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.P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
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 H
5 7.180, P 5
1
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.
I .P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
215
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, H
5 0.467,
1
P 5 0.494, or between the two tag types H 5 1.954, P 5 0.162. The median detection
1 21
21
rate was higher for lightweight tags 186.2 signals tag day
than for the original
21 21
models 141.7 signals tag day
, H 5 5.852, P 5 0.016, but did not differ between
1
species H 5 2.594, P 5 0.107.
1
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, F
5 3.952, P , 0.0005, with
23,69
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, F
5 1.780, P 5 0.033, with greatest
23,73
activity at around sunset and minimum activity at midday Fig. 5.
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.P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
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 n
5 4 individuals tracked in August and September.
I .P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
217
Fig. 4. Mean frequency distribution of activity levels of a crabs, Cancer pagurus n 5 4 individuals, 8913
records, b lobsters, Homarus gammarus n 5 8 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 type
3 size class interaction, F 5 2.435, P 5 0.132 or activity indicated by the
1,24
tilt switch F 5 0.031, P 5 0.862. In addition, tag type did not appear to influence
1,24
the apparent diel pattern of movements tag type 3 hour interaction F
5 0.767,
23,685
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.P. Smith et al. J. Exp. Mar. Biol. Ecol. 247 2000 209 –222
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.
P 5 0.775 or activity F
5 0.751, P 5 0.793, nor were there first order effects of
23,727
tag type on movement F 5 1.008, P 5 0.330 or activity F
5 0.712, P 5 0.411.
1,17 1,17
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
