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Journal of Experimental Marine Biology and Ecology 247 2000 209–222
www.elsevier.nl locate jembe
Digital electromagnetic telemetry system for studying behaviour of decapod crustaceans
I.P. Smith , K.J. Collins, A.C. Jensen
School of Ocean and Earth Science , University of Southampton, Southampton Oceanography Centre,
European Way , Southampton SO14 3ZH, UK
Received 21 July 1999; received in revised form 2 December 1999; accepted 18 December 1999
Abstract
A telemetry system for studying decapod crustacean behaviour is described which uses low frequency, digitally encoded electromagnetic tags whose signals 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 telemetry.
Digital encoding allows many individuals to be monitored simultaneously and one or more behavioural, physiological or environmental variables to be telemetered. In the present system, tag
signals convey identity and a measure of activity derived from an integral tilt switch. Transloca- tional movements are indicated by detection of tags with different aerials. A central data logger
located on the seabed decodes and records tag signals and environmental measurements. Design life of tags is
. 1 year and the receiving system batteries are replaced by divers at intervals of up to 4 weeks. In field tests, crab Cancer pagurus L. and lobster Homarus gammarus L. activity
was monitored at an artificial reef for 14 months. Examples of the type 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, the need for cables on the seabed and the size and shape of the transmitting tag.
2000 Elsevier
Science B.V. All rights reserved.
Keywords : Activity; Crab; Electromagnetic telemetry; Lobster; Movements; Techniques
1. Introduction
Information about movement and activity patterns of decapod crustaceans is required
Corresponding author. Tel.: 1 44-23-8059-6268; fax: 1 44-23-8059-6642.
E-mail address : philip.smithsoc.soton.ac.uk I.P. Smith
0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 2 2 - 0 9 8 1 0 0 0 0 1 4 9 - 0
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.P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
to understand important aspects of their ecology, such as habitat use, foraging patterns, resource limitation and inter- and intraspecific interactions. In addition, ecological
studies often require population density to be estimated Sutherland, 1996 and variation in activity or spatial distribution of mobile species biases most census techniques, by
altering the proportion of individuals in the sampling area likely to be detected with a given level of sampling effort Greenwood, 1996. This has particular relevance in
commercial fisheries, where the success of management measures depends in part on the reliability of information about the abundance, composition and population dynamics of
the fishable stock Addison and Bannister, 1998. The design and interpretation of population surveys may be improved by an understanding of the determinants of
systematic variation in behavioural aspects of catchability Arnold et al., 1990; Miller, 1990. Unfortunately, gaining this understanding is hindered by the cryptic, often
nocturnal habits of marine decapods and the relative inaccessibility of their habitats, which limit the range of methods available to measure activity on appropriate spatial and
temporal scales.
Movements of semi-terrestrial and freshwater decapods have been studied by radio- tracking e.g. Gherardi et al., 1990; Wolcott, 1995; Barbaresi et al., 1997, but radio
energy is severely attenuated by seawater. Ultrasonic tracking has been used to study a range of crab and lobster species Infraorders Astacidea, Palinura, Brachyura and
Anomura and, in some studies, environmental, physiological, or behavioural data have
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been telemetered reviews: Wolcott, 1995; Friere and Gonzalez Gurriaran, 1998. Movements within a confined area can be recorded in great detail using a fixed
hydrophone array with automated, continuous position fixing Urquhart and Smith, 1992, but the accuracy and precision of this technique are adversely affected by
attenuation and reflection of ultrasonic signals by seabed features Smith et al., 1998. The signal can be lost altogether when the animal moves into a crevice or burrow in the
seabed Chapman et al., 1975; Collins and Jensen, 1992; van der Meeren, 1997. Ultrasonic tracking with a fixed hydrophone array is therefore unsuitable for decapods
inhabiting uneven rocky seabed, particularly when they spend a large proportion of their time within shelters or among dense vegetation.
Electromagnetic tracking relies on magnetic inductive coupling between the coil of a low frequency transmitter and that of the receiving aerial. Electromagnetic signals can be
detected in this way through seawater, rock or sediment and the power requirement of transmitters is relatively low, permitting long transmitter life and extended monitoring
periods. However, the range of detection is much shorter than in acoustic tracking, so the position of transmitters is determined by presence within the detection zone of individual
aerials Wolcott, 1995.
Electromagnetic tracking has previously been used to study movements of spiny and clawed lobsters, initially using portable aerials carried by a boat or diver Ramm, 1980
and later using a grid of loop aerials laid on the seabed Phillips et al., 1984; Jernakoff, 1987a,b; Jernakoff et al., 1987; Jernakoff and Phillips, 1988; Collins et al., 1994. In
each of these systems, only one carrier frequency was used and tags were individually identified by their pulse repetition rate. This limited the number of number of tags that
could be distinguished when received simultaneously by the same aerial. The present paper describes a telemetry system based on digital encoding of electromagnetic signals,
I .P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
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which is potentially capable of tracking many more individuals simultaneously, and which permits additional behavioural, physiological or environmental data to be
conveyed in the tag signals. Information from tracking brown crabs Cancer pagurus L. and lobsters Homarus gammarus L. is presented to illustrate the capabilities of the
system. Its utility, long term reliability and prospects for further development are assessed.
2. Materials and methods
