Translocation of desert tortoises Gopherus agassizii to the Large-Scale Translocation Study site, Nevada, USA

Background

The Mojave Desert population of the desert tortoise Gopherus agassizii north and west of the Colorado River (USA) is considered threatened. Many desert tortoises were held captive at the Desert Tortoise Conservation Center (DTCC) in Las Vegas, Nevada, after removal from land undergoing urban development. In this study, translocations were undertaken to assess whether these tortoises could be used to bolster a wild population in an area where the species was thought to occur at low density due to vehicle-caused mortality; fencing was installed prior to the translocation to prevent this in future.

From previous translocations, a potential problem identified was that captive and free-ranging animals differ considerably in their access to water. Infrequent rainfall in the Mojave Desert allows few opportunities to drink, whereas tortoises at the DTCC had daily access to water. Captive tortoises conditioned to plentiful water may therefore experience functional drought conditions upon release to the wild. During this translocation therefore, the effects of discontinuing water supplementation in autumn prior to spring release on subsequent survival was tested.

Action

Study area: The 90 km² translocation site, the ‘Large-Scale Translocation Study’ (LSTS) site, is located in southern Nevada, southwest USA. The wild tortoise density was estimated at 15–20/km²;  likely to be low due to mortality inflicted by vehicles on an adjacent highway.

Release animals and preconditioning: Tortoises selected for release comprised 32 adults kept in outdoor pens for 7 yr and 10 juveniles held at the DTCC for 2 yr. They were housed in four pens (15.2 m × 15.2 m) with artificial burrows for preconditioning before translocation. Two pens received water daily for 15 min (07:45 to 08:00 h) via sprinklers beginning 28 March 1997; saucers were placed to collect water for the tortoises to drink. These tortoises are referred to as ‘water-supplemented’ (WS). Two pens received no water i.e. not supplemented (NS).

Releases: Thirteen burrows (0.3 m long, 19–49 m apart) were dug and their position recorded using a Global Positioning System (GPS) unit. All tortoises (each fitted with a radio transmitter) were given access to water in their plastic transportation tubs for 20 min prior to release into the burrows. A total of 28 tortoises were released betwe 23 April to 23 May 1997: six females, eight males, and one juvenile from the WS group; seven females, five males, and one juvenile from the NS group. Their behaviour was observed for about 4 h on the days of release, recording items eaten and distance moved. High temperatures prevented releases of 14 tortoises planned for 6–19 May.

Weight and carapace length were recorded on day of release, 15 days after release, and once a month thereafter.

Tortoise movements: Tortoises were located as best as possible by radio-tracking through July 1997 to November 1998.

Consequences

Despite harsh conditions, most of the translocated tortoises adapted to life in the wild. Mortality was 21% (6 of 28 released) in 1997. Data suggests that drought rather than the translocation itself negatively affected the tortoises but mortality was not significantly between WS and NS groups (female deaths: NS 2, WS 3; males deaths: WS 1).

None of the tortoises died during the second season after release. Water supplementation was correlated with high rates of carapace growth and greater movements by males after release. The authors consider that initial success of this translocation ( over 2 years) demonstrates potential for longer-term success.

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.sciencedirect.com/science/journal/00063207