4.2.2.1 Transmitter attachment
Small, single-stage radio transmitters (20 mm x 10 mm, with a 150 mm whip antenna; Sirtrack Ltd., New Zealand) were attached to 20 H. duvaucelii (10 geckos per island) for this study. Ten individuals from each island carried transmitters immediately following release. Transmitters were replaced if batteries were reaching the end of their life (within 20% of the total battery life or at 83 days). Transmitters were attached to all re-captured animals throughout the study period, except animals with reduced body condition or signs of previous backpack abrasion.
Transmitters were attached to geckos using a backpack harness design (Plate 4.1) modified from Fisher (1995) and Richmond (1998). At first, rubber tubing was used for creating the harnesses however, later in the study the rubber was replaced with the more flexible Co-Flex®
material. Captive transmitter attachment trials, prior to release, indicated that geckos moved freely and normally while wearing backpack harnesses.
Harnesses were cut out from the Co-Flex® sheet using a cardboard template. The harness consisted of two shoulder straps branching from a central pad. A rubber disc was glued onto the central pad to provide a smoother attachment surface for the transmitter. The transmitters were positioned with the antenna pointing posterior towards the tail and fixed with cyanoacrylate glue (SuperglueTM). The complete harness including transmitter weighed approximately 3.5 g and accounted for less than 5% of H. duvaucelii body weight.
The harness was placed across the individual’s shoulders with shoulder straps pointing anterior along each side of the head. The straps were wrapped over the shoulders, across the chest in a ‘X’ pattern, and then back up behind the opposite foreleg. The ends were subsequently attached to the dorsal surface of the transmitter with glue and excess strap material was cut off. The straps crossing the chest were glued together to reduce movement. A further two straps were applied diagonally over the top of the transmitter and crossing the chest to add further rigidity. The backpack harnesses were fitted neatly but did not constrict the chest and interfere with breathing (Plate 4.3). Care was taken to avoid glue coming in to contact with the individual’s skin and the procedure was performed in open air to avoid excessive inhalation of vapours that could be harmful to the animals.
Plate 4.2 Backpack harness made from Co-Flex® material, with a radio transmitter attached and ready for mounting (left) and mounted on a gecko (Hoplodactylus duvaucelii) (right).
Plate 4.3 Adult female Hoplodactylus duvaucelii shown with a Co-Flex® transmitter harness attached. (Photograph by author).
4.2.2.2 Movements and activity
4.2.2.2.1 Radio-tracking
Radio-tracking was conducted using a hand-held R-1000 telemetry receiver (Communication Specialists Inc.) and a 3-Element Yagi aerial tuned to 160MHz operating frequency (Sirtrack, Havelock North, New Zealand). Radio-tracking was conducted between December 2006 and February 2008.
Post-release telemetry monitoring occurred in three stages following release. These included:
1) Intensive monitoring (first week): Geckos were located every two hours throughout the
night (3-4 fixes per night) and once during the day. This continued for five days on Motuora but only two days on Tiritiri Matangi due to the logistics of concurrent releases. Geckos were located using both triangulation and homing-in methods (both described in sections 4.2.2.2.2 and 4.2.2.2.3).
2) Weekly monitoring (two months following stage one): Each gecko was located during
the day, twice a week. This continued for approximately two months following release and allowed for both islands to be monitoring concurrently. Efforts were made to visually locate geckos during this stage.
3) Monthly monitoring (following stage 2): After two months, the monitoring regime
changed and daily locations were recorded for up to five days each month. New transmitters were attached to recaptured individuals and morphometric measurements were recorded.
4.2.2.2.2 Triangulation
Triangulation was used to determine the geckos’ positions during the initial dispersal phase to minimise any disturbance to their behaviour. This involved recording the distances and compass bearings from multiple point-locations (i.e. monitoring grid reference points, see Chapter 5, section 5.2.2.1) to each radio-transmittered gecko. More point-locations results in a reduction in size of the error polygon and a higher precision in determining the position of a gecko. In the present study, a minimum of three point-locations were used to calculate the estimated position of each gecko. If the gecko was sighted during the triangulation procedure the position was recorded and triangulation abandoned. Individual gecko locations were recorded on GPS, to an accuracy of five metres.
4.2.2.2.3 Homing-in
This method involves ‘homing-in’ on a signal until the animal’s location is determined. The starting point for each animal was at the site where the gecko was last seen. The antenna was moved from side to side to determine which direction the strongest signal was coming from (i.e. the loudest signal pulse). The size of the movements was decreased while moving in the direction of the strongest signal and slowly decreasing the gain and volume to increase the precision of location. Individual gecko locations were recorded on GPS, to an accuracy of five metres.
4.2.2.3 Morphometric measurement
For details on the procedures for obtaining morphometric measurements, refer to Chapter 2 (section 2.4.1).