tags are implanted inside a fish and the data recovered when the fish is recaptured. Another kind is attached to the outside of the fish and programmed to automatically 'pop-off' and float to the surface where it makes radio contact with a satellite and downloads its data via the satellite to the desktops of the programme's researchers. This method is particularly useful with fish like great white sharks or stingrays that are unlikely to be caught again.

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