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King Threadfin (2020)

Polydactylus macrochir

  • Anthony Roelofs (Department of Agriculture and Fisheries, Queensland)
  • Fabian Trinnie (Department of Primary Industries and Regional Development, Western Australia)
  • Mark Grubert (Department of Industry, Tourism and Trade, Northern Territory)
  • Stephen Newman (Department of Primary Industries and Regional Development, Western Australia)
  • Olivia Whybird (Department of Agriculture and Fisheries, Queensland)

Date Published: June 2021

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Summary

King Threadfin in the WA and NT jurisdictions, as well as the QLD east coast management unit are classified as sustainable. In contrast, the Gulf of Carpentaria biological stock is classified as a depleting stock.

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Stock Status Overview

Stock status determination
Jurisdiction Stock Stock status Indicators
Northern Territory Northern Territory Sustainable

Stock assessment, biomass estimate, fishing mortality, catch, catch rate

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Stock Structure

King Threadfin have numerous populations across northern Australia that are separated by 10–100s km or by large, coastal geographical features [Moore et al. 2011, Welch et al. 2010]. With the exception of the Gulf of Carpentaria, there is a lack of information on the degree to which this separation indicates separate biological stocks, and on boundaries between possible stocks.

Here, assessment of stock status is presented at the jurisdictional level—Western Australia and Northern Territory; at the biological stock level—Gulf of Carpentaria (Queensland); and the management unit level—East coast (Queensland).

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Stock Status

Northern Territory

Knowledge of the genetic stock structure of King Threadfin in Northern Territory (NT) waters is limited. Welch et al. [2010] detected two different stocks in the NT from two locations sampled (i.e. Chambers Bay and Blue Mud Bay). Finer-scale sampling conducted in Queensland and Western Australia revealed stocks separated by distances of tens to hundreds of kilometres or by large, coastal geographical features [Welch et al. 2010, Moore et al. 2011]. The existence of multiple biological stocks in these states suggests that the stock structure of King Threadfin in the NT is likely to be more complex than currently described. 

In addition to genetic traits, the year class strength (i.e. productivity) of King Threadfin is affected by freshwater flow and coastal rainfall [Halliday et al. 2008]. Therefore, differences in these environmental drivers between adjacent catchments (as is evident in the NT; BOM 2020) may over-ride genotypic differences in productivity between neighbouring stocks.

Given uncertainties regarding the actual number of biological stocks of King Threadfin in NT waters, and current management arrangements for this species [as a single management unit], the assessment presented here was undertaken at the jurisdictional level.

The most recent assessment of King Threadfin in the Northern Territory (using data to the conclusion of 2019) indicated that the stock was impacted by high fishing pressure in the late 1970s and early 1980s, falling to 47 per cent of the unfished (1950) biomass [Grubert and Saunders, unpublished]. However, there has been a strong recovery since that time, with the annual biomass as a proportion of virgin biomass exceeding 60 per cent for the last two decades, reaching 98 per cent by the end of 2019. The above evidence indicates that the biomass of the stock is unlikely to be depleted and that recruitment is unlikely to be impaired.

The current (2019) fishing mortality rate, as a proportion of fishing mortality at maximum sustainable yield (MSY), was estimated at 16 per cent, roughly one sixth of the rate required to achieve MSY [Grubert and Saunders unpublished]. The standardised catch per unit effort (CPUE) in 2019 was also at a historical high, following a significant increase in this indicator over the last decade (noting that this trend may in part be driven by an increase in targeting of King Threadfin). The above evidence indicates that the current level of fishing pressure is unlikely to cause the stock to become recruitment impaired.

On the basis of the evidence provided above, King Threadfin in the Northern Territory is classified as a sustainable stock.

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Biology

King Threadfin biology [Welch et al. 2010]

Biology
Species Longevity / Maximum Size Maturity (50 per cent)
King Threadfin 22 years, 1 600 mm TL  Males 2 years, 610 mm TL Females 6 years,1 000 mm TL
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Distributions

Distribution of reported commercial catch of King Threadfin
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Tables

Fishing methods
Northern Territory
Commercial
Gillnet
Indigenous
Spearfishing
Hook and Line
Net
Charter
Hook and Line
Recreational
Hook and Line
Management methods
Method Northern Territory
Charter
Gear restrictions
Limited entry
Passenger restrictions
Possession limit
Spatial closures
Spatial zoning
Temporal closures
Commercial
Gear restrictions
Limited entry
Mesh size regulations
Spatial closures
Spatial zoning
Temporal closures
Vessel restrictions
Recreational
Gear restrictions
Possession limit
Spatial closures
Spatial zoning
Temporal closures
Catch
Northern Territory
Commercial 237.75t
Charter 1.5 t
Indigenous Unknown
Recreational 9 t (2010)

Western Australia – Recreational (Catch) Boat-based recreational catch is from 1 September 2017–31 August 2018. These data are derived from those reported in Ryan et al. [2019]. It is important to note that catches of King Threadfin are underestimated as shore-based fishers were out of scope of the survey. Shore based catches of King Threadfin are not known.

Western Australia – Recreational (management methods) A Recreational Fishing from Boat Licence is required for the use of a powered boat to fish or to transport catch or fishing gear to or from a land-based fishing location.

Western Australia – Indigenous (management methods) Subject to application of Section 211 of the Native Title Act 1993 (Cth), and the exemption from a requirement to hold a recreational fishing licence, the non-commercial take by Indigenous fishers is covered by the same arrangements as that for recreational fishing.

Queensland – Indigenous (management methods) for more information see https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing

Northern Territory – Charter (management methods) In the Northern Territory, charter operators are regulated through the same management methods as the recreational sector but are subject to additional limits on license and passenger numbers.

Northern Territory – Indigenous (management methods) The Fisheries Act 1988 (NT), specifies that “…without derogating from any other law in force in the Territory, nothing in a provision of this Act or an instrument of a judicial or administrative character made under it limits the right of Aboriginals who have traditionally used the resources of an area of land or water in a traditional manner from continuing to use those resources in that area in that manner”.

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Catch Chart

Commercial catch of King Threadfin
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References

  1. Bayliss, P, Buckworth, R and Dichmont, C (Eds) 2014, Assessing the water needs of fisheries and ecological values in the Gulf of Carpentaria. Final Report prepared for the Queensland Department of Natural Resources and Mines (DNRM), CSIRO, Australia.
  2. Bibby, JM, Garrett, RN, Keenan, CP, McPherson, GR and Williams, LE 1997, Biology and Harvest of Tropical Fishes in the Queensland Gulf of Carpentaria Gillnet Fishery, Department of Primary Industries, Brisbane.
  3. Bureau of Meteorology (BOM) 2020, Regional Water Information.
  4. Garrett, R 1992, Biological Investigation of King Salmon Polydactylus sheridani in the Gulf of Carpentaria: A Summary Report. In: Healy, T (ed) Gulf of Carpentaria Fishery Review Background Paper No. 1, QFMA, Brisbane.
  5. 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 I Halliday and J Robins (eds) Environmental flows for sub-tropical estuaries: understanding the freshwater needs for sustainable fisheries production and assessing the impacts of water regulation, Department of Primary Industries and Fisheries, Brisbane, 92–106.
  6. Halliday, IA, Robins, JB, Mayer, DG, Staunton-Smith, J and Sellin, MJ 2008, Effects of freshwater flow on the year-class strength of a non-diadromous estuarine finfish, king threadfin (Polydactylus macrochir), in a dry-tropical estuary. Marine and Freshwater Research 59: 157–164.
  7. Moore, BR 2011, Movement, connectivity and population structure of a large, non-diadromous tropical estuarine teleost. PhD thesis, James Cook University.
  8. Moore, BR, Stapley, JM, Williams, AJ, Welch DJ, 2017. Overexploitation causes profound demographic changes to the protandrous hermaphrodite king threadfin (Polydactylus macrochir) in Queensland’s Gulf of Carpentaria, Australia. Fisheries Research, Volume 187, 199–208p.
  9. Moore, BR, Welch, DJ and Simpfendorfer, CA 2011, Spatial patterns in the demography of a large estuarine teleost: king threadfin, Polydactylus macrochir. Marine and Freshwater Research 62: 937–951.
  10. Newman, SJ, Mitsopoulos, G, Skepper, C and Wiberg, L 2020, North Coast Nearshore and Estuarine Resource Status Report 2019. pp. 153–159. In: Gaughan, DJ and Santoro, K (eds.) 2020. Status Reports of the Fisheries and Aquatic Resources of Western Australia 2018/19: The State of the Fisheries. Department of Primary Industries and Regional Development, Western Australia, Perth, Australia. 291p.
  11. QFish, Department of Agriculture and Fisheries, www.qfish.gov.au
  12. Robins, JB, Halliday, IA, Staunton-Smith, J, Mayer, DG and Sellin, MJ 2005, Freshwater flow requirements of estuarine fisheries in tropical Australia: a review of the state of knowledge and application of a suggested approach, Marine and Freshwater Research, 56: 343–360.
  13. Ryan, KL, Hall, NG, Lai, EK, Smallwood, CB, Tate, A, Taylor, SM, Wise, BS 2019, Statewide survey of boat-based recreational fishing in Western Australia 2017/18. Fisheries Research Report No. 297. Department of Primary Industries and Regional Development, Government of Western Australia, Perth. 
  14. Teixeira, D, Janes, R, and Webley, J 2021, 2019–20 Statewide Recreational Fishing Survey Key Results. Project Report. State of Queensland, Brisbane.
  15. Webley, J, McInnes, K, Teixeira, D, Lawson, A and Quinn, R 2015, Statewide Recreational Fishing Survey 2013–14. Department of Agriculture and Fisheries, Brisbane.
  16. Welch, D, Gribble, N and Garrett, R 2002, Assessment of the Threadfin Salmon Fishery in Queensland–2002. Department of Primary Industries, Brisbane.
  17. Welch, DJ, Ballagh, A, Newman, SJ, Lester, RJ, Moore, B, van Herwerden, L, Horne, J, Allsop, Q, Saunders, T, Stapley, J and Gribble, NA 2010, Defining the stock structure of northern Australia’s threadfin salmon species. Final Report to the Fisheries Research and Development Corporation, Project 2007/032. Fishing and Fisheries Research Centre, James Cook University, Townsville, Australia.
  18. Whybird, O, Trinnie, F, Saunders, T, Newman, S, 2018, King Threadfin Polydactylus macrochir, in Carolyn Stewardson, James Andrews, Crispian Ashby, Malcolm Haddon, Klaas Hartmann, Patrick Hone, Peter Horvat, Stephen Mayfield, Anthony Roelofs, Keith Sainsbury, Thor Saunders, John Stewart, Simon Nicol and Brent Wise (eds) 2018, Status of Australian fish stocks reports 2018, Fisheries Research and Development Corporation, Canberra.

Downloadable reports

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