BTC - Technical Paper

TECHNICAL PAPER

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Power. The ARS units were in a relatively remote location and had to run maintenance free for less than 1 month. This required a renewable energy source that would function reliably during the approximately 8 months of the year (Mar-Oct) when tortoises were not hibernating. To accommodate cloudy periods of up to 7 days and seasonal changes in solar radiation, we chose a solar-rechargeable battery (ExideTM [Exide Corp., Bloomfield Hills, Mich.] 12 volt, vented, leadacid, 18 amp-hr [Ah] at 10-hr rate). To enable recharging of the battery on a late autumn or early spring day with average sunlight, each ARS used either 4 0.5-amp photovoltaic (solar) panels (Edmund Scientific model 35,438 [Edmund Sci. Co., Barrington, NJJ) or 1 2-amp unit (Siemens self-regulating model M-22 [Siemens Solar Industries, Camarillo, Calif.]). To reduce energy needs and because tortoises move slowly, we programmed the transceiver's duty cycle for 80 ms on and 100 ins off. We tested this duty cycle with captive tortoises, adjusting it to provide the minimum time and repetition interval required to read a tortoise passing the reading coil at maximum speed. Desert tortoises are largely diurnal; therefore, we also programmed the unit to turn on about 30 min before sunrise and off 30 min after sunset.

Fig. 1. Reading coils for detecting passage of desert tortoises bearing passive integrated transponder tags were placed at the distal ends of the aprons at each end of 2 storm drain culverts.

Length of reading coil. Entrances to the culverts were 1.6 m in diameter with a metal apron extending out to a length of 2.4 in and 2.4 in wide (Fig. 1). Because the reading coil had to be flush with the ground and the metal from the apron would alter the E-M field, thereby reducing the reading distance, we placed the reading coil at the distal edge of the apron. Thus, the reader had to cover an area 2.4-m wide. Because of short-distance limitations of the RF-ID technology, we designed the reading coil to cover the entire 2.4-m width.

Reading distance. A basic technological challenge was to ensure a PIT tag could be read from an optimal distance. An independent evaluation of commercially available PIT tags indicated the mean maximum reading distances for small PIT tags (10- 14 mm) ranged from 26 (SD=1) to 107 (SD=4) mm using standard handheld readers (Anonymous 1991). To ensure that our largest tortoises would be detected when they crossed our reading coils, a reading distance of 75 mm over the entire 2.4-m length was required.

Also, reading distance was affected by the PIT tag's position relative to the reading coil's E-M field (F=48.48; 1, 42 df; P < 0.0001; Table 1; see also Camper and Dixon 1988). Vertical placement (perpendicular to the axis of the coil) of the cylindrical tags and horizontal placement (parallel to the axis of the coil) of the disc tags, with respect to the ground, provided the best responses (for position X size interaction in 2-factor ANOVA: F=3.24; 2, 42 df, P 0.049; Table 1).

	Read Distance (mm)

	14-mm tag		18-mm cyl tag		18-mm disk tag

Date of test	Hor	Vert	Hor	Vert	Hor	Vert

9 Apr 1994	15	98	30	150	180	140
10 Apr 1994	20	50	30	150	160	90
11 Apr 1994	15	50	50	140	190	150
12 Apr 1994	70	80	120	140	220	95
13 Apr 1994	40	75	105	115	190	90
15 Apr 1994	70	60	140	135	240	75
16 Apr 1994	50	70	100	130	220	160
17 Apr 1994	15	50	50	140	190	150
Mean	36.9	66.6	78.1	137.5	199.8	118.8
SE	8.55	6.15	15.23	4.01	9.15	12.13

Table 1. Read distance of 3 types of passive integrated transponder tags (PIT) held horizontally and vertically with respect to the reader coil.

Burying the reading coil reduced reading distance because of absorption of the E-M field by minerals in the soil. To solve this problem we surrounded the reading coil with closed-cell foam. The reading coil dimensions were about 230 X 15 x 2.5 cm high. The surrounding structure was 240 x 46 x 13 cm.

Use of culverts.  To determine travel direction and duration, 2 automated reading systems per culvert were deployed, I on each end (Fig. 1). The direction of motion was determined by comparing the calibrated time stamp of the reading at each culvert.

Security from environmental hazards.  The electronic components of the ARS required special attention to prevent damage from environmental hazards. The primary hazards in the area were mild flash floods, high summer temperatures (often >40 degrees C), and contamination by dirt and dust.

The reading coil housing was constructed like a surfboard, with coil and insulating foam covered by layers of fiberglass cloth and epoxy resin. A cavity was inlaid into the top of the structure to receive the reading cod. After placement of the reading coil, it was covered with an additional plate of fiberglass epoxy and sealed with silicone rubber to prevent water from leaking in.

The electronic equipment (reader board, battery, control panel, and datalogger) were enclosed in a locking, compression-molded, fiberglass box (Rose Enclosures Model 3040 [Rose Enclosures Sys., Inc., Frederick, Md]) measuring 40 X 30 X 20 cm. To further protect the equipment from dirt when the box was opened for access, the box was placed in an irrigation-control box. The nested boxes were then placed in the ground, flush with the surface, to reduce damage from heat, theft, or vandalism (Fig. 2).

Security from human-caused hazards.  The equipment was placed along a heavily traveled California State Highway (average daily traffic=8,500 vehicles; Calif. Dep. of Trans. 1993), and our surveys of culverts for animal tracks revealed occasional use of culverts by people. Therefore, we had to protect all equipment from damage, vandalism, and theft. The surfboard technology used to make the reading coil housing provided sufficient strength. The top surface was finished with a fine coating of glue and desert soil taken from the location where the readers were installed. This camouflaged the unit and provided a rough surface, which kept a thin layer of loose soil on top of the structure.

To protect the reader and data logger from theft and vandalism, we buried the system flush with the surface and covered the lid with 10-15 mm of loose soil (Fig. 2). Wires were buried out of sight and both boxes were locked.

The solar panels could not be hidden. The first one was placed on top of a steel fence post (3 in high, 37 mm diameter). It was stolen within 1 month. We placed the new ones on a 6.4-m high, 130-mm diameter, heavy gauge pole padlocked onto another pole that was sunk 1.5-2 in into concrete in the ground (Fig. 2). One such panel was stolen 9 months later. That one was replaced with an 8.4-m long pole of similar material sunk directly into concrete 2 in deep; no further losses have occurred.

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