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
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 0.4 3 0.2 3 0.2 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 15 3 35 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
o
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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.P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
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, n 5 41, carapace length 72–139 mm and crabs
Cancer pagurus, n 5 8, 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 45
8 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
I .P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
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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
