Twospine Blackfish (Gadopsis bispinosa)

Conservation status

Australian Capital Territory, Nature Conservation Act 2014: Vulnerable.

Breeding season

The calendar below describes the typical pattern documented for the species. Timing can vary with water temperature, fish size and local conditions.

The supplied documents do not provide a complete month-by-month movement or recruitment dataset. All months should be treated as potentially occupied in suitable habitat.

• Breeding and spawning: Spawning occurs in November and December. Larger fish spawn earlier than smaller fish.
• Eggs and guarded larvae: Eggs hatch after approximately 16 days at 15°C. The male guards and fans the egg mass, and the larvae leave the spawning site after approximately 3 weeks.
• Juvenile recruitment: Young-of-year and juveniles use aquatic insect larvae, especially mayflies and midges. The documents do not define a fixed recruitment window across the full range.
• Nocturnal feeding movement: The species is nocturnal throughout the year. In a Cotter reservoir population, adults moved from daytime shelters to fringing reedbeds at night and returned before day.
• Highest clearing risk: Risk is highest during November and December spawning, when egg masses and guarding males occupy gaps between cobbles and boulders. Extend precautions into January where eggs or larvae remain at the site, and apply sediment controls year-round.

Identification

Two-spined blackfish are small to medium-sized native fish with a long dorsal fin and distinctive mottled colouring. They are generally encountered in rocky, cool-water streams or reservoirs rather than in shallow headwater channels.

The back and sides are yellowish-brown to olive green, often with prominent irregular giraffe-like blotches. The belly is creamy to light grey. The body has very small scales and a thick mucous coating.

The head is relatively large, the mouth is large, and the tail is rounded. The dorsal fin is low and long, extending almost to the tail. The pelvic fins are reduced to a pair of fine, white, divided filaments below the throat.

Typical length: Usually less than 240 mm total length.

Maximum length: 325 mm total length.

Maximum recorded weight: 195 g.

Identification: Key field character

The dorsal fin has 1 to 3 spines, usually 2. This is the main feature separating the species from Northern river blackfish, which lacks these dorsal spines.

• Check the dorsal fin for one to three spines, usually two, when handling is authorised and safe.
• Record the mottled body pattern, reduced filament-like pelvic fins, long dorsal fin and rounded tail.
• Use a wet measuring board or other approved method if the fish must be measured, and minimise handling because the species has a heavy mucous coating.

Identification: Field signs and sex

There are no documented tracks, scats, calls, nests, hollows, sloughs or eyeshine to use as field signs. Detection is generally by targeted aquatic survey, visual observation in suitable rocky habitat, or capture and release under the relevant permit and method.

The sources do not describe reliable external differences between males and females. Males can be identified functionally during breeding because they guard and fan the adhesive egg mass until the larvae are nearly free-swimming.

• In the upper Cotter River, medium and large fish may carry numerous small red bite marks, particularly on the lower sides and belly, associated with a small leech.
• Juveniles may occur in large groups, while adults are generally solitary.

Distribution: States and catchments

The species is restricted to the Murray-Darling Basin in south-eastern Australia. Records are concentrated in upland, slopes and montane river valleys, with very few records from lowland areas.

In Victoria, the species occurs in the upper Goulburn, Broken, Ovens, Mitta Mitta and upper Murray catchments. In New South Wales, it is known from the Goodradigbee, upper Murray above Lake Hume, Tumut and Goobarragandra catchments.

In the ACT, it is currently present only in the Cotter catchment. It was formerly recorded from the Murrumbidgee and Paddys rivers, and possibly the Naas and Gudgenby system.

Known Australian range: North-eastern Victoria, the ACT and south-eastern New South Wales, all within the Murray-Darling Basin.

Highest recorded elevation: 1070 m in the Cotter catchment, ACT.

Distribution: Strongholds and subpopulations

Important known areas include the upper Cotter River in the ACT, the upper Murray and associated tributaries, the upper Tumut and Goobarragandra systems, and upland Victorian catchments including the Goulburn, Broken, Ovens and Mitta Mitta.

The ACT population is currently confined to the Cotter catchment. The upper Cotter River population has been severely reduced following post-fire sedimentation, although a strong recruitment pulse was recorded in 2021.

Distribution: Abundance and trend

Where present, the species can be abundant, but it has declined in several ACT and New South Wales catchments. A recent study also reported a second, undescribed close relative currently known only from the Goulburn catchment in Victoria, so survey identifications in that area should be checked carefully.

Sustainable Rivers Audit records: 2520 individuals recorded from 2004 to 2013 across 7 river valleys.

MDB Fish Survey records: 1803 individuals captured from 7 river valleys between 2014 and 2021 to 2022.

Elevational distribution of audit records: 52% from upland zones, 29% from slopes, 18% from montane zones and 1% from lowlands.

Recent ACT fire response: Blackfish abundance in the upper Cotter River declined by approximately 80% after post-fire rainfall and sedimentation events.

Habitat: Stream and reservoir habitat

Two-spined blackfish use cool, clear upland and montane streams with abundant instream cover. They are most associated with forested catchments that have low sediment input and structurally diverse rocky beds.

Typical habitat is in medium to large streams with boulder and cobble cover. The species is generally absent from very small headwater creeks that are shallow and have very fast flows.

It also occurs in upland reservoirs where rocky margins and suitable shelter remain available.

Preferred elevation context: Upland and montane streams, with records up to 1070 m.

Preferred substrate: Boulders and cobbles, including spaces between rocks.

• Retain boulders, cobbles, pools, undercut cover and other instream structure within and downstream of work areas.
• Prevent soil, spoil, ash, sand, silt and gravel from entering occupied or connected waterways.
• Treat clear, cool water and low sediment input as key habitat attributes when assessing a site.

Habitat: Riparian setting

The species is usually found in forested catchments. In the upper Cotter project area, nearby riparian communities include Black Sallee grass-herb woodland, Snow Gum and Candlebark tall grassy woodland, River Tussock, Kangaroo Grass and Rush wet tussock grassland, fen sedge aquatic herbfield and sub-montane moist tussock grassland.

• Keep machinery, access tracks, stockpiles and refuelling areas away from stream banks and connected drainage lines where practicable.
• Use erosion and sediment controls before ground disturbance starts, and maintain them until exposed surfaces are stabilised.

Foraging habitat: Diet and feeding sites

Two-spined blackfish feed on or near the stream bed, mainly at night. Adults and juveniles use different prey sizes, and reservoir fish may move between daytime shelter and night-time feeding areas.

The diet is dominated by aquatic insect larvae, particularly mayflies, caddisflies and midges. Fish and crayfish are taken occasionally. Adults also consume larger prey, including caddisfly larvae and terrestrial invertebrates.

Young-of-year and juvenile fish eat proportionally more mayfly and midge larvae than adults. In a Cotter River reservoir population, adults moved at night from daytime shelters to fringing reedbeds, where they were assumed to feed, then returned to shelter before day.

Adult stream home range: Approximately 15 to 23 m.

Reservoir movement: Greater than in streams, including nocturnal movement to fringing reedbeds.

• Protect rocky stream margins, cobble beds, pools and fringing aquatic vegetation that support aquatic invertebrate prey.
• Avoid fine sediment inputs that can cover insect-bearing cobble surfaces and reduce habitat complexity.
• Schedule noisy or bright night works away from occupied water where practicable, because the species is nocturnally active.

Breeding habitat: Breeding biology

Breeding occurs in late spring and early summer. Spawning sites are closely associated with stable cobbles and boulders that provide a protected surface for the egg mass.

Females lay one adhesive egg mass in gaps between cobbles or boulders, with the eggs attached to the upper surface of a rock. The male guards and fans the eggs until the larvae have nearly used their yolk reserves.

Spawning season: November to December.

Egg mass size: Approximately 80 to 420 eggs.

Egg diameter: Approximately 3.5 mm.

Hatching time: Approximately 16 days at 15°C water temperature.

Post-hatching period: After approximately 3 weeks, the male and larvae leave the spawning site.

Age at first breeding: Females commence breeding in their second or third year, at more than approximately 120 mm length.

Breeding timing by size: Larger individuals spawn earlier in the season than smaller individuals.

Breeding habitat: Features to protect

Breeding habitat requires clean, stable cobble and boulder substrates with gaps that allow an egg mass to be attached to the upper surface of a rock. Sediment deposition, rock removal and rapidly fluctuating reservoir levels can damage or expose egg masses.

• Inspect rocky margins and instream boulder and cobble habitat before works between November and January, especially where water levels are stable and fish are present.
• Do not disturb suspected egg masses, guarding males or occupied rock gaps.
• Prevent sediment pulses and sudden changes in water level downstream of works.

Sheltering habitat: Daytime refuges

Shelter is provided by instream boulders, cobbles and other complex rocky cover. Adults use daytime refuges and leave them at night to feed.

The species is associated with abundant instream cover, usually boulders and cobble. Spawning and sheltering sites are likely to include gaps between rocks, although the sources do not provide dimensions for individual refuges.

Adult fish in streams have small home ranges of approximately 15 to 23 m, indicating that removal or disturbance of local shelter can affect fish within a short reach.

Known refuge type: Gaps among boulders and cobbles, with additional structural cover in rocky stream habitat.

Refuge dimensions: Not specified in the supplied documents.

• Retain boulder fields, cobble beds, pools and rock gaps in occupied reaches.
• Avoid placing sediment, spoil, temporary crossings or construction materials in rocky instream cover.
• If habitat must be modified, obtain species-specific aquatic ecology advice and approvals before works.

Sheltering habitat: Reservoir behaviour

In the Cotter River reservoir population, adults used daytime shelter habitat and made nocturnal movements to fringing reedbeds before returning to shelter. Both the rocky shelter and connected feeding margins should therefore be considered when planning reservoir-edge works.

• Avoid isolating rocky shelter from nearby fringing vegetation and shallow feeding margins.
• Manage lighting, access and water-level changes around reservoir margins to reduce disturbance and exposure of habitat.

Threats: Construction and clearing risks

The main project risks are sedimentation, loss of instream structure, altered water levels and water-temperature change. Fire-related sediment pulses have caused major recent declines in the upper Cotter River.

Sediment can smother eggs and spawning sites, infill pools and reduce substrate diversity. Direct removal or burial of boulders and cobbles removes shelter and breeding habitat.

Machinery access, vehicle crossings, bank disturbance, spoil placement and poorly controlled drainage can deliver sediment to occupied streams. Works in or beside waterways should be planned to avoid direct contact with rocky habitat and to maintain effective erosion and sediment controls.

• Keep clearing and access footprints outside riparian areas and drainage lines where practicable.
• Do not ford occupied streams unless specifically authorised and designed to prevent bed disturbance and sediment release.
• Use clean, local or virgin materials for any approved habitat works, and control weeds and erosion around access routes.
• Inspect controls after rainfall and remove accumulated sediment before it reaches the stream.

Threats: Fire and sedimentation

The 2019 to 2020 bushfires were followed by rainfall events that caused severe sedimentation in affected streams. In the upper Cotter River, blackfish and Rainbow trout abundance declined by approximately 80% immediately after these events. Large adults mainly survived, and a strong recruitment pulse occurred in 2021, but substrate diversity and pool depth remained affected two years after the fire.

• Treat recently burnt catchments as high-risk areas for post-fire sediment delivery, especially before forecast heavy rain.
• Maintain drainage and sediment controls on fire trails, temporary access routes and disturbed banks after fire.
• Include downstream habitat and sediment transport in post-fire project risk assessments.

Threats: Water quality, flow and introduced fish

Cold-water pollution is a significant threat. Trout can prey on Two-spined blackfish and compete for food. Rapidly fluctuating reservoir levels can expose egg masses, while sediment and altered flow conditions can reduce suitable spawning and shelter habitat.

• Assess temperature, turbidity, sediment release and flow changes for works connected to occupied streams or reservoirs.
• Avoid rapid drawdown or other water-level changes during the November to December spawning period and the following egg and larval development period where practicable.
• Report fish kills, unusual turbidity, exposed egg masses or major habitat disturbance through the project incident process and relevant government authority.

Threats: Conservation and regulatory context

The species is not listed under the EPBC Act but is Vulnerable under the ACT Nature Conservation Act 2014. In the ACT, works affecting the species can trigger planning and environmental assessment requirements, particularly in reserves such as Namadgi National Park.

• Confirm state and territory approval, survey and handling requirements before aquatic works.
• Use the ACT Vulnerable status as a project screening trigger for works in the Cotter catchment and other known ACT locations.
• Consider cumulative effects where several activities affect the same short stream reach, because adult movement in streams is limited.

Survey methodology: 1. Nocturnal habitat inspection

Two-spined Blackfish is a nocturnal, bottom-dwelling fish of cool, clear upland and montane streams. Surveys should target rocky instream habitat and use methods suited to fish sheltering between boulders and cobble, with timing adjusted around the November to December spawning period.

Inspect representative reaches after dark because adults are nocturnally active and may move from daytime shelters to feeding areas.

Search pools, boulder margins, cobble beds, undercut banks and other instream cover, particularly where sediment input is low.

Record waterway condition, water temperature, flow, depth, substrate, cover, sediment deposition, barriers and nearby disturbance.

Do not rely on a single visual inspection, because the species has a thick mucous coating, mottled colouring and a restricted home range of about 15 to 23 metres in streams.

Preferred habitat: Cool, clear upland or montane streams with abundant boulders and cobble cover.

Activity period: Mainly nocturnal.

Adult stream home range: Approximately 15 to 23 metres.

Survey methodology: 2. Targeted fish capture survey

Use a qualified aquatic ecologist to select and operate backpack electrofishing, seine, fyke, bait or other approved fish sampling gear suitable for the waterway and its management requirements.

Sample all available habitat units within the proposed disturbance area and comparable retained habitat upstream and downstream, including boulder and cobble cover, pools and deeper margins.

Set effort before fieldwork and report the number and type of passes, nets or traps, trap nights, reach length, sampled area, water temperature, flow and weather conditions.

Use extra care during the November to December spawning period, when eggs may be attached to the upper surfaces of rocks in gaps between cobbles or boulders.

Measure captured fish where authorised, photograph distinguishing features where practical, and release them promptly into the capture location unless an authorised fauna handler directs otherwise.

Spawning period: November to December.

Typical adult size: Usually less than 240 millimetres total length, with a maximum recorded length of 325 millimetres.

Egg attachment site: The upper surface of rocks in gaps between cobbles or boulders.

Survey methodology: 3. Reproductive habitat assessment

Survey rocky margins and cobble and boulder deposits for suitable spawning crevices, adhesive egg masses and male guarding behaviour.

Avoid moving rocks during the breeding period unless the work is authorised, the risk has been assessed and an aquatic ecologist is present.

If eggs or guarding fish are found, record the location, substrate, water depth, flow, water temperature and disturbance risk, then establish a work exclusion area around the site under the fauna management plan.

Treat egg masses as sensitive aquatic habitat, because hatching takes about 16 days at 15 degrees Celsius and larvae remain associated with the spawning site for about three weeks.

Egg size: Approximately 3.5 millimetres in diameter.

Hatching time: Approximately 16 days at 15 degrees Celsius.

Post-hatching association: The male and larvae leave the spawning site after approximately three weeks.

Survey methodology: Minimum effort and reporting

• Use a competent aquatic ecologist to define survey effort where no species-specific guideline or prescribed effort is available in the project documents.
• Survey the full area that could be affected, plus suitable retained habitat needed for comparison and relocation decisions.
• Record survey dates, start and finish times, surveyors, methods, effort, reach lengths, sampled areas, habitat observations, weather, flow, water temperature, captures, non-detections and photographs.
• Map every detection and sensitive habitat feature, including spawning crevices, boulder fields, cobble beds and refuge pools.
• Submit confirmed records to the relevant state wildlife or biodiversity database and provide records to the project approval authority where required.
• Retain permits, animal ethics or scientific licences, chain of custody details and fauna handling records with the project environmental records.

Before habitat disturbance: Avoid and minimise

Plan the work around the species' dependence on rocky instream cover and its restricted distribution. The ACT population is listed as vulnerable under the Nature Conservation Act 2014, and the supplied information identifies decline in several ACT and New South Wales catchments.

• Keep works out of cool, clear upland and montane streams wherever practicable.
• Retain boulder fields, cobble beds, deep pools, undercut margins, stable banks and other instream shelter.
• Prevent sediment, concrete washout, fuel, chemicals and turbid runoff from entering the waterway.
• Maintain connected stream habitat and avoid creating barriers to fish movement, particularly in reaches linking pools and rocky refuge habitat.
• Do not place temporary crossings, pumps, access tracks or stockpiles on spawning substrate or within retained rocky habitat unless the approved method requires it.

Before habitat disturbance: Approvals and planning

EPBC status in supplied material: Not listed.

ACT status in supplied material: Vulnerable.

• Confirm the state or territory conservation status, scientific licence requirements, fish collection approvals, animal ethics requirements and waterway work approvals before survey or disturbance.
• The supplied information identifies the species as not listed under the EPBC Act and vulnerable in the ACT, so complete the current EPBC Act self-assessment and check the relevant state or territory legislation before relying on that status.
• Prepare a fauna management plan covering survey, aquatic fauna handling, relocation, water quality, sediment control, spawning habitat protection, stop-work triggers and incident reporting.
• Schedule instream works outside November to December where practicable, and obtain aquatic ecologist advice before working during the spawning period.
• Stage works so that undisturbed rocky refuge habitat remains available while each work area is isolated and completed.

Before habitat disturbance: Site preparation

• Complete a pre-clearance aquatic survey immediately before works, with timing and repeat surveys set by the aquatic ecologist and the approved fauna management plan.
• Mark no-go areas around boulder and cobble habitat, pools, spawning sites, retained banks and riparian vegetation.
• Install exclusion fencing, sediment controls and access controls where they can be placed without damaging stream banks or aquatic habitat.
• Brief all operators on the species' appearance, nocturnal behaviour, sensitive habitat and the stop-work procedure.
• Inspect the work area for stranded fish, isolated pools, exposed egg masses and changes in flow before machinery enters the site.

During habitat disturbance: Daily controls

Keep machinery, sediment and changes in flow away from occupied rocky habitat. A fauna spotter catcher must work under the approved plan and must have authority to stop work when fish or sensitive habitat are at risk.

• Check exclusion fencing, sediment barriers, flow controls, pumps, bypasses and access routes at the start of each shift and after rainfall or flow changes.
• Keep fuels, chemicals, spoil, concrete, wash water and fine sediment out of the channel.
• Use clean equipment and prevent transfer of aquatic weeds, pathogens and sediment between waterways.
• Maintain sufficient water depth and flow in retained habitat, and check downstream and upstream areas for stranded or isolated fish whenever flow is diverted or a pool is dewatered.
• Limit night lighting, noise and in-channel movement to what the approved method requires, particularly near nocturnal feeding and shelter areas.

During habitat disturbance: Spotter and machinery procedure

• The fauna spotter catcher must remain in communication with the supervisor and plant operator and must have a clear view of the active work area.
• Stop plant before any rock, cobble, pool, bank or flow-control structure is disturbed if the spotter cannot confirm that the area is clear.
• Inspect material before it is lifted, moved or stockpiled, because fish may shelter between boulders and cobble.
• Use an aquatic fauna specialist for fish capture, handling and relocation; a terrestrial fauna spotter catcher should not undertake these tasks unless separately authorised and competent.
• Keep a safe recovery container, suitable water from the capture location and approved handling equipment available where dewatering or rock movement could strand fish.

During habitat disturbance: Animal finds and stop-work triggers

• Stop work immediately if a Two-spined Blackfish, egg mass, guarding male, stranded fish or potentially occupied spawning crevice is found in the disturbance area.
• Only an authorised and appropriately trained person may capture, handle or relocate the fish.
• Relocate an animal only under the approved plan, to nearby suitable retained habitat with comparable cool, clear water, depth, flow, boulder or cobble cover and shelter.
• Do not move fish across catchments or release them into reservoirs or streams without written approval from the relevant authority.
• Send injured or distressed fish to the nominated aquatic wildlife carer, veterinarian or government wildlife authority, using the approved transport method.
• Resume work only after the aquatic ecologist or other authorised person confirms that the trigger has been resolved and records the decision.
• Record the location, time, condition, measurements, photographs, action taken, release location, personnel and authority for every animal interaction.

After habitat disturbance: Completion checks

Clearance is not complete when machinery leaves the channel. Check for stranded animals, unstable habitat and sediment impacts, then maintain controls through the next flow and rainfall events.

• Inspect the entire work footprint, access route, bypass, dewatered area and downstream margin for stranded or injured fish and exposed egg masses.
• Confirm that retained boulders, cobble, pools, undercut margins and connected flow paths remain available and are not buried by spoil or sediment.
• Remove temporary pumps, crossings, pipes, fencing and stockpiles when the approved method allows, without disturbing retained habitat.
• Photograph and map the completed condition, including any habitat removed, retained or reinstated.

After habitat disturbance: Monitoring and reporting

• Monitor turbidity, sediment movement, flow, pool persistence and the condition of reinstated rocky habitat during works and after significant rainfall or flow changes.
• Undertake post-work fish surveys where required by the approval or fauna management plan, using the same reach boundaries and comparable methods as the pre-work survey where practicable.
• Report captures, mortalities, injuries, egg impacts, fish relocations, exceedances, failed controls and stop-work events to the project manager and relevant authority within the time required by the approval.
• Keep controls in place until banks, water quality, flow paths and instream habitat are stable and the environmental representative signs off the area.
• Review monitoring results and amend the work method if sedimentation, habitat loss or fish decline is detected.

After habitat disturbance: Offsets and residual impacts

• Assess residual loss of occupied habitat, spawning substrate and connectivity against the applicable approval and threatened species requirements.
• Apply any approved offset, compensatory habitat measure or recovery action required by the relevant jurisdiction.
• Do not treat constructed habitat as a substitute for avoiding damage to existing boulder and cobble habitat unless the competent authority has accepted that approach.

Rehabilitation actions: Restore stream habitat

Rehabilitation should restore the physical conditions used for shelter, feeding and breeding. The priority is clean, connected, structurally varied stream habitat, followed by monitoring of fish use and habitat stability.

• Reinstate stable boulders and cobble to create gaps, overhead cover, pool margins and potential spawning crevices.
• Use clean, local or otherwise approved virgin rock and cobble that matches the existing substrate and does not introduce fine sediment.
• Restore pool depth, substrate diversity, low-flow connections and stable banks where these were affected by the project.
• Do not leave loose spoil, sand or fine gravel over egg attachment surfaces or between retained boulders.
• Use constructed shelter and breeding structures only where they are designed, installed and managed by the relevant aquatic authority; the ACT has trialled anchored rock bases with an upper timber section in the Upper Cotter River.

Rehabilitation actions: Riparian and catchment recovery

• Re-establish native riparian vegetation appropriate to the local stream community and protect it from trampling, grazing and vehicle access.
• Control weeds and erosion so that rainfall cannot deliver sediment to spawning and refuge habitat.
• Manage fire recovery and post-fire erosion in the catchment, because sedimentation after the 2019 to 2020 fires severely affected Upper Cotter River habitat.
• Assess trout interactions where relevant, because trout predation and competition for food are identified threats to Two-spined Blackfish.

Rehabilitation actions: Success measures and maintenance

• Set measurable success criteria before rehabilitation, including stable boulder and cobble placement, maintained pool depth, connected flow, low sediment deposition, functioning erosion controls and persistence of riparian cover.
• Inspect reinstated and constructed habitat after high flows, during the November to December spawning period and at the monitoring intervals specified in the approval.
• Use targeted fish surveys to test whether Two-spined Blackfish occupy the restored shelter and breeding habitat, and compare results with nearby retained habitat.
• Remove, repair or redesign constructed structures that trap debris, divert flow, become unstable or create a risk to fish or downstream habitat.
• Record fish detections, egg masses, habitat condition, water quality, maintenance actions and weed or predator control outcomes in the rehabilitation monitoring report.

Sources

• Two-spined blackfish - Gadopsis bispinosa (previously G. ..., mdb.fish: https://www.mdb.fish/fish-fact-sheets/two-spined-blackfish
• Gadopsis bispinosa (Two-Spined Blackfish) - Canberra Nature Map, canberra.naturemapr.org: https://canberra.naturemapr.org/species/5611
• [PDF] Threatened Two-spined Blackfish habitat rehabilitation project, ACT Government: https://www.planning.act.gov.au/__data/assets/pdf_file/0008/2462615/ESO202400020-threatened-two-spined-blackfish-habitat-rehabilitation.pdf