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Crocodylus johnstoni 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Reptilia Crocodylia Crocodylidae

Scientific Name: Crocodylus johnstoni Krefft, 1873
Common Name(s):
English Australian Freshwater Crocodile, Freshie, Johnson's Crocodile, Johnstone's Crocodile, Johnston's Crocodile
Synonym(s):
Crocodylus johnsoni Krefft, 1873
Taxonomic Notes:

The specific name of this species is variably given as Crocodylus johnsoni or C.  johnstoni.  The species was named by Krefft after a Mr Johnstone, but as C. johnsoni (see discussion in King and Burke 1989, Tucker 2010). Crocodylus johnstoni (with a 't') has received widespread use, particularly by Australian workers and Cogger et al. (1983) proposed formalising this use, notwithstanding that the correct name based on Johnstone would be 'johnstoneii'. Tucker (2010) argues, using the new edition of the International Code of Zoological Nomenclature, that the use of C. johnstoni be the prevailing spelling based on the intention of Krefft to quickly correct the spelling (Gray 1874). As such, for Red List purposes, C. johnstoni will be used predominantly but it should be clear that C. johnsoni and C. johnstoni are the same species and the terms can be used interchangeably.

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2017
Date Assessed: 2016-09-25
Assessor(s): Isberg, S., Balaguera-Reina, S.A. & Ross, J.P.
Reviewer(s): Somaweera, R., Brien, M.L., Limpus, C., Fukuda, Y., Manolis, C. & Webb, G.J.W.
Justification:

The Australian Freshwater Crocodile (Crocodylus johnstoni) is endemic to mainland northern Australia. It has a wide range and a large global population that is currently considered to be stable. Harvest rates for this species are low/negligible and its habitats are largely intact. Therefore, it remains in the Least Concern category.

Previously published Red List assessments:

Geographic Range [top]

Range Description:Crocodylus johnstoni (Australian Freshwater Crocodile) is endemic to mainland northern Australia (Western Australia, Northern Territory and Queensland). Its distribution has been extended throughout eastern Queensland from Burnett River northward with the intentional release and escape from captive colonies since the 1950s.
Countries occurrence:
Native:
Australia (Northern Territory, Queensland, Western Australia)
Additional data:
Estimated area of occupancy (AOO) - km2:100000Continuing decline in area of occupancy (AOO):No
Extreme fluctuations in area of occupancy (AOO):NoEstimated extent of occurrence (EOO) - km2:955166
Continuing decline in extent of occurrence (EOO):NoExtreme fluctuations in extent of occurrence (EOO):No
Upper elevation limit (metres):950
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Webb et al. (1987) estimated the Northern Territory (NT) population of Australian Freshwater Crocodiles (C. johnstoni) to comprise 30,000-60,000 non-hatchlings. In Western Australia (WA), the populations in the Fitzroy River, Ord Rivers, Lake Argyle and Lake Kununurra were estimated to be at least 47,000 individuals (McNamara and Wyre 1994, DEC 2009). Population size in Queensland is unquantified (Read et al. 2004), but the species is widespread and abundant (Miller 1993).

Crocodylus johnstoni is considered to be at low risk of extinction. The population is large and widely distributed across the three northern Australian states, harvest rates are low/negligible and habitats are largely intact.

The biggest potential risk to C. johnstoni is the Cane Toad (Rhinella marina). Crocodylus johnstoni are more sensitive to the Cane Toad toxin than Saltwater Crocodiles, C. porosus (Smith and Phillips 2006), and often die after ingesting them. Cane Toads were initially introduced to Queensland in 1935 and have been spreading west, reaching the Northern Territory in the early 1980s (Letnic and Ward 2005), and Western Australia (Lake Argyle) in early 2009 (Webb and Manolis 2010). The impact of Cane Toad invasion on C. johnstoni populations appears to vary between regions, and between habitats within catchments, with some populations showing significant localised population declines and others showing negligible impact (Somaweera et al. 2013). For example, population declines of up to 77% have been reported in the Victoria (Letnic et al. 2008) and Daly Rivers (Fukuda et al. 2015) of the Northern Territory, and in Bullo River in Western Australia (Britton et al. 2013). In contrast, there has been no decline of mature individuals reported in the Roper, McArthur and Daly Rivers of the Northern Territory (Catling et al. 1999, Doody et al. 2009) or Lake Argyle in Western Australia (Somaweera and Shine 2012). That healthy C. johnstoni populations remain in areas of Queensland that have been colonised by Cane Toads for several decades, where they  have been observed ingesting adult sized Cane Toads with no associated mortality (Tucker et al. 1996, C.J. Limpus pers. comm. 2016), suggests that extinction is not a primary concern (Webb and Manolis 2010).

Of the studies that showed a significant decline in C. johnstoni numbers, only the small and intermediate size classes (<1.5 m) were affected, since larger C. johnstoni are more tolerant of Cane Toad toxin (Smith and Phillips 2006). With the exception of the stunted escarpment or “pygmy” crocodiles (Webb 1985, Britton et al. 2013), adult C. johnstoni are defined as >1.5 m (Delaney et al. 2010). If we accept this size to be those capable of sexual reproduction, the impact of Cane Toads on the effective population size of C. johnstoni is non-significant. Therefore, whilst it is recognised that there will be an impact on the recruitment of intermediate-sized animals into the breeding population, this is only believed to be short-term, as there is evidence that some younger C. johnstoni  learn to avoid cane toads (Somaweera et al. 2011a). Furthermore, the invasion of Cane Toads has meant less predation on C. johnstoni eggs by varanids allowing a higher hatchling recruitment (Chibeba 2003, Webb and Manolis 2010) to the detriment of varanid lizards (Doody et al. 2009).

Although this IUCN Red List assessment has been made based on the effective population size, the threat to sub-populations within the range should not be overlooked. Particular reference is made to the described stunted escarpment or “pygmy” C. johnstoni in the upper reaches of the Liverpool River (Webb 1985) and Bullo River, Northern Territory (Britton et al. 2013). The current (unpublished) population genetics studies for C. johnstoni do not identify any genetic differentiation of "pygmy" C. johnstoni (N. FitzSimmons pers. comm. June 2016). Although not a subspecies, these crocodiles grow slowly and become sexually mature at much smaller sizes, proposed to be due to limited food availability (Webb 1985). Given the scarcity of prey, there is a higher incidence of Cane Toad consumption by pygmy C. johnstoni, and Britton et al. (2013) reported significant and detrimental declines in this population. Due to their smaller size, even sexually mature “pygmy” crocodiles are vulnerable to cane toad toxin, thus decreasing their effective population size and recruitment ability. On a regional level, this population should be considered critically endangered, particularly if subspecies status is granted.

Interspecies competition with the larger Estuarine or Saltwater Crocodile (C. porosus) could attribute, in part, to observed declines in C. johnstoni in some areas where their distribution overlaps. It appears C. johnstoni expanded into upper estuarine habitats when C. porosus abundance  was severely reduced. There are observations of large C. johnstoni preying on hatchling C. porosus in such sites (Webb pers com). As C. porosus abundance increased, C. johnstoni numbers in such areas declined (Webb et al. 1983e; C.J. Limpus pers. com). However, as C. porosus populations continued to recover, some individuals spread upstream into freshwater areas, previously considered exclusive habitats for C. johnstoni, and such animals prey on C. johnstoni.

There appear to be few other significant threats to C. johnstoni. Entanglement and drowning of C. johnstoni in fishing nets occurs in the catfish fishery in Lake Argyle (Webb and Manolis 2010, Somaweera and Shine 2012). However, long-term survey data (1996-2008), prior to Cane Toad invasion, indicated a 3.8% per annum increase in C. johnstoni numbers despite these losses (Mawson 2004, WMI 2008). During the 1970s and early 1980s, illegal net fishing for barramundi in freshwater rivers of the Gulf plains of Queensland caused localised depletion of C. johnstoni at some locations accessible to the fishers. However, for most areas, local property (cattle station) owners excluded illegal net fishing from their properties (C.J. Limpus pers. comm. 2016). Nest predation by dingoes has been shown to be significant in the Lake Argyle/Ord River systems (Somaweera et al. 2011) as well as feral pigs and dogs, albeit unquantified, in the western Cape York Peninsula and Southern Gulf plains of Queensland (C.J. Limpus pers. comm. 2016).

Loss of riparian habitat, erosion of nesting areas, water diversion for irrigation and land clearing may require monitoring in some areas (Delaney et al. 2010, Webb and Manolis 2010). At a local level, some populations are experiencing a reduction of optimal nesting beaches due to weed invasions. A long-term study on this aspect is underway (Webber et al. 2014).

Crocodylus johnstoni are not considered a threat to humans, and very few cases of unprovoked attacks have been reported (Lindner 2004, Caldicott et al. 2005, Somaweera, 2011). In the Northern Territory and Queensland, C. johnstoni, particularly large individuals, are occasionally removed from recreational areas to alleviate public concerns about safety (Delaney et al. 2010, M.L. Brien pers. comm. 2016, C.J. Limpus pers. comm. 2016).
Current Population Trend:Stable
Additional data:
Continuing decline of mature individuals:No
Extreme fluctuations:NoPopulation severely fragmented:No
Continuing decline in subpopulations:No
Extreme fluctuations in subpopulations:NoAll individuals in one subpopulation:No

Habitat and Ecology [top]

Habitat and Ecology:The Australian Freshwater Crocodile (C. johnstoni) is a small to medium sized crocodile restricted to tropical northern Australia (Western Australia, Northern Territory, Queensland). It has a very narrow snout, and can be readily distinguished from other freshwater species of crocodilians in the Asian region (e.g., C. mindorensis, C. novaeguineae, C. siamensis, C. porosus). It is much more similar in appearance to Mecistops cataphractus from Africa and C. intermedius from the Orinoco River drainage of Venezuela and Colombia.

Maximum size of males exceeds 3 m, and females may exceed 2 m. Throughout its range, C. johnstoni is generally restricted to freshwater habitats upstream of tidal influence (Webb et al. 1987). This includes almost any type of permanent freshwater habitat including rivers, creeks, swamps and floodplain lakes and lagoons (“billabongs) (e.g., see Webb et al. 1983c, Tucker et al. 1997a). In some rivers, C. johnstoni extend into tidal, saline waters. Although sympatric with C. porosus, they may be restricted from colonizing such areas through predation by C. porosus (Messel et al. 1981, Webb et al. 1983e). The historical distribution in Queensland has expanded with “introduced” populations as far south as the Burnett River (25°S) (Read et al. 2004, S.C. Manolis pers. obs. 2016, C.J. Limpus pers. obs. 2016).

Populations of C. johnstoni in upstream escarpment streams in the Northern Territory can be appreciably stunted in size, with adult males and females some 10% of the weight of counterparts in downstream areas. This stunting appears to be due to limited food resources. Individuals in one “stunted” population in Arnhem Land (Webb 1985) could be distinguished from “normal” freshwater crocodiles on the basis of morphometric attributes (Edwards 1983). This stunting of a population has not been observed in the upstream escarpment reaches of rivers in Queensland (Tucker et al. 2006).

The diet of C. johnstoni includes a wide variety of invertebrates and small vertebrates, with prey size increasing as body size increases (Webb et al. 1983d, Tucker et al. 1996). In the McKinlay River area of the Northern Territory, and the upper Mitchel River Catchment in Queensland, the mean clutch size is 13 eggs, which are laid in a hole nest in sand bars exposed in the dry season (Webb et al. 1983b, Tucker and Limpus 1997). Probably the largest population inhabits a man-made lake in Western Australia where the population has adapted to nest in completely different types of substrate compared to the natural habitats (Somaweera and Shine 2013). Incubation normally lasts 75-85 days (Webb et al. 1983b, Webb and Smith 1984). Prior to colonization by Cane Toads (Rhinella marina), annual egg predation by monitor lizards (Varanus spp.) exceeded 90% in some areas (Smith and Webb 1985). The marked reduction in monitor lizards that has followed the spread of cane toads into the Northern Territory has decreased egg predation rates substantially. However, at some locations, dingoes. feral pigs and feral dogs remain a significant nest predator (Somaweera et al. 2011c, C.J. Limpus pers. comm. 2016). Annual hatching success at seven C. johnstoni nest sites monitored before and after toad colonization increased from 4-8 nests (2002-2003) to 26-27 nests (2005-2006) respectively (Chibeba 2003, WMI, unpublished data) – an average of 342% increase in hatchling recruitment.

Since the late 1970s, extensive research has been undertaken on C. johnstoni in Queensland and the Northern Territory, and its basic biology is now well known (e.g., Cooper-Preston and Jenkins 1993; Jessop et al. 2003; Kennett and Christian 1993; Limpus 1984; Smith and Webb 1985; Somaweera and Shine 2012; Somaweera et al. 2011b; Taplin et al. 1993; Tucker 1995; Tucker et al. 1997a,b, 1998; Webb and Gans 1982; Webb and Manolis 1983; Webb and Smith 1984; Webb et al. 1983a,b,c,d,e; Whitehead et al. 1992).
Systems:Freshwater
Continuing decline in area, extent and/or quality of habitat:No
Generation Length (years):25
Movement patterns:Not a Migrant

Use and Trade [top]

Use and Trade: Crocodylus johnstoni were hunted for their skins and for producing stuffed specimens from the late 1950s until the 1970s. Although the number of animals harvested in unknown, it is estimated that between 60,000 to 80,000 skins were traded until protection in Western Australia in 1962 (Burbidge 1987), Northern Territory in 1964 (Webb et al. 1987) and Queensland in 1974 (Taplin 1987). A ranching program based on hatchlings was established in the Northern Territory in 1982 (Webb and Smith 1984, Webb et al. 1987), but by the early 1990s farming had largely ceased due to the relatively low commercial value of the skin (Delaney et al. 2010). Harvesting of C. johnstoni is permitted in Western Australia and the Northern Territory through management programs (DEC 2009, Delaney et al. 2010, PWCNT 2000). Some limited numbers of C. johnstoni were taken from the wild to stock several crocodile farms under permit in Queensland in the 1980s. No use of the wild resource is currently permitted in Queensland. A limited pet market exists in some States and Territories whose legislation allows for the keeping of crocodiles (Webb and Manolis 2010).

Threats [top]

Major Threat(s): Whilst no reduction in mature C. johnstoni numbers has been identified, the intermediate size classes in the Northern Territory have been significantly reduced (up to 77%) since the invasion of Cane Toads (Rhinella marina). Simultaneously, hatchling recruitment has increased due to reduced nest predation by varanids which have also decline in prevalence since cane toad arrival. It will be the learned ability of these hatchlings to avoid Cane Toads (Somaweera et al. 2011a) which will ultimately determine if they will be recruited into the breeding population. Although the Queensland population appears to be abundant despite cane toad colonisation for several decades (Tucker et al. 1996), there are no recent surveys to confirm the continued well-being of this population. Thus, it is recommended that the Northern Territory and Western Australia continue/reinstate annual C. johnstoni surveys to ensure that no reduction in the mature population eventuates and C. johnstoni retain their Least Concern status. Potential other unquantified threats include: a) nest predation by dingoes, feral pigs and feral dogs, b) nest availability due to weed invasion, erosion of nesting areas, water diversion for irrigation projects and land clearing, c) climate change and encroaching saline waters that may result in upstream areas, and d) Saltwater Crocodiles (C. porosus) inhibiting the expansion of C. johnstoni populations due to their larger size and competition for resources.

Conservation Actions [top]

Conservation Actions: The highest priority conservation action noted by Webb and Manolis (2010) was to (re)initiate C. johnstoni surveys. Compared to C. porosus, there is very low demand for their skin so little to no wild harvesting occurs. As a result, population surveys ceased in the Northern Territory in the mid-1990s with no systematic monitoring ever undertaken in Queensland.

Despite multiple reports of no detriment to mature C. johnstoni (>1.5 m) from cane toad ingestion, and thus to the effective population size, continued monitoring should be continued to ensure there is no effect on future recruitment given the large decreases in the intermediate age classes. Population surveys of C. johnstoni populations, where Cane Toads have been in existence for some time, such as in Queensland, may provide important information regarding the long-term effects on the species from Cane Toads.

Citation: Isberg, S., Balaguera-Reina, S.A. & Ross, J.P. 2017. Crocodylus johnstoni. The IUCN Red List of Threatened Species 2017: e.T46589A3010118. . Downloaded on 12 December 2017.
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