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King Threadfin Salmon (Polydactylus macrochir) year class strength

About Polydactylus macrochir

Polydactylus macrochir are a long-lived species of marine fish occuring in shallow coastal waters. King threadfin are protandrous hermaphrodites, beginning life as a male, changing into females at around 6-10 years maturity and completing their entire lifecycle in estuarine and associated marine foreshore environments. The Fitzroy River estuary in Queensland particularly serves as an important spawning ground for this species, with King threadfin salmon population persistence depending upon the provision of suitable freshwater flows in the estuary at crucial times of year. Young-of-the-year enter estuaries during the wet season, enabling them to take advantage of salinity gradients and the seasonal blooms in prey species such as Acetes spp. and juvenile penaeids that are accentuated in wet years. Freshwater flows in spring and summer are important drivers of the year-class strength of estuarine finfish, increasing salmon production and recruitment via an increase in biological productivity within estuaries. The increased turbidity resulting from flow events also reduces the threat of predation. A reduction in these timely freshwater flows, through the development of water infrastructure and abstraction or long-term climate change, will potentially reduce the size of the population of estuarine fish available for human harvest. It has been selected as an ecological asset for the Fitzroy Water Plan because it has a critical link to flows, which provide benefits for salmon production and recruitment, and is deemed to be potentially at risk from changes to the flow regime.

Water requirements

Freshwater flows reaching estuaries increases salmon production and recruitment via an increase in biological productivity within estuaries. The increased turbidity resulting from flow events also reduces the threat of predation.

Model purpose

The purpose of the model is to analyses long times series of modelled flow to determine if the risk to year class strength of King Threadfin Salmon is adversely affected by proposed water resource development for the Fitzroy River (Qld).

Development context

This model, describing year class strength of King Threadfin Salmon is based on an empirical model developed for the Fitzroy River Catchment.

It was developed to support the Queensland Department of Environment and Resource Management’s ecological risk assessment for the Fitzroy River Water Resource Plan (WRP) reviews.

Spatial application

This model and its default parameters were created for application in the Fitzroy River estuary, Queensland.

However, the model parameters could be edited to suit other locations if the underlying observation data exists to create a model of the same form.

Model description

Ecohydrological rules

Summer flows to the estuary are correlated with King Threadfin Salmon recruitment.

Year class strength (YCS) was determined using the following formula from Halliday et al (2007):

Y = 0.7227x – 4.1011

where y is YCS recruitment index score, and x is Log10 flow volume in ML during Summer

The YCS index output by the computation can be used to determine the degree of recruitment strength in a given year:

  • Strong recruitment = >0.5 YCS index
  • Moderate recruitment = 0.5 to -0.5 YCS index
  • Poor recruitment = <-0.5 YCS index

Strong recruitment years (>0.5 YCS index) were considered to be those required to maintain a healthy salmon population and the greater the number of years between these events, the higher potential risk to salmon populations:

High risk was defined as any year where the number of consecutive years below the threshold of concern exceeded the most common longevity of salmon, 11 years.

A moderate risk was defined as any year where the number of consecutive years below the threshold of concern was between the minimum age of protandry (six years) and the high risk profile.

All other years were deemed as being low risk to salmon populations.

Overall risk from water resource development was interpreted as a change in the risk profiles (i.e. relative proportion of each risk category) between the pre-development and development scenarios.

Assessment method

A binary yearly value is determined by classifying strong years successful, and all other strength as a failure. These results are then aggregated to a binary 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.


  • Daily flow data
Parameter Sections
  • Summer flow period – the period to analyse the data from. Includes a data picker to define the season.
  • Constants – the constants to use when applying the YCS equation, as discussed above. Includes both a and b values.
  • Intermediate assessment – parameters defining the intermediate assessment results calculated by this model. This includes the annual recruitment strength (what YCS is required to provide string, moderate or poor recruitment) and the temporal risk (how many years without strong recruitment is high, moderate or low risk). Assessment results are still provided, separately to these intermediate assessment results.


  • Yearly time series of YCS index score, recruitment strength and risk.
  • Temporal time series of assessment results (This is seperate to the temporal risk which is included in the yearly intermediate 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.


Halliday I, Staunton-Smith J, Robins J, Mayer D and Sellin M  2007, ‘Using age-structure of commercial catch to investigate the importance of freshwater flows in maintaining barramundi and king threadfin populations. In: Environmental flows for sub-tropical estuaries: understanding the freshwater needs of estuaries for sustainable fisheries production and assessing the impact of water regulation. Final Report FDRC Project No. 2001/022 Coastal Zone Project FH3/AF’, Department of Primary Industries and Fisheries, Queensland Government, Brisbane.