Electromagnetic tracking of lobsters on an artificial reef

Abstract

Lobster (Homarus gammarus) behaviour on an experimental reef in Poole Bay, UK has been studied since its construction in 1989. The use of acoustic methods, which are widely used for marine telemetry, are limited by reflections and attenuation of the signal when the lobsters are within passages in the reef. Electromagnetic/inductive coupling telemetry has a limited range but is not so severely attenuated by rock or concrete. A tag attached to the animal emitted 3 ms pulses at 32.7 kHz through a 4 cm diameter coil. The pulses were received by 5 m diameter loop aerials laid on the seabed around each reef unit and joined to a central receiver and data logger on the seabed. Tagged animals were continuously in range within the loop and 3-4 m outside of it. The aerials were sequentially connected to a 3 stage TRF receiver tuned to 32.7 kHz with a BFO to convert to base band output. The output bursts were converted to unipolar pulses which were recorded on a data logger. Animals were identified by the individual tag pulse rate (1-0.5 Hz). The data logger was removed and replaced by divers. The system was deployed on the artificial reef, monitoring the movement of 4 lobsters continuously for 2 months.