Digital electromagnetic telemetry system for studying behaviour of decapodcrustaceans

A telemetry system for studying decapod crustacean behaviour is described w hich uses low frequency, digitally encoded electromagnetic tags whose signa ls are detected with a grid of loop aerials on the seabed. Electromagnetic telemetry can be used to study short range movements of cryptic animals in topographically complex habitats that are not amenable to ultrasonic teleme try. Digital encoding allows many individuals to be monitored simultaneousl y and one or more behavioural, physiological or environmental variables to be telemetered. In the present system, tag signals convey identity and a me asure of activity derived from an integral tilt switch. Translocational mov ements are indicated by detection of tags with different aerials. A central data logger located on the seabed decodes and records tag signals and envi ronmental measurements. Design life of tags is > 1 year and the receiving s ystem batteries are replaced by divers at intervals of up to 4 weeks. In fi eld tests, crab (Cancer pagurus L.) and lobster (Homarus gammarus (L.)) act ivity was monitored at an artificial reef for 14 months. Examples of the ty pe of information acquired are presented to illustrate the capabilities of the system and potential applications are discussed. Limitations of digital electromagnetic telemetry stem mainly from the short range of detection, t he need for cables on the seabed and the size and shape of the transmitting tag. (C) 2000 Elsevier Science B.V. All rights reserved.