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Fukuda Y.,Environment and the Arts and Sport | Webb G.,Wildlife Management International Pty. Ltd | Webb G.,Charles Darwin University | Manolis C.,Wildlife Management International Pty. Ltd | And 4 more authors.
Journal of Wildlife Management | Year: 2011

Saltwater crocodiles (Crocodylus porosus) in the Northern Territory of Australia were protected in 1971, after a severe population decline resulting from 26 yr of intense commercial hunting. By that time wild saltwater crocodiles were rarely sighted anywhere and they were commercially extinct in areas where they had once been abundant. Standardized monitoring by spotlight surveys started in 1975 and provided relative density indices over time (1975-2009) as a unique record of the post-protection recovery of a wild crocodilian population. We examined the survey data for populations at 12 major tidal rivers, individually and as a single subpopulation. The pattern of recovery in the subpopulation in both abundance and biomass was approximated by logistic curves, predicting 5.26 non-hatchling crocodiles weighing 387.64 kg sighted per kilometer of river in 2010. We predicted potential carrying capacity as 5.58 non-hatchling crocodiles (5.73% higher than 2010) weighing 519.0 kg (25.31% higher than 2010). Individual rivers showed largely different abundance and biomass among rivers. The statistical model that best described the recovery in individual rivers was not always logistic. However, where it was logistic, expected carrying capacity of different rivers showed considerable variation in abundance and biomass. The variation indicates different habitat quality among the rivers. Recovery occurred despite various consumptive uses, particularly a widespread egg-harvest program, which has been an integral part of the incentive-driven conservation program for saltwater crocodiles in the Northern Territory since 1983. We suggest that the saltwater crocodile population of the Northern Territory is achieving full recovery from uncontrolled hunting in 1945-1971. Although saltwater crocodiles are considered an important natural resource, their increase in number, size, and distribution is posing management issues for public safety. Continuation of human-crocodile conflict management through public education and strategic removal of problem crocodiles will be essential. Copyright © 2011 The Wildlife Society.

Tingley R.,University of Melbourne | Crase B.,National University of Singapore | Webb G.,Wildlife Management International Pty. Ltd | Webb G.,Charles Darwin University
Animal Conservation | Year: 2016

Invasive predators can cause population declines in native prey species, but empirical evidence linking declines of native predators to invasive prey is relatively rare. Here, we document declines in an Australian freshwater crocodile Crocodylus johnstoni population following invasion of a toxic prey species, the cane toad Rhinella marina. Thirty-five years of standardized spotlight surveys of four segments of a large river in northern Australia revealed that the density of freshwater crocodiles decreased following toad invasion and continued to decline thereafter. Overall, intermediate-sized freshwater crocodiles (0.6-1.2m) were most severely impacted. Densities of saltwater crocodiles Crocodylus porosus increased over time and were generally less affected by toad arrival, although toad impacts were inconsistent across survey sections and size classes. Across the entire river, total freshwater crocodile densities declined by 69.5% between 1997 and 2013. Assessments of the status of this species within other large river systems in northern Australia, where baseline data are available from before the toads arrived, should be prioritized. Our findings highlight the importance of long-term monitoring programmes for quantifying the impacts of novel and unforeseen threats. © 2015 The Zoological Society of London.

Somaweera R.,University of Sydney | Brien M.,Wildlife Management International Pty. Ltd | Brien M.,Charles Darwin University | Shine R.,University of Sydney
Herpetological Monographs | Year: 2013

Although adult crocodilians have few predators (mostly humans and other crocodilians), hatchlings and eggs are killed and consumed by a diverse array of invertebrates, fishes, anurans, reptiles, birds, and mammals. We review published literature to evaluate the incidence of predation in crocodilian populations, and the implications of that mortality for crocodilian life-history evolution. Presumably because predation is size-dependent, small-bodied crocodilian taxa appear to be more vulnerable to predation (across a range of life stages) than are larger-bodied species. Several features of crocodilian biology likely reflect adaptations to reducing vulnerability to predation. For example, the threat of predation may have influenced the evolution of traits such as nest-site selection, maternal care of eggs and hatchlings, crèche behavior in hatchlings, and cryptic coloration and patterning. Even for such large and superficially invulnerable taxa such as crocodilians, the avoidance of predation appears to have been a significant selective force on behavior, morphology, and ecology. © 2013 The Herpetologists' League, Inc.

Manolis C.,Wildlife Management International Pty. Ltd | Zuur A.,Highland Statistics Ltd.
PLoS ONE | Year: 2015

Conflicts between humans and crocodilians are a widespread conservation challenge and the number of crocodile attacks is increasing worldwide. We identified the factors that most effectively decide whether a victim is injured or killed in a crocodile attack by fitting generalized linear models to a 42-year dataset of 87 attacks (27 fatal and 60 non-fatal) by saltwater crocodiles (Crocodylus porosus) in Australia. The models showed that the most influential factors were the difference in body mass between crocodile and victim, and the position of victim in relation to the water at the time of an attack. In-water position (for diving, swimming, and wading) had a higher risk than on-water (boating) or on-land (fishing, and hunting near the water's edge) positions. In the in-water position a 75 kg person would have a relatively high probability of survival (0.81) if attacked by a 300 cm crocodile, but the probability becomes much lower (0.17) with a 400 cm crocodile. If attacked by a crocodile larger than 450 cm, the survival probability would be extremely low (<0.05) regardless of the victim's size. These results indicate that the main cause of death during a crocodile attack is drowning and larger crocodiles can drag a victim more easily into deeper water. A higher risk associated with a larger crocodile in relation to victim's size is highlighted by children's vulnerability to fatal attacks. Since the first recently recorded fatal attack involving a child in 2006, six out of nine fatal attacks (66.7%) involved children, and the average body size of crocodiles responsible for these fatal attacks was considerably smaller (384 cm, 223 kg) than that of crocodiles that killed adults (450 cm, 324 kg) during the same period (2006-2014). These results suggest that culling programs targeting larger crocodiles may not be an effective management option to improve safety for children. © 2015 Fukuda et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Manolis C.,Wildlife Management International Pty. Ltd | Appel K.,Parks and Wildlife Commission of the Northern Territory
Journal of Wildlife Management | Year: 2014

We reviewed the historical records of attacks by saltwater crocodiles (Crocodylus porosus) and the removal of problem saltwater crocodiles in the Northern Territory of Australia. Between 1977 and 2013, 5,792 problem crocodiles were removed, of which 69.04% were males and 83.01% were caught within the Darwin Crocodile Management Zone where suitable breeding habitats were hardly available. The most common size class was 150-200 cm and their mean size did not change significantly over years. This reflected the greater mobility of juvenile males as the majority of problem crocodiles, dispersing from core habitats that were occupied by dominant individuals. Eighteen fatal attacks and 45 non-fatal attacks occurred between 1971 and 2013. The rate of crocodile attacks, particularly non-fatal cases, increased over time. This increase was strongly related to the increasing populations of both humans and crocodiles, and the increasing proportion of larger (>180 cm) crocodiles. The management of human-crocodile conflict (HCC) should incorporate both human (e.g., public education and safety awareness) and crocodile (e.g., population monitoring, removal of problem crocodiles) components. Crocodiles in the 300-350-cm class were most responsible for attacks, and they should be strategically targeted as the most likely perpetrator. Approximately 60% of attacks occurred around population centers including remote communities. Problem crocodile capture and attacks both peak in the beginning (Sep-Dec) and end (Mar-Apr) of the wet season. However, fatal attacks occurred almost all year around. Attacks by crocodiles >400 cm often resulted in death of the victim (73.33%). Local and male victims were much more common than visitors and females, respectively. The most common activity of victims was swimming and wading. Despite the increasing rate of attacks over time, the Northern Territory's management program, and in particular the removal of problem crocodiles from urban areas, is considered to have reduced potential HCC. Public education about crocodile awareness and risks must be maintained. © 2014 The Wildlife Society. © The Wildlife Society, 2014.

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