Abstract
Translocated saltwater crocodiles (Crocodylus porosus) in the Northern Territory (NT) of Australia often return to their original capture sites, which complicates management interventions aimed at reducing human-crocodile conflict. We examined the spatial events implicated in this homing ability, using ARGOS satellite tracking devices. Five large male C. porosus (3.03 m to 4.02 m TL) were shifted and released 100–320 km from their capture sites, and 3 additional ones (3.67 m to 4.23 m TL) were released at their site of capture as controls. Translocated crocodiles were more mobile than the controls, and moved at sea in the direction of their original capture site. However, they were unable or unwilling to swim around a geographic structure, Cobourg Peninsula, which prevented homing being achieved in all five cases. Two control crocodiles remained near their capture sites, but one, after the first year, made a 900km journey for six months, before returning to its original capture and release site. Genetic analysis of tissue samples from nests across the NT coast demonstrated significant genetic structure across the coast, and confirmed that Cobourg Peninsula contributes to genetic differentiation among populations along the NT coast. These results provide new insights into C. porosus movements, which have management significance for the maintenance of public safety.
Highlights
Crocodiles 60682 (3.85 m total length (TL); 268 days of tracking) and 60683 (3.67 m TL; 48 days of tracking) remained in and around their capture site, and when they did venture to the coast, they stayed within 15 km of the mouth of the Mary River and showed no strong directional movement
Unlike the Northern Territory (NT) crocodiles we studied, a 4.5-m C. porosus caught on the western side of Cape York Peninsula, and released on the eastern side, stayed near the release site for three months returned 411 km to its capture site within a month, possibly exploiting local surface currents around Cape York [12,13]
Cobourg Peninsula was not crossed by any of the five translocated crocodiles in our study, nor by the three control crocodiles released at the capture site
Summary
On the western side of the Cobourg Peninsula, five crocodiles were caught in the Mary River, and either released near their site of capture (N = 3), transported 100 km further west and released at sea (N = 1), or moved 320 km to the Barriers to crocodile movement and dispersal eastern side of Cobourg Peninsula and released in the Blyth River (N = 2) (Fig 1). Crocodile 60686 (3.81 m TL; 396 days tracking) was translocated from the Blyth River on the eastern side of Cobourg Peninsula, 320 km west to the Mary River (Fig 4).
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