Understanding the movement and dispersal of saltwater crocodiles (Crocodylus porosus) within and around Australia

Abstract

Human-crocodile conflict (HCC) is becoming a conservation challenge worldwide. The saltwater crocodile (Crocodylus porosus) is the largest and most aggressive living crocodilian species, being responsible for increasing attacks on people in many countries. This species is highly adapted to both freshwater and saline environments and is widely distributed in the Indo-Pacific region. However, their complex movement and dispersal patterns remain largely unknown. In the first chapter, we examined the spatial events implicated in the homing ability of crocodiles that complicate management interventions aimed at reducing HCC. Five large male crocodiles were shifted and released 100-320 km from their capture sites, and three additional ones were released at their capture sites 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, the Cobourg Peninsula, which prevented homing from being achieved in all five cases. Genetic analysis of tissue samples demonstrated significant genetic structure across the coast and confirmed that the Cobourg Peninsula contributes to genetic differentiation among populations along the coast. The second chapter demonstrated environmental influences on crocodiles' dispersal, which comprised emigration, movement and settlement. We found that both environmental resistance and properties of the source and destination catchments were important factors influencing observed dispersal events. Competition for habitat influences emigration and settlement choices, and environmental resistance to movement occurred, in which high-quality habitat was associated with the greatest environmental permeability. Approximately 42% of crocodiles were migrants from populations other than their sampling locations, and some outstandingly productive populations, such as the Goyder River (Arafura Swamp), had a much higher proportion of emigration than immigration. The distance most commonly travelled between source and destination was 150-200 km, although a few travelled much longer (600-700 km). The third chapter identified the natal origins of 95 crocodiles caught as problem animals in Darwin Harbour, where habitats suitable for breeding are extremely limited. Population assignments showed that approximately 50% of the crocodiles originated from the Adelaide and Mary Rivers and Kakadu region, 30% from the Daly, Finniss and Reynolds Rivers, and 15% from Tiwi Islands. Attributes such as their total length (most commonly 150-180 cm), sex (75% male), and the year of capture (2015-2017) were not significantly correlated to the distance that they travelled to the harbour. Given that most crocodiles found in Darwin Harbour are migrants from multiple sources, we argue that adaptive measures such as removing crocodiles in the defined management zones would be more efficient at reducing risk to public safety than more proactive approaches, such as interfering with targetted, distant sources. The last chapter measured the population structure of crocodiles in Australia and its neighbouring countries. The analyses showed that populations can be separated at the broadest level into Oceania (Australia and New Guinea) and Southeast Asia (Borneo, Java, Peninsular Malaysia, Mindanao, and Sumatra), broadly aligned with Sunda and Sahul shelves of the Last Glacial Maximum. We found no evidence for gene flow between Australia and Timor-Leste, which has been posited as a possible explanation for the apparently increasing crocodile attacks in Timor-Leste. For a more comprehensive mapping of the genetic connectivity across the range, more extensive sampling will be required.r The findings from this study shed light on a better understanding of the movement of crocodiles and provided implications for HCC management in Australia and other countries. Show more