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Mitchell's Water Monitor (Varanus mitchelli)

Mitchell's Water Monitor is a medium-sized, semi-aquatic goanna found from the Kimberley through the Northern Territory to north-west Queensland. It uses permanent or semi-permanent fresh and brackish water, dense riparian vegetation and rocky waterway edges, so clearing near waterways can remove feeding, basking, nesting and refuge habitat.

Conservation status

Commonwealth, EPBC Act 1999: Critically endangered.

Queensland, Nature Conservation Act 1992: Critically endangered.

Western Australia, Biodiversity Conservation Act 2016: Critically endangered.

Northern Territory, Territory Parks and Wildlife Conservation Act 1976: Vulnerable.

Breeding season

Mitchell’s Water Monitor breeds mainly in the dry season, with wild egg laying recorded from April to June. Clearing during this period may affect gravid females, nests and eggs. Eggs may remain in nests for several months, but wild hatching dates are not recorded.

Reproductive mode: Oviparous. Females lay eggs.

Clutch size: Wild clutches of 3 to 12 eggs have been reported. Captive clutches ranged from 13 to 27 eggs, with a mean of 20 eggs.

Incubation: Captive eggs hatched after 157 to 289 days. Wild incubation length is not recorded.

Size at hatching: Hatchlings were recorded at less than 8 cm snout to vent length. A separate source reports neonates at approximately 20 to 25 cm total length.

Age at maturity: Sexual maturity is attained at about 22 cm snout to vent length. Maturity within 3 to 4 years is also reported, but age data are limited.

Nest or egg sites: Wild nest sites are poorly documented. Reported sites include excavated nests in sandy soil near water and termite mounds. In captivity, eggs were laid in a nest box 400 mm by 400 mm by 400 mm, 300 mm deep, containing approximately 250 mm of moist sand and peat mix.

Breeding frequency: Double and triple clutches occurred in captivity. Multiple clutches were 41 to 60 days apart, with a mean interval of 48 days.

Confirm timing with local records, rainfall and temperature conditions, because dry-season onset, flooding and captive breeding dates vary across the species’ range.

  • Mating: Wild mating months are not recorded. In captivity, copulation to oviposition took 18 to 36 days, with a mean of 28 days, and captive breeding occurred mainly around the documented April to June wild laying period, although captive timing varied from January to September and November to March.
  • Gravid females: Gravid females have been recorded from April to June. The captive interval from copulation to oviposition was 18 to 36 days, with a mean of 28 days, so females may also be carrying developing eggs before April.
  • Egg laying: Wild breeding usually occurs in the dry season, with females depositing eggs between April and June. Captive laying was more variable, with records from January to March, April to July, June to September and November to January.
  • Incubation: Wild incubation timing is not recorded. Applying the captive incubation period of 157 to 289 days to April to June laying indicates that eggs may remain in nests from about September to March, but this seasonal extension is an estimate from captive data.
  • Hatchlings: Wild hatching and juvenile emergence months are not recorded. Based on April to June laying and the documented captive incubation period of 157 to 289 days, hatchlings may occur from about September to March.
  • Active period: No calendar months for adult activity are recorded. Adults rest on branches over water during the day and night, and wet-season flooding expands their habitat use, so retained water-edge habitat should be treated as potentially occupied year round.
  • Highest clearing risk: Risk is highest from April to June because this is the documented wild egg-laying period and gravid females may be present. Nests and eggs may remain at risk from about September to March based on the 157 to 289 day captive incubation period, while wild hatchling timing remains unconfirmed.

Identification: Appearance and size

This is a medium-sized water monitor with strong adaptations for swimming and life around waterways. Identification should combine body pattern, tail shape, head position and habitat.

Total length is 60 to 96 cm. The upper body varies from dark brown or bluish black to blackish grey or olive grey, with yellowish or cream flecks, spots or ocelli.

The neck and throat are bright yellow to orange with black flecks, while the underbelly is pale. The tail is laterally compressed, double keeled and 1.7 to 2.1 times the head and body length.

The nostrils face upwards and outwards, an adaptation associated with its semi-aquatic habits.

Total length: 60 to 96 cm

Tail proportion: 1.7 to 2.1 times head and body length

Identification: Telling it from similar monitors

Mitchell's Water Monitor is one of the aquatic or semi-aquatic monitors included in the Tropical Rivers study. Merten's Water Monitor is another aquatic species in the same study area, while the mangrove monitors are associated with mangrove habitat and aquatic prey.

Use the combination of a laterally compressed, double-keeled tail, upward and outward facing nostrils, yellow to orange flecked throat, body ocelli and occurrence beside fresh or brackish water. The supplied documents do not provide a full diagnostic key separating Mitchell's Water Monitor from Merten's Water Monitor, Varanus semiremex or other local monitors.

Sex-specific field characters are not described in the supplied documents.

Identification: Field signs

The supplied documents do not describe wild tracks, scats, calls, eyeshine or sloughs for this species. They also do not describe wild nest entrances or nest mounds.

During site work, record direct sightings, photographs, the waterbody used, nearby dense vegetation and any potential nest or refuge feature. Do not treat an absence of visible signs as evidence that the species is absent.

Distribution: National range

The documented Australian range is concentrated in the tropical north. The supplied documents do not provide mapped bioregion boundaries, population estimates or a quantified population trend.

The range extends from the Kimberley in Western Australia through the Northern Territory to extreme north-west Queensland. This corresponds broadly with tropical northern Australian waterway systems, but the supplied documents do not name individual bioregions.

The species is associated with permanent or semi-permanent fresh and brackish water, so records should be interpreted in relation to waterways, wetlands and connected riparian vegetation.

  • In Western Australia, search and assess suitable habitat in the Kimberley.
  • In the Northern Territory, assess suitable permanent and semi-permanent waterways and associated riparian vegetation.
  • In Queensland, the documented range is the extreme north-west of the state.

Distribution: Records and population information

The Tropical Rivers Project list reports 75 records within its study area and 76 records in total for Varanus mitchelli. These are database records, not an estimate of the number of animals.

No population size, density, stronghold, subpopulation structure or population trend is provided in the supplied documents.

OZCAM records within study area: 75

Total OZCAM records listed: 76

Population estimate: Not provided

Population trend: Not provided

Habitat: Water and riparian habitat

Mitchell's Water Monitor uses waterbodies with dense cover and nearby basking or rocky edges. Habitat can be fresh or brackish and may include both natural and modified water features if suitable cover and water quality remain.

The species inhabits permanent or semi-permanent waterbodies. These may contain fresh or brackish water.

Important vegetation includes dense riparian stands of Pandanus, Melaleuca, freshwater mangroves and bamboo. Rocky habitat along waterway edges is also used.

The supplied documents do not provide elevation limits, minimum waterbody size, home-range size or measured riparian buffer distances.

Water regime: Permanent or semi-permanent water

Water type: Fresh or brackish

Recorded vegetation: Pandanus, Melaleuca, freshwater mangroves and bamboo

Other edge habitat: Rocky waterway margins

Habitat: Features to retain during works

Retain connected riparian vegetation, overhanging or dense cover, rocky waterway margins and access to permanent or semi-permanent water.

Avoid isolating small waterbodies from surrounding riparian vegetation. The supplied documents do not quantify the distance the species travels between habitat features.

  • Map all permanent and semi-permanent waterbodies before clearing.
  • Mark dense Pandanus, Melaleuca, freshwater mangrove and bamboo cover for retention where practicable.
  • Inspect rocky edges and vegetated margins before machinery enters the work area.

Foraging habitat: Diet

Mitchell's Water Monitor is an active forager that feeds in and around aquatic habitat. Its diet includes aquatic and terrestrial prey, and the mix changes with wet-season flooding.

Wild food consists of arthropods, fish, frogs, small reptiles and their eggs, small mammals and birds.

The diet changes seasonally when wet-season flooding begins. This indicates that changes in water availability can alter prey access and foraging conditions.

Foraging mode: Active forager

Recorded prey: Arthropods, fish, frogs, small reptiles and eggs, small mammals and birds

Seasonal change: Diet changes with the onset of wet-season flooding

Foraging habitat: Foraging habitat and range

Foraging habitat includes permanent or semi-permanent fresh and brackish water, dense riparian vegetation and rocky edges. The species may use both aquatic and adjacent terrestrial prey sources.

No foraging range, home-range size, foraging distance from water or prey density is provided in the supplied documents.

  • Retain access between water, dense riparian cover and rocky edges.
  • Prevent spoil, sediment, rubbish and contaminants from entering waterways used by the species.
  • Schedule fauna inspections around waterbodies and dense riparian vegetation before clearing.

Breeding habitat: Breeding biology

Breeding information is limited. The strongest available evidence indicates dry-season breeding and egg laying from April to June, with nest-site details documented mainly from captive management.

Breeding usually occurs in the dry season, and females have been recorded depositing eggs between April and June. Wild clutch sizes have been reported as 3 to 11, 7 to 12 and 12 eggs.

At Perth Zoo, one female produced 11 clutches from 2002 to 2005, ranging from 13 to 27 eggs, with a mean of 20 eggs. Double and triple clutches were 41 to 60 days apart, with a mean interval of 48 days.

Captive eggs hatched after 157 to 289 days. This incubation range should not be assumed to represent all wild nests. Age at maturity and lifespan are not provided in the supplied documents.

Breeding period: Usually the dry season

Egg laying: April to June

Wild clutch size: 3 to 11, 7 to 12, or 12 eggs in reported records

Captive clutch size: 13 to 27 eggs, mean 20, from one female

Multiple clutches: 41 to 60 days apart, mean 48 days

Captive incubation: 157 to 289 days

Age at maturity and lifespan: Not provided in the supplied documents

Breeding habitat: Nesting habitat

The supplied documents do not describe wild nest structures or wild nest-site selection. At Perth Zoo, a concrete nest box was filled with approximately 250 mm of moist sand and peat mix and provided for oviposition.

The captive enclosure also provided permanent water, branches, basking sites and cover. These conditions show the value of access to water, suitable nesting substrate, thermal opportunities and nearby refuge, but they do not establish the dimensions of wild breeding habitat.

Captive nest substrate depth: Approximately 250 mm of moist sand and peat mix

Captive nest box: Triangular box, 400 mm by 400 mm by 400 mm, with a 300 mm depth

  • Protect undisturbed friable soil and other potential nesting substrate near retained water and riparian cover.
  • Avoid clearing, vibration and heavy vehicle movement around suitable water-edge habitat during April to June.
  • Use a qualified fauna spotter catcher if a suspected nest or egg site is found.

Sheltering habitat: Known refuge features

The species shelters and moves within complex water-edge habitat, but the supplied documents do not identify a particular wild den, hollow or burrow type.

Suitable sheltering habitat includes dense riparian vegetation, Pandanus, Melaleuca, freshwater mangroves, bamboo and rocky edges beside water.

Captive animals were provided with branches for climbing, bark hides and a mock-rock heat pad. These captive features support the use of elevated, covered and thermally varied refuges, but wild refuge dimensions are not documented.

  • Retain dense ground-level and low-growing cover beside waterways.
  • Retain fallen timber, bark-like cover and rocky edge features where they occur within the work area.
  • Inspect vegetation, rock piles and water edges before disturbance.

Sheltering habitat: Features not documented

The supplied documents do not give dimensions for wild refuges, hollows, burrows or dens. They do not report communal sheltering, winter torpor or hibernation.

Do not fill, collapse or remove an apparent refuge until it has been assessed by the fauna spotter catcher.

Threats: Development risks

The supplied documents confirm a high conservation listing but do not provide a formal threat assessment or quantify the causes of decline. On development sites, the main foreseeable risks are the loss and isolation of water-edge habitat, damage to nesting substrate and changes to water conditions.

Clearing permanent or semi-permanent waterbodies, dense riparian vegetation or rocky waterway margins can remove habitat used for feeding, basking, shelter and breeding.

Fragmentation can separate water from riparian cover and reduce movement between habitat features. Roads and construction access can also increase direct mortality where they cross waterways or riparian habitat.

The supplied documents do not quantify road mortality, fragmentation thresholds or the effect of specific clearing methods.

  • Avoid clearing riparian vegetation and rocky waterway margins where practicable.
  • Fence or clearly mark retained habitat and keep machinery out of water-edge refuge areas.
  • Use fauna spotter catcher supervision during clearing near suitable waterways.
  • Inspect culverts, drains, rock piles, vegetation and other refuges before removal.

Threats: Fire, predators, disease and hydrology

The supplied documents do not provide species-specific evidence on fire, introduced predators, disease or hydrology change as threats. Hydrology is still a key site consideration because the species depends on permanent or semi-permanent water and its diet changes when wet-season flooding begins.

Changes that dry out waterbodies, remove seasonal connectivity or degrade water quality may reduce the habitat and prey conditions described for the species. Site-specific assessment is required before altering drainage, floodplain flows or waterbody levels.

  • Prevent sediment, fuel, concrete washout and other pollutants from entering occupied or suitable waterways.
  • Maintain natural drainage and avoid unnecessary isolation of waterbodies from riparian cover.
  • Record and report any injured, dead or displaced monitors found during works.

Threats: Conservation listing

The supplied conservation information lists Mitchell's Water Monitor as Critically Endangered under the EPBC Act, Vulnerable in the Northern Territory, and Critically Endangered in Queensland and Western Australia.

EPBC Act: Critically Endangered

Northern Territory: Vulnerable

Queensland: Critically Endangered

Western Australia: Critically Endangered

Survey methodology: 1. Daytime habitat inspection

Mitchell’s Water Monitor is a semi-aquatic goanna associated with permanent or semi-permanent fresh or brackish water, dense riparian vegetation and rocky edges. The supplied documents do not provide a species-specific impact assessment survey guideline or prescribed survey effort, so survey design should be set by the project approval, the relevant state guidance and the likely habitat present.

Inspect all permanent and semi-permanent water bodies, drainage lines, billabongs, wetland margins and rocky waterway edges within and beside the proposed disturbance area.

Search riparian vegetation, including Pandanus, Melaleuca, freshwater mangroves and bamboo, and inspect trees, banks, rocks, logs and other features that could provide cover or access to water.

Record the waterbody type, whether water is fresh or brackish, riparian vegetation condition, rocky margins, potential basking sites and any monitor observations or signs.

Survey methodology: 2. Active searches at water edges

Conduct careful visual searches along the full accessible edge of each waterbody and along connected riparian habitat, with the search area and distance walked recorded for each visit.

Include checks of basking sites, overhanging branches, rock piles, dense vegetation, banks and shallow margins, because the species uses water and adjacent riparian or rocky habitat.

Approach quietly and avoid repeated disturbance, as Mitchell’s Water Monitors are described as generally timid.

Survey methodology: 3. Camera and trapping methods

Use remote cameras at likely access points, basking sites and narrow movement routes where direct observation is difficult or where repeated visits could disturb the animal.

Use cage trapping, funnel trapping or other approved methods only under a project-specific fauna survey or handling permit and with an experienced reptile ecologist responsible for animal welfare.

Keep a separate record of trap locations, trap nights, checking times, weather, water conditions, bait, captures and non-target animals; the supplied documents do not specify a standard trap-night effort for this species.

Survey methodology: 4. Survey timing and conditions

Prioritise surveys when water is present and access to riparian habitat is possible, and record whether conditions are fresh water, brackish water, flooded or dry.

Plan additional coverage around the species’ documented breeding period, when females usually deposit eggs from April to June, if works could disturb likely nesting or refuge areas.

Record date, start and finish time, temperature if measured, rainfall, wind, cloud, water level, observer effort and all detection locations; do not treat the absence of a detection as evidence that the species is absent.

Survey methodology: Minimum effort and reporting

  • Use the effort required by the relevant approval, state survey guidance and project risk assessment, because no species-specific survey effort is provided in the supplied documents.
  • Survey every suitable waterbody and connected riparian section that could be affected directly or indirectly by the works.
  • Map all observations and potential habitat features using coordinates, photographs and a clear description of the location.
  • Report confirmed records and significant survey results to the relevant state wildlife database and provide records to the project environmental officer and approval agencies where required.
  • Retain survey data, negative results, weather records, effort calculations, photographs, permits and observer details with the project environmental records.

Before habitat disturbance: Avoid and minimise

Avoid clearing suitable water-associated habitat wherever practicable. The species depends on permanent or semi-permanent water and associated riparian or rocky habitat, so the work footprint should be designed around these features.

  • Retain permanent and semi-permanent water bodies, wetland margins, riparian vegetation and rocky edges within and beside the work area.
  • Retain mature Pandanus, Melaleuca, freshwater mangroves and bamboo where they form dense cover beside water.
  • Maintain movement links between retained water bodies and adjacent riparian habitat, and do not isolate retained habitat with roads, stockpiles, fencing or sediment controls.
  • Keep plant, spoil, lighting and access routes out of retained water and riparian areas unless the approved design specifically requires work there.

Before habitat disturbance: Approvals and work planning

  • Confirm the EPBC Act requirements before work starts because the species is identified in this profile as nationally Critically Endangered.
  • Confirm permits, approvals and handling requirements for the relevant state or territory, noting the profile’s state listings for the Northern Territory, Queensland and Western Australia.
  • Have the ecologist check the final footprint for suitable water-associated habitat before mobilisation and update the fauna management plan if habitat or animals are found.
  • Plan staging so that the smallest practicable area is opened at one time and retained habitat remains connected throughout construction.
  • Consider the April to June egg-laying period when setting the work program, and obtain species-specific advice if works may affect likely nesting areas.

Before habitat disturbance: Site preparation

  • Complete a pre-clearance survey of all waterbodies, riparian vegetation, rocky margins and other suitable habitat within the disturbance footprint.
  • Mark retained water, riparian vegetation, mature trees, rock features, logs and access routes before machinery enters the site.
  • Install and maintain exclusion fencing where it can keep machinery and people out of retained habitat without blocking the animal’s access to retained water or creating a trapping hazard.
  • Brief all workers on the species’ appearance, habitat, stop-work requirements, reporting chain and the prohibition on unauthorised capture or handling.

During habitat disturbance: Daily controls

Treat all water edges and connected riparian habitat as potential monitor habitat during clearing and construction. A fauna spotter catcher should control the interface between the animal, machinery and retained habitat.

  • Inspect exclusion fencing, water controls, access tracks, stockpiles and retained habitat before work begins each day.
  • Keep machinery, spoil, fuels, concrete washout and sediment controls out of retained water and riparian habitat.
  • Use a qualified fauna spotter catcher during clearing or other activities that could injure an animal or remove habitat.
  • Stop or alter work if an animal is detected, if fencing fails, or if water quality, flow or access to retained habitat is being affected.

During habitat disturbance: Spotter catcher and machinery

  • Keep the spotter catcher in a position where the active clearing face, machinery operator and escape route to retained habitat can be seen.
  • Clear slowly towards retained habitat only where the approved fauna management plan allows it, and leave a clear route away from machinery and into suitable retained habitat.
  • Do not chase, corner, pick up or move a monitor unless the authorised fauna handler considers handling necessary for its safety.
  • Suspend the relevant plant operation immediately when the spotter catcher calls stop, and recommence only after the animal and habitat risk have been assessed.

During habitat disturbance: Habitat features and animal encounters

  • Treat banks, dense riparian vegetation, rocks, logs, tree hollows and other cover beside water as potential refuge features and inspect them before removal.
  • Do not remove or fill burrows, cavities, rock piles or other suspected refuge features until they have been checked by the authorised fauna handler.
  • If a Mitchell’s Water Monitor is found, stop nearby work, keep people and machinery back, and allow an unobstructed route to nearby retained habitat where safe.
  • Only an appropriately authorised and experienced reptile handler should capture or relocate the animal, and relocation should be to nearby suitable retained habitat agreed in the fauna management plan.
  • Take injured animals to a veterinarian or contact an authorised wildlife carer through the project’s fauna response procedure.
  • Record the location, time, activity, animal condition, action taken, handler, release location and photographs where this can be done without delaying treatment.

During habitat disturbance: Stop-work triggers

  • Stop work if the species is found in the active work area or if a suspected nest, refuge or access route is identified.
  • Stop work if an animal is injured, trapped, surrounded by machinery or unable to reach retained habitat safely.
  • Stop work if clearing reveals water-associated habitat not assessed in the approved plan, or if works damage a retained waterbody or riparian exclusion area.
  • Notify the project environmental officer and approval contact before restarting work where the fauna management plan or approval requires notification.

After habitat disturbance: Post-clearance checks

Post-clearance checks should confirm that animals have not been trapped or injured and that retained water and riparian habitat remain usable. Continue controls until the risk from construction has ended.

  • Inspect the cleared area, retained habitat edges, fencing, pits, trenches, drains and other artificial hazards at the frequency required by the fauna management plan.
  • Check that no monitor is trapped in excavations, behind fencing, in pipes or in sediment and water-control structures.
  • Recheck waterbodies and riparian margins after staged clearing, major earthworks, flooding or changes to access routes.
  • Close, cover or fauna-proof artificial pits, trenches and pipes when they are not being actively worked, subject to safety requirements.

After habitat disturbance: Monitoring and reporting

  • Continue targeted searches or camera monitoring where the approval, risk assessment or ecologist identifies a residual risk to the species.
  • Compare post-clearance detections with pre-clearance records and map any change in access to water, riparian cover or rocky habitat.
  • Report all captures, injuries, deaths, relocations, habitat damage, stop-work events and corrective actions to the project environmental officer.
  • Submit confirmed records and required incident reports to the relevant state wildlife database and approval agencies.

After habitat disturbance: Controls and offsets

  • Keep exclusion fencing, water-quality controls and habitat protection measures in place until the environmental officer confirms that construction risk has ended.
  • Repair damaged riparian areas, banks and waterbody margins as soon as practicable under the approved rehabilitation plan.
  • Apply any approved offset, conservation action or compensatory habitat measure required by the EPBC Act or state approval.

Rehabilitation actions: Water and riparian habitat

Rehabilitation should restore connected water-associated habitat rather than only replacing cleared vegetation. The strongest species-specific information supplied is its association with permanent or semi-permanent water, dense riparian cover and rocky margins.

  • Reinstate permanent or semi-permanent water features where they were temporarily disturbed and where this is consistent with the approved design and hydrology.
  • Replant locally appropriate riparian vegetation, including Pandanus, Melaleuca, freshwater mangroves and bamboo where these occur naturally at the site.
  • Create continuous vegetated links between retained waterbodies and rehabilitation areas instead of isolated planting patches.
  • Stabilise banks with natural vegetation and suitable rock or woody material, while avoiding designs that create entrapment points for fauna.

Rehabilitation actions: Cover and refuge features

  • Retain or reinstate natural rocky margins, logs, branches and dense vegetation beside water where they do not create a safety or flood risk.
  • Replace removed habitat features with locally sourced material approved by the ecologist, and record the location and type of each installed feature.
  • Provide nest boxes only where the ecologist identifies a demonstrated shortage of suitable cavities and the design is appropriate for this species; the supplied documents do not establish a nest-box design or requirement for Mitchell’s Water Monitor.
  • Avoid removing potential refuge features during the April to June egg-laying period unless the fauna management plan provides for an authorised inspection and response.

Rehabilitation actions: Threat and site management

  • Control weeds so that native riparian cover and access to water can develop and persist.
  • Prevent sediment, chemicals, litter and altered drainage from degrading rehabilitated fresh or brackish water.
  • Manage domestic, feral and pest animals in accordance with the approved site plan where they threaten rehabilitated habitat or fauna.
  • Use fire planning that protects established riparian vegetation and avoids unnecessary burning of dense cover beside water.

Rehabilitation actions: Monitoring and success measures

  • Inspect rehabilitation after planting, major rainfall and flooding to check survival, erosion, water persistence, weed cover, connectivity and the condition of logs and rocks.
  • Use repeat habitat inspections, active searches and remote cameras where required by the approval or rehabilitation plan.
  • Measure success against the approved targets for water availability, riparian vegetation establishment, connected cover, stable banks and absence of fauna hazards.
  • Adapt weed, erosion, water and access controls if monitoring shows that rehabilitated habitat is not functioning as intended.

Sources