Directory UMM :Data Elmu:jurnal:J-a:Journal of Experimental Marine Biology and Ecology:Vol247.Issue2.May2000:

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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.: 144-23-8059-6268; fax: 144-23-8059-6642.

E-mail address: philip.smith@soc.soton.ac.uk (I.P. Smith)

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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,

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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 2.1. Study site

Field trials of the telemetry system were conducted at an artificial reef constructed in 1989 in Poole Bay on the south coast of England. The site is 2.2 km from land, with a mean water depth of 12 m and mean tidal range varying from 0.5 m (neaps) to 1.7 m (springs). The reef consists of eight piles of blocks (0.430.230.2 m) made of concrete or cement-stabilised pulverised fuel ash (Collins et al., 1991). The piles (5 m diameter, 1 m high), subsequently referred to as reef units, are arranged in two rows of four, aligned east-west, occupying an area of sedimentary seabed 15335 m.

2.2. Telemetry system 2.2.1. Transmitters

The tags consisted of a transmitting coil (42 mm diameter, primary coil 15 turns, secondary coil 270 turns), driven by a timepiece crystal oscillator, controlled by an individually programmed microcontroller (Microchip PIC16C54LP), with power sup-plied by a 3.6-V, 600 mA h, Saft lithium half AA size battery. Digitally encoded tag signals were transmitted every 30 s on a carrier frequency of 32.67 kHz. Signals consisted of a train of five 1-ms pulses: the first inter-pulse interval (5 ms duration) signified the start of a tag signal, the second and third intervals (6–15 ms in 1-ms increments) indicated the identity of the tag (potentially 1–99) and the fourth interval (6–14 ms in 1-ms increments) indicated activity level (0–8). Activity level was derived from interrogating a tilt switch, incorporated into the tag, at one second intervals. If the

tag had tilted sufficiently (.20 from the horizontal) since the last interrogation, the microcontroller incremented an 8-bit counter by one. At 10-min intervals, activity level was set to the position of the most significant digit of the binary number of tilts and the counter was reset to zero (activity level, number of tilts: 0, 0; 1, 1; 2, 2–3; 3, 4–7; 4, 8–15; 5, 16–31; 6, 32–63; 7, 64–127; 8, 128–255). Anticipated lifetime of the tags was in excess of 1 year, during which time the animal was likely to moult, shedding the exoskeleton and tag.

The microcontroller, oscillator, tilt switch and associated electronic components were mounted on a printed circuit board contained within the transmitting coil, with the battery. This assembly was encapsulated in epoxy resin, giving a disc of diameter 45 mm, depth 13 mm, weight in air 36 g and weight in water 13 g. Later versions incorporated glass fibre filler (microscopic glass bubbles) in the resin, to reduce tag

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density, and had a slightly thicker layer of resin (2 mm) on the upper surface to provide greater protection from abrasion (38 g in air, 10 g in water).

Lobsters (Homarus gammarus, n541, carapace length 72–139 mm) and crabs (Cancer pagurus, n58, carapace width 120–175 mm) were caught in baited pots placed adjacent to each of the reef units and brought to the surface by divers. Transmitting tags were attached to the dorsal surface of the cephalothorax of both species with quick-setting epoxy resin (Devcon 5 min epoxy), after drying the carapace with propanone. The tag was attached with the integral tilt switch level and with its long axis at a horizontal angle of 458 to the animal’s sagittal axis, so that pitch and roll would be detected equally. Divers returned tagged animals to the reef unit where they were caught. Crabs and lobsters were maintained in cool, dark, moist conditions during the tagging procedure, which involved aerial exposure of approximately 10 min. Four lobsters were re-tagged after recapture following tag loss.

2.2.2. Receiving system

Tag signals were detected with 5-m diameter loop aerials laid on the seabed around each of the eight reef units (Fig. 1). The aerials were made of three-core electrical flex with the cores connected to give three turns, and were tuned with capacitors for peak response at 32.7 kHz. Screened coaxial cable connected the aerials to a central three-stage tuned radio frequency receiver (Mariner Radar) via a selector switch (CMOS analog switch). The precise frequency of the tags permitted the use of a very narrow bandwidth receiver. The maximum range of detection was approximately 10 m, but this was reduced to 2–3 m by reducing the gain of the receiver, so that there was only a small overlap in the detection zone of different aerials on the open seabed between reef units.

The receiving system was controlled by a computer (PC / 104 core module, 8088 processor, Ampro Computers Inc.), with control and data input achieved through the parallel port and data recorded on magnetic disk (2.5 inch 400-Mb hard drive). A shift register provided the required number of lines for connecting the receiver to different aerials and for switching power to peripherals. Analog signals from environmental sensors were passed through a 12-bit analog to digital (A-D) converter (Texas Instruments TLV2543). The environmental variables measured were temperature (Na-tional Semiconductor LM35 temperature sensor), light (R.S. Ltd 305-462 general purpose photodiode in a linear photometer FET op-amp circuit), hydrostatic pressure (Sensor Technics SSC3000 temperature compensated silicon stainless steel pressure sensor) and current speed (modified Braystoke current meter with impeller magnet / reed switch pulsing). In addition, surface wave height was estimated four times each day from 2050 readings from the pressure sensor taken at a sampling frequency of 4 Hz.

The system operated on a 10-min cycle controlled by a program written in Turbo Pascal (Borland International Inc.). Power was switched to environmental sensors long enough to record measurements, then the electromagnetic receiver was connected to each of the eight aerials in turn for 1 min. If a stream of five pulses was received with inter-pulse intervals conforming to a valid tag signal, it was decoded to give identity and activity level. Pulse intervals were timed with a counter in the control program calibrated in milliseconds. Valid signals were stored in volatile memory with their associated time

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Fig. 1. Schematic diagram of the electromagnetic telemetry system used at an artificial reef in Poole Bay, southern England: (a) analog aerial selector switch, (b) tuned radio frequency receiver (32.7 kHz), (c) shift register, (d) 12-bit analog to digital converter.

and aerial number, until the end of the 8-min listening period, when they were written to disk. The computer then marked time until the start of the next 10-min cycle. Because tags transmitted every 30 s, they could be detected twice per aerial per listening cycle. Duplicate signals were recorded, but were not included in the analysis, unless a different activity level was transmitted in the second signal.

The computer, receiver, shift register, A-D converter, light and pressure sensors (constituting the ‘data logger’) were contained in a waterproof housing (Seapro, Greenaway Marine Ltd) clamped to a pole driven into the seabed in the centre of the artificial reef. A sachet of silica gel was placed in the housing to minimise humidity. The system was powered by one or two sets of six 12-V sealed lead acid batteries (Yuasa NP15-12), each set in a separate polypropylene waterproof case (modified Pelican 1550, Pelican Products Inc.). The receiving / recording system drew 250 mA at 12 V and two battery cases powered the system for up to 4 weeks. The aerial array, environmental sensors and batteries were connected to the data logger housing with underwater pluggable connectors (Subconn, MacArtney A / S), enabling divers to replace the

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batteries and data logger. On retrieval of the data logger, data were downloaded to a desktop computer for analysis.

The system was deployed from 9 August 1996 to 28 September 1997. From 4 to 30 October 1997, data transmission from the study site to a laboratory 45 km away via a radio data network was tested. The data logger was connected by a RS232 serial cable laid along the seabed to a radio-PAD (packet assembler and disassembler, Paknet CA8001) mounted in a buoy marking a historic shipwreck site 100 m south of the reef (Fig. 1). The telemetry system computer was programmed to switch power to the radio-PAD and transmit compressed sensor data files to another radio-PAD in the laboratory twice per day.

3. Results

3.1. System and tag reliability

The system was operated nearly continuously for 415 days, completing 56 666 operating cycles and recording 493 651 signals, of which 42.9% were accepted, 26.0% were double detections of a tag within a single aerial listening period, 29.6% were from shed tags and 1.4% were from test deployment of tags. During tests with tags placed by divers, the system recorded tag identity and location correctly. Occasionally during the main study period, recorded signals were attributed to tag identities not in use (0.04% of the total number of records). False signals from tags known not to be in the water were easily filtered out. If the incidence of false signals was the same among the tag identities actually in use, then approximately 0.02% of the accepted records may have been spurious. A small proportion of signals with valid identity codes had invalid activity levels (0.04%) and were also easily filtered. The incidence of invalid activity levels was lower among the lightweight tags which had a thicker layer of resin protecting the transmitting coil, than the original version (Non-parametric ANOVA H157.180, P5 0.007).

On two occasions the batteries for the receiving system failed before they were replaced, resulting in a total of 8 days of data loss. Data loss also resulted from damage to the aerial array, thought to have been caused by recreational craft dragging their anchors through the site. The lead to one aerial was severed on 4 November 1996 and on another on 15 February 1997. Fortunately, the tagged animals present on the site during the period of damage occupied other reef units and were therefore detected by other aerials. The aerial array was repaired when weather and underwater conditions permitted on 26 March 1997, but the replacement array was faulty and was itself replaced on 9 May 1997. As a result, no reliable movement or activity data were obtained during the intervening period. An aerial lead was again severed on 22 June 1997 and replaced on 11 July 1997.

Sensor data were transmitted over the Paknet radio-data network from the study site to the laboratory, but some transmissions were unsuccessful. The length of cable required to reach the buoy from the batteries and data logger at the artificial reef led to electrical losses, which impaired the power supply and serial data transmission to the radio-PAD.

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Of the 53 electromagnetic tags used, 21 were functioning after the animal was recaptured, the tag was shed, or the study ended; eight were recovered inoperative from recaptured animals or the seabed; and 24 were lost as a result of the animal eventually moving outside the range of the telemetry system, tag failure, or unreported recapture. Eleven of these lost animals were tagged before the receiving system was operational. Unless the animal or the tag was recovered, it was not possible to determine whether signal loss was due to tag failure or the animal leaving the study site. Among the 29 tags accounted for, 27.6% failed, but the failure rate was lower among the better-protected lightweight tags (11.1%).

3.2. Track durations and signal detection rate

Notwithstanding tag loss or failure, up to 13 individuals were monitored simul-taneously, 65% of tags in use after the receiving system was fully operational provided track durations greater than 15 days (encompassing a spring / neap tidal cycle), the median track duration was 38.7 days, and 15% of tags were monitored for 100 days or more. One individual lobster was monitored for 344 days. Track duration did not differ significantly between crabs and lobsters (Non-parametric two-way ANOVA, H150.467, P50.494), or between the two tag types (H151.954, P50.162). The median detection

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rate was higher for lightweight tags (186.2 signals tag day ) than for the original

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models (141.7 signals tag day , H155.852, P50.016), but did not differ between species (H152.594, P50.107).

3.3. Movement between aerials

Detection of tags with different aerials allowed the movements of individuals around the artificial reef to be plotted (Fig. 2) and the timing of movements to be analysed. The diel distribution of movements was assessed by calculating for each individual the percentage of the total number of movements between reef units by that animal occurring in each hour of the day. In crabs, for example, means of these individual hourly percentages varied significantly with hour of the day (Randomized block ANOVA with crab identity as the blocking variable, F23,6953.952, P,0.0005), with most, but by no means all, movement occurring at night (Fig. 3).

3.4. Activity indicated by tilt switch

Information about body movements indicated by the tilt switch provided a second measure of activity that did not rely on animals moving between reef units (aerials). The distribution of activity levels differed between crabs and lobsters, with crabs actuating the tilt switch less often than lobsters (see Section 2.2.1 for the derivation of activity levels). The majority of crab records indicated no tilts within the preceding 10-min period (Fig. 4). Nevertheless, there was significant diel variation in the mean activity level of crabs (Randomized block ANOVA, F23,7351.780, P50.033), with greatest activity at around sunset and minimum activity at midday (Fig. 5).

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Fig. 2. Schematic diagram of the artificial reef, showing numbers of movements between numbered reef units by a female crab, Cancer pagurus (120 mm carapace width), in the period 15 August to 28 September 1997. The crab spent 77% of the time on reef unit 1, 22% on reef unit 2 and the remainder on units 3, 6, 7 and 8.

Fig. 3. Diel distribution of movements between reef units by crabs (Cancer pagurus). Two daily cycles are shown for clarity; black and white bars indicate night and day, respectively. Hourly means and standard errors of the percentage of the total number of movements by individuals calculated from arc-sine transformed data (n54 individuals tracked in August and September).

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Fig. 4. Mean frequency distribution of activity levels of (a) crabs, Cancer pagurus (n54 individuals, 8913 records), (b) lobsters, Homarus gammarus (n58 individuals, 20 833 records). Means and 95% confidence intervals calculated from arc-sine transformed data.

3.5. Tag effects on behaviour

Aquarium observations of tagged lobsters did not indicate impairment of balance, locomotion, shelter seeking, feeding or ecdysis. The field data did not show a decline in movement or activity over time since tagging that was attributable to a debilitating effect of carrying the tag. Reduction in activity in winter was apparently related to declining water temperatures and, in those animals tracked through to spring or beyond, activity increased again as temperatures rose (Smith et al., 1999). In lobsters, there were no significant interactive effects of tag type and body size (less than or greater than 95 mm carapace length) on either mean daily number of inter-reef unit movements (ANOVA, tag type3size class interaction, F1,2452.435, P50.132) or activity indicated by the tilt switch (F1,2450.031, P50.862). In addition, tag type did not appear to influence the apparent diel pattern of movements (tag type3hour interaction F23,68550.767,

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Fig. 5. Activity of Cancer pagurus in relation to time of day. Two daily cycles are shown for clarity; black and white bars indicate night and day, respectively. Activity level was derived from a tilt switch in the telemetry tag (see Section 2.2.1). Error bars are standard errors calculated from hourly mean activity levels for four to five individuals tracked in August and September.

P50.775) or activity (F23,72750.751, P50.793), nor were there first order effects of tag type on movement (F1,1751.008, P50.330) or activity (F1,1750.712, P50.411). Thirteen tagged lobsters and one tagged crab were recaptured up to five times each in baited traps set at the study site, suggesting that their motivation to seek food and their ability to enter traps was not greatly impaired. Three lobsters were recaptured by commercial fishermen at distances of 3–12 km from the study site and two crabs at distances of 2–41 km, indicating that tagged animals were also capable of long distance movements.

4. Discussion

Digitally encoded electromagnetic telemetry allowed movements and activity of individual crabs and lobsters to be monitored in detail for long periods (.11 months in one case) over a wide range of environmental conditions, generating data that could not be obtained by other techniques. In principle, the main advantages of digital encoding over previous electromagnetic tracking systems are that a greater number of individuals can be monitored simultaneously, information from a sensor on the animal can be transmitted in addition to tag identity, and tag lifetime is greater. In practice, extended tag battery endurance was achieved and transmission of tilt switch data demonstrated the

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potential for telemetry of other variables (Wolcott, 1995). However, the limited abundance of lobsters and crabs in the small artificial reef site and the tendency of some tagged animals eventually to leave the site did not allow us to test adequately the ability to track a large number of individuals at once.

The self-contained nature of the system allows it to be deployed at sites remote from land and avoids the problem of wave damage to cables crossing the shore (Jernakoff, 1987a). The feasibility of radio data transmission from the study site to the laboratory was established, although difficulty was encountered with electrical losses in the cable connecting the data logger at the artificial reef site to the buoy containing the radio-PAD. This would be less of a problem at sites where a dedicated radio buoy could be placed next to the telemetry system. At remote sites, a radio data network, or satellite communication where no network exists (e.g. Eiler, 1995), could be used to receive telemetry data more frequently than with site visits, or could be used simply to indicate the status of the system and the need for intervention.

The main drawback of electromagnetic telemetry is the short range of detection, which means that the position of tagged animals can only be indicated by presence within the reception area of an aerial. This places logistical restrictions on the area that can be monitored with a static aerial array, in terms of the amount of cable required, and imposes a trade-off between size of study area and spatial resolution of tracking. Automated electromagnetic telemetry is therefore suited to species, such as Homarus gammarus (Dybern et al., 1967), that spend prolonged periods within confined areas, or that return predictably to particular locations. By comparison, species with less site fidelity or that undertake seasonal migrations, such as Cancer pagurus (Bennett and Brown, 1983; Hall et al., 1991; Skajaa et al., 1998), are likely to be amenable to electromagnetic telemetry for short periods only.

Larger areas could be monitored by electromagnetic telemetry than in the present study with a greater number of individual aerials (Jernakoff, 1987a), or, in shallow water, by manual tracking with portable aerials (Ramm, 1980), which would be labour intensive and weather-dependent. Using a larger aerial array with a single receiver and digitally encoded tags would involve either a longer listening cycle, which would reduce the temporal resolution of tracking, or a shorter listening period for each aerial combined with an increased tag transmission rate, which would reduce the tag battery life. Since track duration was not limited by tag battery life in most cases in the present study, the latter option may not result in substantial loss of data.

An array of cables laid on the seabed is vulnerable to damage from agents such as anchors, fishing gear and storms. In the present study, the aerials were connected in such a way (for waterproofing) that the entire array had to be replaced when one element was damaged. Use of underwater pluggable connectors on individual aerials would facilitate rapid repair of the array, but would add to the cost of the system.

The size and shape of the present transmitting tag restricts its use to relatively large benthic invertebrates with an exoskeleton. However, there is scope for reducing the size of the tag, so that a greater size range of animals could be tagged. The size of the tag is largely determined by the diameter of the transmitter coil, which was chosen to maximise range of detection. However, in practice, the receiver gain had to be reduced to minimise overlap of the reception areas of adjacent aerials. A smaller, less-optimal,

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coil diameter may therefore be practicable. A smaller tag would also require a smaller battery, implying shorter operational life, but, as indicated above, this may be an acceptable compromise.

As with many telemetry studies (Wolcott, 1995), it was not possible to assess directly whether electromagnetic tags affected the behaviour of study animals, since there was no practicable, less obtrusive, method of observation available. In a shallow, subtropical lagoon, Jernakoff et al. (1987) were able to use diving observations to compare activity of individually marked Panulirus cygnus fitted with an operating electromagnetic tag, a dummy tag or no tag. No effects of the presence of the tag or the transmitted signal were evident, although only 18% of marked lobsters were resighted. In the present study, the limited abundance of lobsters and crabs at the study site precluded such an experiment. Nevertheless, preliminary inferences can be made about the possible effects of tags (Wilson et al., 1986). The extra loading from the tag presumably increased the energy cost of locomotion, but aquarium observations, field telemetry data and recaptures of lobsters and crabs in baited traps at the study site and elsewhere suggested that their mobility and important aspects of their routine behaviour were not seriously impaired. The first model of tag weighed 30% more in water than the later version. The absence of interactive effects of tag weight with lobster size or with environmental variables of interest, and the absence of first order effects of tag weight, suggest that the observed patterns of movement and activity were not artefacts of the observation technique (Peterson and Black, 1994). However, controlled experiments in conditions where tagged and untagged individuals can be monitored (e.g. in an aquarium) are desirable to test the effects of tags on mobility, energy expenditure and aspects of behaviour, in relation to body size (e.g. Newland and Chapman, 1993).

Acknowledgements

This study was funded by the UK Ministry of Agriculture, Fisheries and Food. We are grateful for support and advice from R.C.A. Bannister and J.T. Addison (CEFAS Lowestoft). F. Elston, M. Markey, J. Mallinson, E. Mattey and volunteer divers gave valuable fieldwork assistance. J. French and M. Wilkin helped with telemetry hardware and software design, respectively. [AU]

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