Mary River cod (Maccullochella mariensis)
About Maccullochella mariensis
The Mary River cod (Maccullochella mariensis) is one of Australia’s most endangered fish. Natural populations of the species are now restricted to suitable areas of habitat in the Mary River system. It is estimated that Mary River cod now occur in less than 30% of their former known range in the Mary River system. Remnant populations may have become isolated from each other due to habitat fragmentation and the impoundment of streams.
The Mary River cod prefers shaded pool habitats with abundant instream cover (ie. logs, log jams, rock ledges, boulders, undercut banks). The species occurs from high gradient upland streams to slow flowing lower catchment reaches. Submerged logs are thought to be used as nest sites. Individual cod may move long distances during periods of high water flow. Movements tend to be upstream in summer and downstream in autumn. Between periods of movement, cod occupy a restricted home range which they maintain for up to several years.
Water requirements
The Mary River cod has three flow requirements and one temperature requirement for a successful breeding year. Firstly, pre-spawning activities from July to August require sufficient water levels to provide a connectivity event between males and females and suitable nesting habitats. The spawning season then begins when water temperature rises above 18°C in spring, ending by 31st December (S.Brooks, S.Poole, R.Manning, D.Knowles pers. comms.). Successful spawning during this time requires a temperature window of 18°C – 25°C and sufficient water levels for a second connectivity event, this time linking females with spawning sites. Subsequent development of free-swimming larvae may take up to 28 days from the time of spawning. During this time, stable flows are required to prevent developing eggs and larvae from being washed out.
Model purpose
The purpose of the model is to assess the impacts of alternative water management strategies on the spawning opportunities of Mary River Cod.
Development context
This model, describing the flow requirements for spawning and egg and larval survival, has been developed using quantitative information from the literature and expert opinion.
It was developed to support the Queensland Department of Environment and Resource Management’s ecological risk assessment for the Mary River Water Resource Plan (WRP) review.
Spatial application
This model and its default parameters were created for application in the Mary River, Queensland.
However, the model parameters could be edited to suit other locations or species with similar requirements.
Model description
Ecohydrological rules
The flow requirements for spawning success can be summarised as connectivity and flow pulses for access to spawning habitat and partner selection followed by stable water levels for egg and larval development. Recruitment success is assumed to occur when the following events/conditions are sequentially met:
- Connectivity event 1: water flow 25 cm above the CTF level for at least 1 day for male movement and partnering (July–August)
- Water temperature of 18°C – 25°C
- Connectivity event 2: water flow 25 cm above the CTF level for at least 1 day for female spawning movements (July – December while temperature rule is satisfied)
- Stable flows (below 1.5 year ARI) for 28 days following connectivity event 2 to prevent egg/larvae washout
Assessment method
This model produces binary daily results (daily spawning success). These results are then aggregated to a yearly result, and then further to a temporal result based on the defined assessment parameters.
The temporal results are then analysed across locations to report an overall landscape risk by considering the simultaneous occurrence of failures across the system.
Inputs
Data
- Daily depth data
- Daily flow data
- Daily temperature data
Parameter Sections
- Connectivity event 1 – define the parameters relating to the success of connectivity event 1. This includes a season (defined by dates), the cease to flow (CTF) threshold and an amount above CTF threshold.
- Connectivity event 2 – define the parameters relating to the success of connectivity event 2. This includes a season (defined by dates), a temperature range, the cease to flow (CTF) threshold and an amount above CTF threshold.
- Period without disturbance – define the parameters relating to the requirement of a period without disturbance. This includes a flow threshold (or a calculated ARI threshold) and a duration.
Outputs
- Daily time series of recruitment opportunities, including intermediate results. These results are, if the day is in connectivity event 1 season, if the connectivity event 1 depth threshold is exceeded, if the connectivity event 2 depth threshold is exceeded, if there is suitable time to assess the period without disturbance and finally if the max flow remain below the threshold during the period without disturbance.
- Yearly time series of assessment results
- Temporal time series of assessment results
- Spatial time series of assessment results
User interface
Underlying code
This plugin is written in Python and its underlying code is publicly available from the Eco Risk Projector computation repository.
Eastern snake-necked turtle. Source: Nicolai, R. NSW Government. 
Leafy Elodea, Egeria densa. Source: Biocontrol of priority environmental weeds in NSW. Copyright CSIRO Australia, New South Wales Government
Australian bass (Macquaria novemaculeata) spawning and juvenile movement
Northern snake-necked turtle (Chelodina oblonga)
Mary River turtle (Elusor macrurus) and white-throated snapping turtle (Elseya albagula)