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Little rainbowfish (Melanotaenia wilsoni)

The Little rainbowfish is a very small freshwater rainbowfish endemic to shallow, spring-fed streams, pools and swamp channels below the Tabletop Range in Litchfield National Park, Northern Territory. Its narrow range and dependence on clear, cool, weakly mineralised water make shallow aquatic habitat, hydrology and dense vegetation important considerations during clearing and construction works.

Conservation status

Northern Territory, Territory Parks and Wildlife Conservation Act 1976: Near threatened.

Breeding season

A reliable wild annual cycle has not been documented for the Little rainbowfish. The calendar below records the evidence available and highlights the need to manage suitable shallow habitat throughout the year rather than relying on a confirmed low-risk season.

Month markings should be refined with local hydrology, survey results and advice from a freshwater fish ecologist. Any proposed work in shallow spring-fed habitat should be assessed for year-round habitat and water quality effects.

  • Dry-season refuge use: Seasonal contraction into refuge holes is reported, but the months are not specified. The marked period is indicative only and is not a documented calendar for this species.
  • Highest clearing risk: No lower-risk clearing period can be supported because wild breeding timing is unknown and suitable habitat may be used year round, including refuge holes when swamp and channels contract.

Identification

The Little rainbowfish is the smallest known rainbowfish species. It is a small, laterally compressed fish with strongly contrasting dark stripes and a restricted Northern Territory range.

The body is yellowish-brown to whitish or silvery-white below, with 7 to 8 dark lateral stripes. The midlateral stripe is usually the most conspicuous and can have a dark purple hue. A second dark stripe is commonly prominent along the lower side.

The iris is silver to orange. A small orange to red spot may be present on the upper gill cover, with orange sometimes extending onto the breast and the front of the pelvic fins.

The dorsal and anal fins have a broad greyish to black, somewhat diagonal band, a pale yellow area above the band, and often a whitish to light blue margin. Males commonly have darker markings on the second dorsal fin.

Body form: Compressed, deep-bodied fish.

Dorsal fin formula: Usually 5 to 7 spines and 7 to 9 soft rays.

Anal fin formula: 1 spine and 13 to 16 soft rays.

Lateral scales: 30 to 34, with 30 to 32 reported in the original description.

Identification: Size and sex

Adults and subadults are very small. Specimens used in the species description measured 19.3 to 33.3 mm standard length, and the maximum recorded size is 3.3 cm standard length.

Male and female fin morphology is similar. Nuptial males have more pronounced dark markings on the second dorsal fin, but sexing small field-caught fish may be difficult.

Species description specimens: 19.3 to 33.3 mm standard length.

Maximum size: 3.3 cm standard length, or less than 35 mm standard length.

Weight: No species-specific weight information is provided.

Identification: Similar species and field checks

The Little rainbowfish was formerly confused with forms now recognised as McCulloch's Rainbowfish, Melanotaenia maccullochi, and Sahul Rainbowfish, Melanotaenia sahulensis. The Little rainbowfish is geographically isolated, has a more prominent purplish midlateral stripe, and generally has fewer vertebrae, lateral scales, cheek scales, procurrent caudal rays and anal rays than the other two species.

Reliable confirmation requires close examination of fin and scale characters, and may require an experienced ichthyologist or specialist identification service. Do not rely on colour alone, especially where fish are stressed or held temporarily.

No tracks, scats, calls, nests, burrows, sloughs or eyeshine are expected field signs for this fish. The useful field evidence is live or preserved fish, photographs, habitat observations and accurate location data.

  • Record the waterbody, depth, flow condition, water clarity, vegetation and substrate at each capture location.
  • Photograph the whole fish and both sides where possible, while minimising handling time.
  • Treat small striped rainbowfish from suitable Litchfield habitat as potential Little rainbowfish until an appropriately qualified person confirms the identification.

Distribution: National range

The species is a narrow-range endemic of the Northern Territory. All documented localities are below the Tabletop Range escarpment in Litchfield National Park.

The known Australian range is restricted to the Northern Territory. The species is recorded from small streams and pools in sandy areas fed by springs off the Litchfield escarpment.

Documented catchments are Burton Creek in the Finniss River system, and Tolmer Creek and Sandy Creek in the Reynolds River system.

State or territory: Northern Territory only in the supplied records.

Bioregional setting: Below the escarpment of the Tabletop Range, within Litchfield National Park.

Known river systems: Finniss River system and Reynolds River system.

Distribution: Strongholds and population information

The supplied sources identify the three creek areas above as the known distribution, but do not provide abundance estimates, a total population size, population density or a measured population trend.

The species was described from 50 specimens. This is a taxonomic sample and should not be interpreted as a population estimate.

Specimens in species description: 50 specimens.

Global conservation assessment: Vulnerable, criterion D2, assessed 1 February 2019.

Northern Territory listing: Near Threatened.

Population trend: Not reported in the supplied sources.

Habitat: Creeks, pools and swamps

Little rainbowfish habitat is shallow, structurally complex freshwater habitat associated with springs, sandy soils, Pandanus and peat swamp vegetation. Habitat can contract seasonally into refuge holes and connected channels.

The species occurs in small spring-fed creeks, shallow pools, side channels and densely vegetated peat swamp. Streams are lined with Pandanus and may have abundant aquatic vegetation and woody debris.

Water is clear in some streams and heavily stained in some swamp and channel habitats. Swamp and channel habitat may be seasonal or may retract to refuge holes during dry conditions.

Typical depth: Less than 50 cm.

Substrate: Sandy soils.

Water temperature: 23 to 24°C.

pH: Usually 6.0 to 6.5, with one additional value of 5.0 noted in the source.

Conductivity: 15 to 30 microSiemens per centimetre.

Hardness: General and carbonate hardness below 10 ppm.

Habitat: Structural features

Important physical and biological features include spring inflows, shallow flowing or pooled water, lateral connections to peat swamp and side channels, dense aquatic vegetation, Pandanus, woody debris and sandy margins.

The known sites are tropical and low lying below the Tabletop Range escarpment. No elevation limit is provided in the supplied sources.

Elevation limit: Not reported.

Climate setting: Tropical, with water temperatures of 23 to 24°C.

  • Map and protect spring heads, shallow pools, side channels and wet peat depressions within and downstream of work areas.
  • Maintain the connection between main creek channels, swamp habitat and side channels where these features occur.
  • Avoid treating shallow water as insignificant habitat, because the species is specifically associated with very shallow streams and pools.

Foraging habitat: Known information

The supplied sources do not report the Little rainbowfish diet, prey items, feeding rate or foraging range. Field assessment should therefore focus on retaining the shallow, vegetated aquatic structure used by this small freshwater fish.

No species-specific food items or feeding observations are provided. The species is described as a freshwater, benthopelagic fish, meaning it uses the water column and areas close to the bottom.

Its habitat contains aquatic vegetation, woody debris, sandy soils and connected swamp and channel areas. These features should be retained because they form the known physical setting in which feeding occurs.

Diet: Not reported in the supplied sources.

Foraging range: Not reported in the supplied sources.

Feeding position: Benthopelagic.

Foraging habitat: Site practice

Avoid clearing aquatic vegetation, woody debris and dense marginal vegetation in occupied or potentially suitable shallow habitat. Prevent changes that reduce spring flow, disconnect side channels or increase sediment and staining in shallow pools.

Any fish handling, salvage or relocation method should be prepared by a suitably qualified freshwater fish ecologist because the species is very small and extremely shy in captivity.

  • Inspect spring-fed creeks, pools, swamp margins and side channels before works begin.
  • Keep machinery and spoil away from shallow water, spring outlets, sandy margins and connected peat swamp.
  • Record water depth, clarity, temperature and flow condition during surveys so that habitat changes can be compared over time.

Breeding habitat: Breeding facts

Wild breeding biology is poorly documented. Captive observations include spawning and behaviour, but the supplied sources do not provide a wild breeding season or quantified reproductive parameters.

The species has been observed spawning in captivity. No reliable information is supplied on wild spawning dates, egg number, egg size, incubation time, larval habitat, age at maturity or lifespan.

The small, shallow, densely vegetated streams, pools, swamp margins and side channels used by the species are the habitat features most likely to require protection during the breeding period, but a species-specific breeding microhabitat has not been reported.

Breeding season: Not known from the supplied sources.

Spawning: Recorded in captivity; wild spawning details are not provided.

Clutch or egg number: Not reported.

Incubation period: Not reported.

Age at maturity: Not reported.

Lifespan: Not reported.

Breeding habitat: Not specifically identified; known habitat is shallow, vegetated spring-fed creek, pool, swamp and side-channel habitat.

Breeding habitat: Protection during works

Because breeding timing is unknown, do not assume that works outside a conventional breeding season are low risk. Retain shallow water, aquatic vegetation, woody debris, spring inputs and connections to peat swamp and side channels throughout the year.

If fish are detected, stop or modify works that disturb occupied water until the site is assessed by a qualified freshwater fish ecologist.

  • Survey suitable shallow habitat before clearing, dewatering or stream crossings.
  • Avoid dewatering refuge holes, pools and connected side channels.
  • Keep sediment, concrete washout, fuels and other contaminants out of spring-fed habitat.

Sheltering habitat: Known refuges

The species uses shallow aquatic habitat with dense vegetation and woody debris. During seasonal contraction, swamp and channel habitat may retract to refuge holes.

Potential shelter includes shallow pools, refuge holes, side channels, densely vegetated peat swamp, aquatic vegetation and woody debris. The supplied sources do not give dimensions for refuge holes, pools or cover objects.

The species is described as extremely shy in captivity, a behaviour considered an adaptation to shallow habitat.

Known shelter features: Refuge holes, shallow pools, side channels, peat swamp, aquatic vegetation and woody debris.

Cover dimensions: Not reported.

Behaviour: Extremely shy in captivity.

Sheltering habitat: Construction controls

Protect refuge holes and wetted depressions even when adjacent swamp or channel habitat appears seasonal. Avoid filling, isolating or diverting these features during access construction, trenching, drainage works or dewatering.

Maintain dense aquatic and marginal vegetation where it occurs in suitable habitat. Do not remove woody debris unless it is necessary for safety and the habitat consequences have been assessed.

  • Delineate all wet depressions, spring outlets, shallow pools and side channels before works start.
  • Keep exclusion zones around refuge holes and connected wetland habitat.
  • Use fish rescue and dewatering procedures prepared by a freshwater fish ecologist if occupied water must be disturbed.

Threats: Known conservation concerns

The species has a very small known range and specific environmental requirements. Direct loss or alteration of shallow spring-fed habitat is therefore a primary project concern.

The species is a narrow-range endemic restricted to a small area below the Tabletop Range. It has been assessed as Vulnerable under criterion D2 and is listed as Near Threatened in the Northern Territory.

The supplied sources do not quantify the effects of individual threats, but they identify specific requirements for shallow, spring-fed streams, peat swamp, side channels, aquatic vegetation and particular water conditions.

Range condition: Small, geographically isolated range.

Environmental requirements: Shallow spring-fed streams and pools, dense vegetation, connected swamp and channel habitat, and low conductivity water.

Conservation status: IUCN Vulnerable, NT Near Threatened, not listed under the EPBC Act in the supplied information.

Threats: Project-related risks

Clearing can remove Pandanus, aquatic vegetation, swamp vegetation and woody debris around occupied creeks and pools. Excavation, filling, drainage or crossings can remove shallow habitat, isolate side channels and destroy refuge holes.

Changes to spring flow, groundwater, drainage or wet-season connectivity can alter the shallow water conditions on which the species depends. Sediment, staining or other water quality changes may be particularly important where clear spring-fed water is affected.

The supplied sources do not provide species-specific evidence on roads, fire, introduced predators, disease or vehicle strike. These issues should still be considered where a project changes access, water quality, aquatic connectivity or predator access.

  • Avoid clearing spring-fed creeks, shallow pools, Pandanus-lined margins, peat swamp and side channels.
  • Maintain dry-season refuge holes and wet-season lateral connectivity.
  • Prevent sediment and contaminated runoff from entering occupied or connected habitat.
  • Do not introduce fish, move water between catchments or release aquarium fish near the known range.
  • Use a project-specific aquatic fauna plan where works may alter flow, water depth, water quality or wetland connectivity.

Survey methodology: 1. Map and inspect aquatic habitat

Melanotaenia wilsoni is a very small freshwater fish restricted to shallow spring-fed creeks, pools, peat swamps and side channels below the Tabletop Range in Litchfield National Park, Northern Territory. Survey design should target shallow, densely vegetated waterbodies and refuge holes, and should be led by an appropriately licensed freshwater fish ecologist.

Identify all shallow spring-fed creeks, pools, peat swamp margins, side channels and refuge holes within and downstream of the project area.

Record water depth, flow, connectivity, water clarity, aquatic vegetation, woody debris, Pandanus, substrate and evidence that habitats are seasonal or contracting to refuge holes.

Give particular attention to water less than 50 centimetres deep, with clear to heavily stained water and low conductivity conditions consistent with the known habitat.

Known water depth: Generally less than 50 cm; FishBase also records a depth range of 0 to 1 m.

Known water chemistry: pH 6.0 to 6.5, temperature 23 to 24°C, conductivity 15 to 30 µS cm-1, and general and carbonate hardness below 10 ppm.

Survey methodology: 2. Targeted fish sampling

Use a combination of small-mesh seine nets, hand nets and other approved low-impact freshwater fish methods suited to shallow streams, pools and dense aquatic vegetation.

Sample representative habitats along the full affected reach, including open shallow water, Pandanus-lined margins, vegetated swamp edges, woody debris and isolated refuge holes.

Use environmental DNA sampling where physical capture is difficult or where the project may affect turbid, densely vegetated or sensitive habitat, and have samples processed by an appropriately qualified laboratory.

Obtain all required Northern Territory permissions before capture, handling, electrofishing, bait trapping or collection of water samples.

Survey methodology: 3. Timing and detection

Survey while the spring-fed creeks, pools and connected swamp habitats contain water, including any seasonal refuge holes that could retain fish during contraction of the wet habitat.

Repeat sampling across more than one survey occasion when initial conditions are unsuitable, water is absent, or connectivity between streams, swamps and side channels has changed.

Record weather, recent rainfall, water level, water temperature, pH, conductivity, visibility, sampling duration, gear type, reach length and the area sampled.

Treat a non-detection as inconclusive where habitat is dry, access is limited, water is heavily stained, or sampling has not covered refuge holes and vegetated margins.

Survey methodology: Minimum effort and reporting

  • Use a qualified freshwater fish ecologist to set the final gear, reach length, replicate and repeat-survey effort because the supplied documents do not provide a species-specific survey guideline or minimum effort.
  • Survey all suitable habitat that may be directly disturbed and connected habitat that could be affected by altered flow, sediment, pollutants or loss of connectivity.
  • Identify records using current taxonomic nomenclature, retain voucher photographs where appropriate, and record the coordinates, date, observer, method and life stage or size of each fish.
  • Submit confirmed records and negative survey results, where requested, to the relevant Northern Territory wildlife or biodiversity database and include the records in the project environmental report.
  • Report any proposed capture, translocation, injury or mortality to the relevant Northern Territory agency and follow permit conditions.

Before habitat disturbance: Avoid and minimise

Avoid disturbance to the small, isolated population and its connected shallow-water habitat wherever practicable. The known range is restricted to Burton Creek and Tolmer and Sandy creeks in Litchfield National Park, so even short works near a spring-fed creek, swamp or side channel require a site-specific check.

  • Retain spring-fed creeks, shallow pools, Pandanus-lined margins, peat swamp, side channels, aquatic vegetation and woody debris within the project footprint.
  • Maintain surface-water and groundwater connections between streams, swamp habitat and refuge holes, and do not isolate pools or divert spring flows.
  • Keep machinery, stockpiles, spoil, concrete washout, fuels and sediment controls out of waterbodies and their drainage paths.
  • Avoid works that increase sediment, alter low flows, change water temperature or introduce pollutants into clear or stained shallow-water habitat.
  • Use existing access routes and the smallest practicable work area, particularly where works occur in or beside shallow water.

Before habitat disturbance: Approvals and work planning

  • Confirm the current EPBC Act status before works commence and obtain advice on whether referral is required; the supplied listing information records the species as not listed under the EPBC Act, while FishBase records a global Vulnerable assessment.
  • Check Northern Territory threatened-species requirements, permits and park approvals before survey, capture, handling, relocation or habitat disturbance.
  • Prepare a fauna management plan that identifies aquatic habitat, exclusion zones, water-quality controls, fish rescue procedures, authorised personnel, stop-work triggers and reporting responsibilities.
  • Stage works to keep connected aquatic habitat available and avoid leaving fish in isolated pools or dewatered work areas.

Before habitat disturbance: Pre-clearance preparation

  • Complete a pre-clearance aquatic habitat inspection and targeted fish survey before finalising the clearing boundary.
  • Mark no-go aquatic habitat, drainage lines, refuge holes, swamp margins and machinery exclusion zones on plans and on the ground.
  • Install exclusion fencing or waterbody barriers where they can prevent machinery, sediment and personnel entering retained habitat without blocking natural flows or fish movement.
  • Brief all workers and spotter catchers on the species' maximum size of less than 35 millimetres standard length, its dark lateral stripes, and the requirement to stop if an unfamiliar small rainbowfish is found.
  • Confirm an approved holding container, aerated water source, clean nets, identification resources, release location, wildlife carer and aquatic fauna veterinarian before any in-water works.

During habitat disturbance: Daily site controls

The fauna spotter catcher should work as part of the construction team, with authority to stop work where aquatic habitat or fish may be harmed. Protection should focus on preventing entry of sediment, machinery and contaminants into shallow spring-fed water and on rescuing fish only under an approved method.

  • Inspect exclusion fencing, sediment controls, creek crossings, pumps, hoses and drainage diversions before work starts each day and after rainfall.
  • Keep machinery, fuels, chemicals, concrete, spoil and wash water outside retained aquatic habitat and prevent dirty water from entering creeks, pools, swamps or side channels.
  • Maintain natural flows and avoid pumping a pool or channel dry unless an approved fish rescue and dewatering method is in place.
  • Use clean equipment between waterbodies to reduce transfer of sediment, pathogens and aquatic weeds.
  • Record water level, turbidity, visible fish, water releases, incidents and corrective actions each day.

During habitat disturbance: Spotter catcher and machinery interface

  • Position the spotter catcher where the operator can see and hear clear stop instructions before machinery enters or works beside aquatic habitat.
  • Use a slow, staged approach to dewatering, excavation, bank access and removal of woody debris, with the spotter catcher checking each work area before the next machine movement.
  • Keep the spotter catcher independent of production pressure and give the nominated environmental officer authority to stop work.
  • Do not allow the spotter catcher to enter deep, unstable, fast-flowing, contaminated or otherwise unsafe water; use a licensed fish ecologist or other authorised person for aquatic capture.

During habitat disturbance: If a fish is found

  • Stop the activity that could affect the fish and keep machinery, pumps and people clear of the water until the authorised aquatic fauna specialist assesses the situation.
  • Do not handle, net, retain or relocate the fish unless the person, method and project approval authorise that action.
  • If capture is approved, use clean, wet equipment and keep handling time and exposure out of water to a minimum, consistent with the approved fish-rescue method.
  • Release the fish into nearby retained habitat with suitable water quality and connectivity, only where the permit or fauna management plan allows relocation and the receiving site is approved.
  • Do not release fish into an unassessed waterbody, another catchment or water with materially different temperature or chemistry.
  • Send injured or moribund fish to an aquatic veterinarian or authorised wildlife carer and notify the relevant Northern Territory authority as required.

During habitat disturbance: Stop-work triggers and records

  • Stop work if a suspected Little rainbowfish is found, a retained waterbody becomes isolated, a pump or diversion threatens to dewater habitat, sediment enters water, or a spill reaches a drainage path.
  • Stop work if works damage Pandanus-lined creek margins, peat swamp, aquatic vegetation, woody debris or refuge holes outside the approved footprint.
  • Photograph and map every find, record the date, coordinates, habitat, water conditions, identification evidence, action taken, release location and people involved.
  • Report injury, mortality, unapproved capture, pollution, fish stranding, loss of connectivity and any breach of permit or management-plan controls to the project environmental officer immediately.

After habitat disturbance: Post-clearance inspection

Post-work checks should confirm that retained aquatic habitat remains connected, clean and suitable for a very small fish that uses shallow streams, pools, peat swamps and side channels. The project should not treat completion of earthworks as completion of the fauna risk.

  • Inspect the work area, retained creek margins, pools, swamp edges, side channels and downstream receiving waters after machinery leaves.
  • Check for stranded fish, blocked flow paths, sediment deposition, bank collapse, fuel or concrete contamination, altered water level and loss of aquatic vegetation or woody debris.
  • Remove temporary barriers, hoses, spoil and waste only when doing so will not release sediment or isolate fish.
  • Complete a final check after the first substantial flow event or other project-defined rainfall response, where the works may have altered drainage.

After habitat disturbance: Monitoring and reporting

  • Monitor water quality and habitat condition at locations upstream, within and downstream of the works where there is a plausible pathway for impact.
  • Repeat targeted fish sampling where the project caused dewatering, changed connectivity, affected a spring, altered a creek crossing or produced a reportable incident.
  • Compare post-work observations with the pre-work record, including water depth, temperature, pH, conductivity, clarity, aquatic vegetation, woody debris and fish detections.
  • Provide the regulator, land manager and project owner with survey data, incidents, fish-rescue records, photographs, maps, permit compliance and corrective actions.

After habitat disturbance: Controls and offsets

  • Maintain sediment, erosion, spill, access and exclusion controls until disturbed ground is stable and retained aquatic habitat is protected from construction traffic.
  • Repair failed controls promptly and escalate any continuing water-quality or connectivity impact to the environmental officer and relevant authority.
  • Apply any approved offset, restoration or conservation measure required by Northern Territory approvals or project conditions; do not use an offset as a substitute for avoiding the known habitat.

Rehabilitation actions: Restore habitat structure

Rehabilitation should recreate the shallow, vegetated and connected freshwater conditions used by Little rainbowfish. The priority is to restore spring-fed flow paths, swamp and side-channel connections, aquatic cover and clean water rather than simply revegetating the bank.

  • Re-establish shallow creek and pool profiles that retain water without creating isolated stagnant pits or severing natural flow paths.
  • Replant locally appropriate native vegetation, including Pandanus where it formed part of the original creek margin, and retain or replace aquatic vegetation and woody debris in accordance with the restoration design.
  • Reinstate stable, gently graded banks and sandy substrates where these were removed, while preventing fine sediment from entering the channel during establishment.
  • Reconnect side channels, peat swamp margins and refuge holes where project drainage or access works interrupted them.

Rehabilitation actions: Water quality, weeds and predators

  • Maintain erosion and sediment controls until groundcover and bank stability are established.
  • Prevent introduction of weeds, pathogens, chemicals and altered water chemistry through clean equipment, controlled access and appropriate wash-down procedures.
  • Inspect restored waterbodies for invasive aquatic plants and fish, and remove or report them under the applicable Northern Territory arrangements.
  • Manage weeds and excessive disturbance around aquatic vegetation, and use fire planning that protects Pandanus, peat swamp margins and riparian cover from damaging high-intensity fire.

Rehabilitation actions: Monitoring and success measures

  • Monitor restored reaches for flow continuity, water retention, depth, pH, temperature, conductivity, clarity, aquatic vegetation, Pandanus recovery, woody debris and bank stability.
  • Use targeted fish surveys and, where appropriate, eDNA to check whether Little rainbowfish is present after habitat has re-established; use the same methods and comparable reaches as the baseline survey.
  • Treat restoration as successful only when the approved design criteria are met, water remains connected through the intended habitat, sediment and pollution controls are no longer needed, and no project-related barrier or degraded refuge hole remains.
  • Continue monitoring for the period required by approval conditions and escalate failed plantings, erosion, weed invasion, altered flows, fish mortality or non-detection from otherwise suitable habitat.

Sources