Population ecology of Crocodylus porosus (Schneider 1801) in the Kimberley region of Western Australia

Crocodylus porosus is the most widely distributed crocodilian species and suffered widespread population decline during the 20th century, principally due to commercial hunting for their hides, but also from habitat destruction and persecution. An estimated 270,000 to 330,000 animals were killed in Australia before the species was protected in the early 1970s. Populations of C. porosus in Western Australia are poorly studied and the number of animals taken during the hunting era is unknown. However, by 1970, hunting was no longer considered to be commercially viable. Over the last two decades, adding commercial value to crocodiles through sustainable use has been an integral part of the management strategy throughout Australia. Despite this, demographic parameters remain unquantified for most life-history stages. This study attempts to provide quantified demographic information for some populations of C. porosus in Western Australia. Patterns of movement are a fundamental component to the dynamics of animal populations. VHF radio tags were attached to 16 estuarine crocodiles, which were tracked between October 2001 and May 2003. Male (n = 12) and female (n = 4) crocodiles exhibited distinctly different patterns of movement. Females occupied a small core linear range (1.3 ± 0.9 km) on the main river channel during the dry season and moved distances of up to 62 km to nesting habitat during the wet season, returning to the same core area the following dry season. Males moved considerable distances along the Ord River throughout the year. The largest range recorded was 87 km for a 2.5 m juvenile male, which had not stabilized. However, male ranges did not appear to be related to body size, with the largest two ranges recorded for the smallest (2.5 m) and largest (4.3 m) males tagged. Rates of male movement did not differ significantly between three size classes of males but there were significant seasonal differences, with the highest mean rates of movement occurring during the summer wet season (4.0 ± 5.4 km/d). The highest rate of sustained movement was 9.8 km/d for a translocated 2.6 m juvenile male, which travelled 118 km in 12 days to return to the area of its capture. Neither males nor females showed exclusive habitat preferences for any of four broad riverine habitats identified on the Ord River. Males had substantial range overlaps with no obvious spatial partitioning, suggesting territoriality is not an important behavioural characteristic of free-ranging male crocodiles along the Ord River. Rates of migration were also examined indirectly using genetic data, which integrates patterns of movement at the population level over many generations. One hundred and twenty three tissue samples were collected from three river systems between April 2001 and September 2002. Levels of genetic diversity and structure were quantified at nine microsatellite loci. Genetic data indicated that C. porosus shows strong site fidelity. However, indirect estimates of migration from fixation indices suggest gene flow is sufficient to maintain genetic diversity and population substructure but not so low as to cause inbreeding. Genetic diversity was similar in all three populations examined with allelic richness ranging from 4.6 to 5.0 alleles per locus and mean observed heterozygosity ranging from 0.63 to 0.74. Inbreeding coefficients indicated there was only moderate differentiation among the populations (FST = 0.08, RST = 0.06) but differences in allele frequencies were highly significant. Assignment tests designated 80% of individuals to their population of origin and identified only five individuals (4%) as first generation migrants. Populations that experience a severe and rapid reduction in size, such as occurred with C. porosus following the Second World War, are susceptible to losing a significant component of their genetic variation. There was compelling evidence for a recent genetic bottleneck in the three populations sampled. Survivorship in C. porosus in unknown for most life-history stages so a mark-recapture study was implemented to investigate the dynamics of the C. porosus population in the King River. A total of 162 animals were marked between June 2001 and July 2002 of which 75 marked animals were recaptured on 123 occasions. The size of the hatchling population (age < 1 year old) was estimated to be 91 ± 2 (SE) in 2001, which was effectively an estimate of successful recruitment for the King River during that year. The size of the non-hatchling population (age >1 year old) was estimated to be 69 ± 13 but the precision of the estimate was low and should be treated cautiously. Survival of the 2001 cohort of hatchlings was high (> 95%) between June and December and did not vary between the sexes. Hatchlings grew rapidly from June 2001 to July 2002, doubling in length and showing a ten-fold increase in body mass. There were no significant differences in growth rates between females and males during the first 18 months of life. Crocodiles showed a significant behavioural response to capture, with capture probabilities decreasing over time for two groups classified by age. Furthermore, differences in capture probabilities between age groups were significant on all occasions. This has important implications for monitoring crocodile populations, particularly if sightability is influenced by human disturbance. This study has provided some insights into important ecological processes operating within Western Australian populations of C. porosus, but the number of populations examined and period of study were small for such a long-lived animal. More work is needed to determine whether the patterns that have emerged are typical throughout the species' range and longer term studies will be required to quantify vital statistics for most life stages