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Mary River Cod (Maccullochella mariensis)

The Mary River cod is a large, endangered freshwater fish found in south-east Queensland, mainly in the Mary River system. On development sites, clearing, bank disturbance, sedimentation, loss of riparian shade, removal of submerged timber and changes to flow or fish passage can damage occupied habitat and breeding sites.

Conservation status

Commonwealth, EPBC Act 1999: Endangered.

Queensland, Nature Conservation Act 1992: Endangered.

Breeding season

Timing varies with water temperature, flow and local conditions. The pattern below is a practical guide for the Mary River system, based mainly on hatchery observations, survey guidance and recent field monitoring.

Survey guidance recommends avoiding sampling during the August to October spawning period where breeding could be affected. Works may still harm cod outside this period if they remove habitat, interrupt fish passage, cause sedimentation or alter flows.

  • Breeding: Spawning is generally reported in spring, with hatchery breeding from August to October. Field nesting activity has been recorded when water temperatures rise towards or above 20°C.
  • Eggs and guarded larvae: Eggs hatch after about 4 to 7 days at 21°C. The male guards the brood until larvae disperse about 7 to 9 days after hatching.
  • Movement: Cod may move into smaller tributaries in late winter. Movements may be upstream in spring and summer and downstream in winter, with some fish travelling more than 30 km during high flows.
  • Home-range use and feeding: Between longer movements, individuals may remain in a 100 to 1000 m home range for years. Feeding and patrolling are most common from dusk to dawn.
  • Highest clearing risk: August to October carries the highest species-specific risk because spawning, egg guarding and early larval development occur then. Clearing, bank disturbance, dewatering, vibration and removal of logs or cavities should be avoided in occupied habitat during this period.

Identification

Mary River cod are large, elongate freshwater fish with a distinctive cod-like profile and mottled colouration. Identification should be confirmed by an experienced freshwater fish ecologist, particularly for juveniles and where other Maccullochella species or stocked fish may occur.

The species has a concave head profile and a protruding lower jaw. The back ranges from golden-yellow, green or dark brown, with black to dark green mottling. Mottling may extend onto the grey or whitish underside. The dorsal, pectoral, caudal and anal fins are clear to dark and mottled near their bases. Soft dorsal, anal and caudal fins have thin whitish margins, and the whitish pelvic fins have white filaments.

Identification: Size and field appearance

Mary River cod are generally smaller than Murray cod. Fish over 70 cm and 5 kg are uncommon, although the largest documented specimen weighed 23.5 kg. Unconfirmed local reports describe fish weighing up to about 36 to 38 kg.

Adults may be around 500 to at least 700 mm long at nesting sites. Size alone is not sufficient for identification.

Documented maximum: 23.5 kg.

Common upper size noted in conservation advice: About 70 cm and 5 kg.

Identification: Similar fish and field signs

The Mary River cod can be confused with Murray cod, trout cod and eastern freshwater cod. Useful features include the concave head, protruding lower jaw, relatively long pelvic fins, deep and short caudal peduncle, and distinctive mottled colouration. Juvenile Mary River cod may be confused with small spangled perch, so expert identification is recommended.

There are no reliable tracks, scats, calls or eyeshine signs. The most useful field sign is a large fish occupying a submerged hollow log, log pile, undercut bank or rock cavity. During breeding, a male may be visible guarding eggs or larvae inside a cavity.

  • Photograph the fish from the side and record its total length, colour pattern, fin margins and exact waterbody location.
  • Do not remove natural logs, root masses, undercut banks or other suspected nesting structures before the area has been assessed.
  • Use appropriate fish survey methods and minimise handling, disturbance and time out of water.

Distribution: Current Queensland range

The Mary River cod is restricted to Queensland. Its remaining natural range is concentrated in the Mary River system, with historical records from several other south-east Queensland coastal catchments.

The species is known from the Mary River system in south-east Queensland. The conservation advice identifies three main areas of occurrence: Tinana and Coondoo creeks upstream of Tinana Barrage, Six Mile Creek downstream of Lake Macdonald, and upper Obi Obi Creek.

Reports also exist from Widgee, Glastonbury, Amamoor and Yabba creeks and other parts of the Mary River, but survey information indicates that numbers in many of these areas are very low.

  • Treat suitable habitat in the Mary River system as potential Mary River cod habitat unless current survey information demonstrates otherwise.
  • Include downstream reaches and connected tributaries in impact assessments because fish may move between pools, tributaries and the main river.

Distribution: Historical range and strongholds

Freshwater cod formerly occurred in the Brisbane, Stanley, Albert, Logan and Coomera river systems. They are now considered very rare or extinct in those systems, although Mary River cod have been stocked into impoundments within and outside the Mary system.

The best documented remaining areas are Tinana to Coondoo Creek, Six Mile Creek and upper Obi Obi Creek. These should be treated as high sensitivity areas for works near watercourses.

State range: Queensland only, based on the supplied sources.

Former known range: The Mary River system and parts of the Brisbane-Stanley, Albert-Logan and Coomera systems.

Remaining range estimate: Less than 30% of the former known range in the Mary River system was estimated in the recovery plan.

Distribution: Population status

A reliable total population estimate is not provided. The species has a small, fragmented population, with very low numbers reported from many former or peripheral locations. Impoundment and degraded habitat have isolated remnant populations.

Stocking has occurred since 1983, but the available sources do not establish whether stocked populations are self-sustaining.

Total population: Not known from the supplied sources.

Population pattern: Small, fragmented remnant populations, with stronger concentrations in a few Mary River tributaries.

Habitat: Stream and pool habitat

Mary River cod use freshwater habitats from high-gradient rocky upland streams to large, slow-flowing lowland pools. They are strongly associated with shaded water, deep pools and complex instream cover.

Preferred habitat includes large, deep, shaded pools with slow-flowing water. The species also occurs in high-gradient rocky upland streams and cooler side branches or tributaries. Survey guidance identifies pools about 0.8 to 3.2 m deep as suitable habitat.

Cod may occur immediately downstream of riffles or other channel constrictions, where flowing water may concentrate prey. Open water without cover is generally avoided.

Reported suitable pool depth: About 0.8 to 3.2 m.

Water conditions: The species occurs across a wide range of pH, conductivity, dissolved oxygen and water clarity conditions in the Mary system.

Habitat: Riparian and instream structure

Important habitat features include overhanging native vegetation, stable shaded banks, submerged logs, log piles, root masses, undercut banks, rock ledges and boulders. Tinana and Coondoo creeks retain substantial riparian vegetation, shading and woody cover, while Obi Obi Creek provides deep rocky habitat with less timber.

Radiotracking observations found cod within 1 m of instream timber in more than 90% of observations. Removing riparian vegetation can reduce shade, bank stability and future recruitment of large wood.

  • Retain riparian vegetation, large trees that can supply future instream timber, natural snags, root masses and undercut banks.
  • Prevent sediment from entering pools and burying logs, rock cavities, macrophyte beds or interstitial spaces.
  • Assess both the work area and connected upstream and downstream habitat.

Habitat: Site setting

The supplied sources do not provide a reliable elevation limit or formal bioregional boundary for the species. Habitat suitability is better assessed from stream type, pool depth, shading, water permanence, flow connection and available cover.

Foraging habitat: Diet

Mary River cod are large predators that feed mainly around cover in pools and slow-flowing stream sections. They are most active from dusk to dawn and may patrol a relatively small home range for long periods.

The diet consists mainly of fish and large crustaceans, including yabbies and shrimp. Angled fish have also been reported to contain small birds, bats and water rats.

Young fish become aggressive predators at about 30 to 50 mm in hatchery conditions.

Main prey: Fish, yabbies and shrimp.

Other reported prey: Small birds, bats and water rats.

Foraging habitat: Foraging locations and behaviour

Cod forage around submerged timber, log piles, rock cover, undercut banks and pool margins. They may occur immediately downstream of riffles or channel constrictions where prey is concentrated by flow.

Feeding and patrolling movements are most common from dusk to dawn. Individuals may remain within a home range 100 to 1000 m long, although some fish move more than 30 km during high flows.

Typical home range length: 100 to 1000 m.

Core areas: Usually 2 to 4 within a home range.

Activity period: Most common from dusk to dawn.

  • Retain cover and avoid lighting, noise, vibration and in-water disturbance around occupied pools, especially from dusk through dawn.
  • Maintain flow connection between pools, tributaries and the main channel so fish can reach feeding areas and seasonal habitat.

Breeding habitat: Breeding biology

Mary River cod form pairs and spawn annually in spring. Breeding depends on a secure cavity or other large interstitial structure, with the male guarding the eggs and early larvae.

Breeding season: Spring, with hatchery spawning generally reported from August to October.

Spawning trigger: Spawning begins after water temperatures rise to about 20°C. Recent field observations followed sustained temperatures above 20°C and a post-winter flow event.

Egg number: Wild clutch size is not well known. Hatchery experience suggests about 2000 eggs per kilogram of female body weight, and some females may spawn more than once in a season.

Egg size: About 3.0 to 3.5 mm diameter.

Incubation: Hatching starts towards the end of the fourth day and is usually complete by the seventh day at 21°C.

Larval dependence: The male guards the brood until larvae disperse to search for food about 7 to 9 days after hatching.

Age at maturity: Not reported in the supplied sources.

Lifespan: Not reported in the supplied sources.

Breeding habitat: Nest habitat

The male selects and guards a nest. Hatchery fish use hollow pipes or purpose-built nesting boxes, and hollow logs are presumed to be the natural nest structure. Natural nesting sites include submerged hollow logs, undercut banks and other large interstitial spaces.

A 2024 field trial recorded four males guarding eggs, including one confirmed spawning event in an artificial hollow log. Natural structures supported more frequent spawning than the artificial structures in that trial.

  • Treat submerged hollow logs, large cavities, undercut banks and similar structures as potential breeding habitat during August to October.
  • Avoid clearing, vibration, dewatering, high turbidity and in-water works near suspected nests during the breeding season.
  • If habitat enhancement is proposed, use a species-specific design and monitoring plan. Artificial structures supplement but do not replace protection of natural habitat.

Sheltering habitat: Natural refuges

Shelter is a defining habitat requirement for Mary River cod. Adults spend much of their time close to large woody or rocky cover, which also provides nesting and refuge sites.

Preferred refuges include large submerged logs, log piles, root masses, undercut banks, rock ledges and boulders. Fish may use relatively shallow water when abundant cover is present, but usually avoid open water.

Radiotracking recorded fish within 1 m of instream timber in more than 90% of observations. A home range commonly contains 2 to 4 core areas centred on large logs or log piles.

Distance from instream timber: More than 90% of radiotracking observations were within 1 m.

Home range: 100 to 1000 m, usually with 2 to 4 core areas.

  • Retain submerged and partly submerged timber, including large individual logs and log piles.
  • Do not straighten, widen or over-clear banks where undercut banks, root masses or rock cavities provide cover.
  • Keep stock, machinery and people away from unstable banks and shallow margins containing cover.

Sheltering habitat: Artificial structures

A 2024 trial used hollow hardwood structures about 1300 mm long and 800 mm in external diameter, with internal cavities about 900 mm long and 400 mm in diameter. Structures were placed at 1 to 1.5 m water depth and tethered to the bank.

Cod used both open and blind artificial logs. One open log contained eggs, while blind logs were occupied more often. The structures also provided refuge for other fish, frogs and reptiles.

  • Use artificial nesting structures only under an approved habitat enhancement and monitoring programme.
  • Place and secure structures so they do not create a snagging hazard, obstruct fish passage or move during high flows.
  • Protect natural hollow logs and other existing structures before considering artificial replacements.

Threats: Clearing and habitat loss

The main threats are loss and fragmentation of aquatic habitat, altered flow and barriers to movement. Clearing and construction can affect the species through direct habitat removal, sedimentation, loss of shade and disturbance of breeding cavities.

Removal of riparian vegetation reduces shade, bank stability and recruitment of large timber. Bank erosion can add sediment, fill pools and bury instream structures. Removal of logs, log piles and other woody debris removes adult shelter, feeding cover and potential nests.

Impoundments and poorly designed fishways can isolate home ranges and prevent movement between tributaries and the main river. Water extraction and urban use can alter flow, temperature, pool and riffle structure, and breeding conditions.

  • Avoid clearing riparian vegetation beside known or potential cod habitat.
  • Fence or otherwise exclude cattle and construction traffic from riparian zones and unstable banks.
  • Use erosion and sediment controls that remain effective during rainfall and high flows.
  • Maintain environmental flows and fish passage where works alter a weir, culvert, dam or channel.

Threats: Construction and hydrology impacts

Dewatering, channel excavation, dredging, bed disturbance, piling, rock placement and machinery access can destroy or disturb occupied pools, refuges and nests. Sediment can smother macrophytes, change substrate, bury cavities and reduce habitat complexity.

High flows can also remove or degrade natural nesting structures. A 2024 study linked the need for artificial nesting habitat to habitat loss from human disturbance and major flooding events.

  • Survey shaded pools, tributary junctions, deep runs and areas with logs or undercut banks before in-water works.
  • Keep works out of the water where practicable and maintain downstream passage and suitable residual pools during any unavoidable isolation.
  • Inspect retained logs, cavities and banks after high flows and after construction to confirm they remain functional.

Threats: Introduced fish and fishing

Introduced fish such as mosquito-fish, guppies and swordtails may compete with larvae for resources. Translocated golden perch, silver perch and saratoga may compete for food or habitat, and golden perch have similar feeding and habitat preferences. Tilapia is also identified as a potential invasive threat in recent research.

Overfishing contributed to historical decline and remains a suspected threat. Mary River cod are subject to no-take protection in some Queensland waters and strict limits apply in approved stocked areas.

  • Do not move, release or transfer fish, water, equipment or sediment between catchments unless authorised and managed under applicable biosecurity controls.
  • Prevent construction equipment from transferring aquatic weeds, fish or contaminated water between sites.
  • Do not capture, retain or relocate Mary River cod without the required approvals.

Threats: Fire, roads, predators and disease

The supplied sources do not identify fire, roads, terrestrial predators or disease as primary species-specific threats. They can still affect cod indirectly where they cause riparian clearing, bank erosion, sediment delivery, loss of shade, altered runoff or contamination of waterways.

Road crossings, temporary access tracks and firebreaks should therefore be assessed for bank damage, sediment runoff, barriers to fish passage and loss of riparian vegetation.

  • Locate crossings outside deep shaded pools and retain fish passage through permanent and temporary crossings.
  • Use clean-water, sediment and spill controls at roads, culverts, bridge sites and fuel storage areas.
  • Treat any fish kill, disease sign, unusual mortality or chemical spill as an incident requiring immediate aquatic fauna advice.

Survey methodology: 1. Site and habitat assessment

Survey Mary River cod habitat before clearing, excavation, in-stream works or changes to flow. Focus on shaded, deep, slowly flowing pools with submerged timber, log piles, undercut banks, rock ledges and other heavy cover, especially in tributaries of the Mary River system.

Map all waterways, pools, tributaries, dams and wetlands within the project area and downstream area that could be affected by the works.

Inspect and record pool depth, flow, water clarity, shading, riparian vegetation, submerged logs, log piles, undercut banks, rock cover and likely movement routes.

Give priority to large, deep pools about 0.8 to 3.2 m deep, with slow flow, shade and woody debris, and to relatively undisturbed tributaries with cooler water than the main river channel.

Record fish passage barriers, including weirs, culverts and temporary construction crossings, because Mary River cod may move more than 30 km upstream or downstream and may return to established home ranges.

Survey methodology: 2. Angling with barbless lures

Use lure fishing with barbless hooks as a low-impact method for presence or absence surveys where authorised.

Survey at dawn, dusk or night, when cod are generally more active, and target submerged or partly submerged trees, logs, snags and deep shaded pools.

Use spring and summer only where possible, but avoid the spawning period from August to October so breeding fish are not disturbed.

Record survey date, start and finish time, waterway, pool or reach sampled, lure effort, captures, water temperature, weather and exact habitat used by each fish.

Survey methodology: 3. Electrofishing

Use boat-based or backpack electrofishing where the waterway, access and depth allow, and operate equipment under the applicable Australian code of practice and state requirements.

Sample deep pools, shaded margins, submerged timber and woody debris, while recognising that cod may hide during daylight and that electrofishing can be less effective in clear water.

In larger dams, night electrofishing may produce better catches, but use it only with the required authority and an approved animal welfare procedure.

Avoid the breeding period from August to October unless the survey is specifically authorised and designed to avoid impacts on spawning fish.

Record equipment type, settings, operator, sampled reach or area, effort, water conditions, fish captured, injuries and release location.

Survey methodology: 4. Daytime snorkelling and visual searches

Use daytime snorkelling or underwater visual observation only where water clarity, depth, flow and safety allow reliable detection.

Search slowly around logs, log piles, undercut banks, rock ledges, boulders and other cover, because radiotracking observations located cod within 1 m of instream timber more than 90 percent of the time.

Do not enter the water during unsafe flows, poor visibility or contaminated conditions, and do not dislodge logs or disturb likely nest sites.

Record transect length, width, duration, visibility, water depth, cover searched, fish observed and whether the observation was made at a natural structure or artificial habitat.

Survey methodology: 5. Non-destructive habitat monitoring

Use remote cameras or endoscopic cameras to inspect hollow logs, undercut banks and artificial nesting structures where direct handling would cause greater disturbance.

During the breeding season, inspect quietly and minimise the time spent at each structure, because males guard eggs and larvae until the larvae disperse.

A recent trial inspected structures weekly for 11 weeks from August to October, usually for less than 30 seconds and for no more than 300 seconds at a structure.

Treat eggs, larvae and a guarding male as a stop-and-reassess observation, not as a reason to open, move or enter the structure.

Recent artificial habitat trial: Twenty-four hollow log structures were installed at six treatment sites and monitored with natural structures at paired control sites.

Artificial log placement in the trial: Logs were placed at 45 degrees to the bank in 1 to 1.5 m of water and tethered to the bank.

Survey methodology: Minimum effort and reporting

  • Use a combination of methods rather than relying on a single survey technique, because no method is consistently effective for Mary River cod.
  • Survey every suitable pool and reach that may be directly or indirectly affected, including downstream habitat where flow, sediment or barriers could alter conditions.
  • Avoid August to October for routine adult surveys and breeding habitat disturbance, unless a suitably authorised survey plan demonstrates that impacts will be avoided.
  • Record negative results as well as detections, with mapped survey boundaries, effort, conditions, habitat observations, photographs and identification confidence.
  • Have uncertain fish identifications checked by a suitably experienced fish ecologist, because small spangled perch can be confused with juvenile Mary River cod.
  • Submit confirmed records and survey metadata to the relevant Queensland wildlife or fisheries database and provide records to the project approval authority where required.

Before habitat disturbance: Avoid and minimise

Treat any suitable Mary River cod waterway as potential habitat until a site assessment shows otherwise. The species is restricted to a small, fragmented range and depends on connected waterways, shaded pools and instream cover.

  • Redesign the project to retain deep, shaded, slowly flowing pools, riparian vegetation, submerged logs, log piles, undercut banks, rock ledges and boulders.
  • Keep the channel connected and avoid new barriers that prevent movement into or out of home ranges.
  • Retain native riparian vegetation and in-stream timber wherever safe, because clearing increases erosion, sediment deposition and loss of shade and cover.
  • Prevent sediment from filling pools, burying logs or changing the substrate around known or potential cod habitat.
  • Do not remove, relocate or anchor large timber in a waterway without a site-specific ecological and waterway assessment.

Before habitat disturbance: Approvals and work planning

National status: Endangered under the EPBC Act.

Queensland status: Endangered.

  • Check whether the action may significantly affect the EPBC Act listed endangered Mary River cod and obtain an EPBC Act referral or approval where required.
  • Confirm Queensland approvals, fisheries requirements, no-take rules and permits for fish capture, handling, electrofishing, in-stream works and relocation.
  • Schedule routine habitat disturbance outside August to October, the reported spawning period, wherever practicable.
  • Prepare a fauna and aquatic habitat management plan covering survey methods, exclusion zones, fish rescue, water quality, incident response, authorised handlers and reporting.
  • Stage works so that retained habitat, movement routes and water quality controls remain functional while each work area is opened.

Before habitat disturbance: Pre-clearance and site preparation

  • Complete a targeted survey before works begin, using habitat assessment plus a combination of angling, electrofishing and daytime visual observation where authorised and suitable.
  • Mark all retained pools, snags, hollow logs, undercut banks, rock ledges, riparian buffers, fish passage routes and no-go areas on project plans and on the ground.
  • Use exclusion fencing or equivalent controls where they can prevent machinery, vehicles and people entering retained riparian or instream habitat without blocking fish passage.
  • Install sediment, erosion, dewatering and dirty-water controls before ground disturbance, and inspect them after rainfall and high flows.
  • Brief all operators and spotter catchers on the species, its habitat, stop-work triggers and the requirement to avoid taking Mary River cod.

During habitat disturbance: Daily controls and supervision

Keep machinery out of retained waterway habitat and maintain a direct communication line between the fauna spotter catcher, site supervisor and machine operators. Stop work before an animal, nest site or habitat feature is placed at risk.

  • Hold a daily pre-start briefing covering mapped exclusion zones, waterway controls, fish rescue equipment, weather, flow conditions and the day’s work sequence.
  • Have the fauna spotter catcher inspect the work area before machinery enters and remain able to stop the operator immediately.
  • Keep plant, fuel, concrete, sediment, spoil and wash-down water out of pools, channels, riparian vegetation and retained woody habitat.
  • Suspend works when rising water, high turbidity, dewatering, bank collapse or sediment movement could harm cod or disconnect habitat.
  • Inspect temporary crossings, culverts, cofferdams and barriers each day to confirm that they are stable, do not trap fish and do not block movement.

During habitat disturbance: Habitat feature handling

  • Do not remove or disturb submerged logs, log piles, undercut banks, rock ledges, boulders or hollow structures unless the approved method specifically requires it and a qualified aquatic fauna specialist is present.
  • Treat any large hollow log, log pile or undercut bank as a possible nesting structure during August to October.
  • If a feature must be moved, inspect it first with a camera or other low-disturbance method and retain it in the same waterway where safe and approved.
  • Keep any salvaged timber wet, shaded and stable, and reinstate it in suitable flowing water with cover and depth comparable to the retained habitat.
  • Use fish passage structures designed for the site where a barrier cannot be avoided, and obtain approval before installation.

During habitat disturbance: If a Mary River cod is found

  • Stop the activity that could affect the fish and keep machinery, people and noise away from the pool or cover being used.
  • Do not chase, net, handle or relocate the fish unless the person is authorised under the relevant approval and has the required fish-handling skills.
  • If capture is authorised, use the least stressful approved method, keep handling time short, support the fish in water, and release it into nearby retained habitat in the same connected waterway unless the approval specifies otherwise.
  • Do not move a fish across a weir, road, catchment boundary or isolated waterbody without specific approval and advice from the relevant Queensland fisheries authority.
  • Take photographs from a safe distance, record the location and habitat, notify the fauna spotter catcher and environmental manager, and report the detection through the project incident process.
  • Send injured fish to an appropriately authorised aquatic wildlife rehabilitator or veterinarian under the approved response procedure, and do not release a dead or injured fish without advice.

During habitat disturbance: Stop-work triggers and records

  • Stop work if a Mary River cod is within the disturbance area, if eggs or larvae are found, or if a guarding adult is observed at a log, hollow structure or undercut bank.
  • Stop work if dewatering, sediment discharge, bank collapse, fish stranding, loss of flow, blocked passage or damage to retained woody habitat occurs.
  • Record the date, time, location, observer, identification evidence, fish size if safely estimated, behaviour, habitat feature, water conditions, action taken and release or treatment outcome.
  • Notify the project environmental officer, site supervisor and relevant approval contact before work resumes, and revise the method if the trigger resulted from an avoidable impact.

After habitat disturbance: Post-clearance checks

Leave the waterway stable, connected and suitable for cod before demobilising. Post-work checks should confirm that retained habitat and water quality controls have not been lost during construction or clearing.

  • Inspect all disturbed pools, banks, channels, crossings and downstream reaches for stranded fish, sediment deposition, bank slumping, blocked passage and damaged woody habitat.
  • Complete a final fish and habitat check after dewatering, cofferdam removal, major flow changes or removal of temporary crossings.
  • Remove temporary barriers, pipes, spoil and materials that could obstruct cod movement or fill pools, unless they form part of an approved permanent design.
  • Confirm that retained riparian vegetation, snags, log piles, undercut banks and rock cover remain intact and are protected from vehicles and stock access.

After habitat disturbance: Monitoring and reporting

  • Monitor water quality, sediment controls, fish passage and retained habitat during the defects period and after the first significant rainfall or flow event.
  • Undertake targeted follow-up surveys where the project removed cover, altered flow, installed a barrier, caused fish rescue, or affected a known cod location.
  • Compare post-work observations with the pre-work habitat map, including pool depth, shade, cover, flow, barriers and evidence of cod use.
  • Report all detections, mortalities, injuries, breeding observations, incidents, corrective actions and survey results to the approval authority and relevant Queensland database.

After habitat disturbance: Maintain controls and offsets

  • Keep exclusion fencing, sediment controls, riparian protection and fish passage measures in place until the environmental manager confirms that the site is stable.
  • Repair failed erosion, sediment or passage controls immediately and inspect downstream habitat for secondary impacts.
  • Implement any approved offset, compensatory habitat or restoration measure in accordance with the EPBC Act and Queensland approval conditions.
  • Do not treat restocking as a substitute for protecting connected habitat, riparian vegetation and suitable nesting cover.

Rehabilitation actions: Riparian and channel restoration

Rehabilitation should restore connected, shaded waterway habitat with stable banks, deep pools and abundant cover. Restocking alone will not address habitat loss if fish cannot move, shelter or reproduce.

  • Replant native riparian species suited to the local Mary River catchment to restore shade, bank stability and future sources of in-stream timber.
  • Fence or otherwise manage livestock access where grazing prevents natural regeneration or disturbs stream banks.
  • Stabilise eroding banks and control sediment so pools are not progressively filled and spawning or refuge structures are not buried.
  • Reinstate or retain large logs, log piles, undercut banks, rock ledges and boulders in deep or slowly flowing pools, using designs approved for the waterway and flood conditions.
  • Remove invasive fish risks from project activities and do not translocate non-endemic fish into Mary River cod habitat.

Rehabilitation actions: Nesting and refuge structures

Recent artificial structure result: Cod occupied six artificial structures, and one confirmed spawning event occurred in an artificial log during the 2024 breeding season.

Trial construction cost: The trial structures cost about $1,750 each and were approximately 1,300 mm long by 800 mm external diameter, with an internal space about 900 mm by 400 mm.

  • Use natural hollow logs wherever safe and available, because Mary River cod are presumed to use hollow logs as nests and depend on large interstitial spaces for breeding.
  • Where natural nesting habitat has been lost, consider artificial hollow logs or nesting structures as a supplement, not a replacement for catchment and riparian restoration.
  • Place artificial structures in suitable water depth, flow and cover, secure them against movement, and monitor occupancy without repeatedly disturbing guarding fish.
  • Use local, durable materials and record structure design, placement, condition, occupancy, eggs, larvae and any evidence of recruitment.
  • Design structures to support broader aquatic habitat where appropriate, because trialled artificial logs were also used by other fish, reptiles and frogs.

Rehabilitation actions: Long-term management and success measures

  • Monitor restored reaches for riparian survival, bank stability, sediment accumulation, pool depth, shade, woody cover, fish passage and invasive fish presence.
  • Use repeat surveys to assess cod presence, size classes, occupancy of natural and artificial cover, spawning, larval dispersal and recruitment where approvals allow.
  • Adapt planting, erosion control, log placement, fish passage and artificial nesting designs when monitoring shows that habitat is not being used or is being damaged by floods.
  • Control weeds and manage fire in riparian areas so native shade, bank cover and future large timber are maintained.
  • Set project success measures around connected habitat, retained or increasing cover, stable water quality, functioning passage and confirmed cod use, rather than stocking numbers alone.

Sources