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King Threadfin

Polydactylus macrochir

  • Olivia Whybird (Department of Agriculture and Fisheries, Queensland)
  • Stephen Newman (Department of Fisheries, Western Australia)
  • Thor Saunders (Department of Primary Industry and Resources, Northern Territory)
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Stock Status Overview

Stock status determination
Jurisdiction Stock Fisheries Stock status Indicators
Queensland East Coast ECIFFF Sustainable Catch, CPUE, length and age frequencies, Mortality estimates 
Queensland Gulf of Carpentaria GOCIFFF Transitional-depleting Catch, MCY, CPUE, length and age frequencies, Mortality estimates
Northern Territory Northern Territory BF Sustainable Catch, CPUE, length and age frequencies
Western Australia Western Australia KGBMF Sustainable Catch
BF
Barramundi Fishery (NT)
ECIFFF
East Coast Inshore Fin Fish Fishery (QLD)
GOCIFFF
Gulf of Carpentaria Inshore Fin Fish Fishery (QLD)
KGBMF
Kimberley Gillnet and Barramundi ManagedFishery (WA)
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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 features1,2. 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)1,2; and the management unit level—East coast (Queensland).

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Stock Status

Western Australia

King Threadfin is landed in the Kimberley Gillnet and Barramundi Fishery of Western Australia. Commercial catches in 2015 were 25 tonnes (t), similar to that reported in 2014. Recent catches are well below the average of 74.5 t for the 10-year period from 2004–13. This is due to low effort levels in the fishery3 following the removal of two fishing licenses from the Broome coast area. The Broome coast area has been closed to commercial fishing since late 2013. This commercial closure in the principal area for King Threadfin landings, in association with their productive life history characteristics, is likely to have substantially increased the spawning stock biomass of this species. In addition, the catch rates for King Threadfin are within the range exhibited by the fishery over the past 20 years. King Threadfin are landed by recreational and charter fishers, but only in small quantities (1 t). The above evidence indicates the biomass of this stock is unlikely to be recruitment overfished, and that the current fishing pressure is unlikely to cause the stock to become recruitment overfished.

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

Northern Territory

Commercial catch in the Barramundi Fishery has declined in recent years, but is still 71 per cent of the long-term (1983–2012) average. However, monitored stocks have a healthy size and age distribution4 and nominal catch per unit effort (CPUE) has increased substantially over the past 10 years, with the 2015 value being the second highest value in the history of the fishery. The above evidence indicates that the biomass of this stock is unlikely to be recruitment overfished. The above evidence indicates that the current fishing pressure is unlikely to cause the stock to become recruitment overfished.

 

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

Gulf of Carpentaria

The only stock assessment for King Threadfin in the Gulf of Carpentaria was conducted was using commercial catch and effort reports in 20025, that produced an estimate of maximum constant yield (MCY) to provide a conservative upper limit for sustainable catch levels. From 1989–2012, the annual commercial catch exceeded the MCY of 236 t5 by an average of 43 per cent. Recently, commercial catches decreased from 313 t in 2012 to 176 t in 2013 and were 138 t in 2015, despite new markets causing higher prices6 The commercial catch was 42 per cent lower than MCY in 2015. Despite increases in fishing power5 recent nominal CPUE (kg per 100 m net) were 45 per cent lower in 2014 and 42 per cent lower in 2015 than the previous 5 year average, which is the lowest rate reported in the history of the fishery. This follows a declining trend in catch and CPUE observed in many other species in the tropical inshore fisheries in Australia, which is thought to be due primarily to reduced wet season flows in recent years. Reduced flow is likely to have a negative impact on King Threadfin year class strength and catchability7. Catches have also declined as a consequence of a decrease in commercial fishing effort8. It is apparent that biomass has declined over the period (2013–15), but the stock is not yet considered to be in a recruitment overfished state.

 

High discard mortality, variable growth, late female maturity and size at transition to female (larger than legal size) make this species susceptible to overfishing9. A temporal commercial closure offers protection during most of the spawning season10. Fishing effort decreased from 97 operators undertaking 8565 days in 1989 to 63 operators fishing 3095 days in 2015. Despite the downward trend in fishing effort, fishing mortality has been estimated to be two to four times natural mortality for the 2007–09 period9. A population structure change appears to have occurred between studies conducted in 1986–9010,11 and 2007–099, with fish in the more recent study having a younger maximum age and changing sex when smaller and younger1. These changes were considered to be caused by fishing pressure9, although differences in the selectivity of sampling methods, poor sample sizes and natural variation make the comparison of size and age frequencies uncertain. The above evidence indicates that the current level of fishing pressure is likely to cause the stock to become recruitment overfished.

 

On the basis of the evidence provided above, the Gulf of Carpentaria (Queensland) biological stock is classified as a transitional–depleting stock.

East Coast

The only stock assessment for King Threadfin in the East coast management unit was conducted with limited data in 2002, producing a conservative MCY of 62 t. Commercial catch has exceeded this estimate every year since 19885. Rainfall events and changes to spatial targeting and minimum size limits have occurred since the assessment and the reference points may no longer be appropriate to the current biomass of the fishery. Catches and the nominal catch rates have shown an increasing trend since 1997 and are currently close to historical high levels (208 t and 17.1 kg per 100 m net respectively)8. Despite a general reduction in recreational fishing12 the East coast recreational catch of King Threadfin for 2013–14 was estimated as 47 t (± 17 t), an increase of more than 80 per cent since the 2010–11 surveys6. Year class strength (based on an analysis of age frequency) has been found to be positively correlated with spring and summer freshwater flows and coastal rainfall in the Fitzroy River region13,7. The central and southern areas, which experienced average or above average rainfall during the summers of 2009–10 to 2012–13, including some significant flood events, display increasing catch and catch rates. In the northern area, where in recent years the seasonal monsoon trough has been weak, bringing below average summer rainfall, catches remain lower than those of the 2007–12 period. The above evidence indicates that the biomass of this stock is unlikely to be recruitment overfished.

 

The fine spatial scale of the genetic population structure, late female maturity, high discard mortality and size at transition to female (larger than legal size) make this species susceptible to overfishing1. However, estimates of total mortality for the Mary, Fitzroy and Brisbane Rivers in 2011 were very low (fishing mortality was less than natural mortality)9. However, fishing pressure in these areas has continued to increase and the estimates of mortality for species with variable annual recruitment may be inaccurate. East coast catch and catch rates are near the historical highs, particularly in central and southern Queensland. The number of active commercial operators has reduced by 24 per cent since 2013 and fishing days have remained stable. In late 2015, several new net closure areas were introduced along with a buy-back scheme for net licenses. These measures are likely to have reduced fishing effort. The above evidence indicates that the current level of fishing pressure is unlikely to cause the stock to become recruitment overfished.

On the basis of the evidence provided above, the East coast (Queensland) management unit is classified as a sustainable stock.

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Biology

King Threadfin biology2

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,1000 mm TL
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Distributions

Distribution of reported commercial catch of King Threadfin

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Tables

Fishing methods
Western Australia Northern Territory Queensland
Commercial
Various
Gillnet
Recreational
Hand Line, Hand Reel or Powered Reels
Spearfishing
Indigenous
Spearfishing
Hand Line, Hand Reel or Powered Reels
Pots and Traps
Management methods
Method Western Australia Northern Territory Queensland
Commercial
Gear restrictions
Limited entry
Size limit
Spatial closures
Spatial zoning
Temporal closures
Vessel restrictions
Indigenous
Gear restrictions
Recreational
Bag limits
Licence
Limited entry
Passenger restrictions
Possession limit
Recreational
Size limit
Spatial closures
Spatial zoning
Temporal closures
Active vessels
Western Australia Northern Territory Queensland
4 in KGBMF 14 in BF 161 in ECIFFF, 63 in GOCIFFF
BF
Barramundi Fishery (NT)
ECIFFF
East Coast Inshore Fin Fish Fishery (QLD)
GOCIFFF
Gulf of Carpentaria Inshore Fin Fish Fishery (QLD)
KGBMF
Kimberley Gillnet and Barramundi ManagedFishery (WA)
Catch
Western Australia Northern Territory Queensland
Commercial 25.49t in KGBMF 237.11t in BF 208.10t in ECIFFF, 138.43t in GOCIFFF
Indigenous Unknown Unknown Unknown
Recreational < 0.5 t, 0.8 t (2013–14) 1.5 t, 9 t (2010), 1.5 t in the FTO Included in recreational estimate, 83 t +/- 24 t (2013–14)
BF
Barramundi Fishery (NT)
ECIFFF
East Coast Inshore Fin Fish Fishery (QLD)
GOCIFFF
Gulf of Carpentaria Inshore Fin Fish Fishery (QLD)
KGBMF
Kimberley Gillnet and Barramundi ManagedFishery (WA)

a Western Australia – Recreational (catch) Boat-based recreational catch from 1 May 2013–30 April 2014b Queensland – Recreational (catch) Survey of Queensland residents only from August 2013–October 2014 12

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Catch Chart

Commercial catch of King Threadfin

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Effects of fishing on the marine environment

  • The majority of King Threadfin are harvested by commercial gillnets. The catch from these nets is quite selective, with bycatch making up only a small proportion of the catch14, therefore the direct impact on the wider environment is limited.
  • The Northern Territory Fishery reported bycatch of less than one per cent of the total catch. This bycatch typically consists of Queenfish, unwanted shark species, catfish and Blue Threadfin15.
  • The most common interactions with threatened, endangered or protected species are with Saltwater Crocodiles and which are highly unlikely to impact populations of this species given that they are considered to have recovered from unregulated hunting. For example, the Northern Territory population of Saltwater Crocodiles is considered to be at carrying capacity in most catchments16.
  • Other species of conservation interest are interacted with in very low numbers (less than 10 per year). These species include sawfish, turtles and dugongs. The low bycatch and interaction levels in the fishery have been supported by fishery observer coverage on-board commercial vessels14,15.
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Environmental effects on King Threadfin

  • The duration, magnitude and timing of the wet season has been shown to influence the catchability of King Threadfin17,18.
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References

  1. 1 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.
  2. 2 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. Draft Final Report to the Fisheries Research and Development Corporation, Project 2007/032. Fishing and Fisheries Research Centre, James Cook University, Townsville, Australia.
  3. 3 Brown, JI, Newman, SJ, Mitsopoulos, G, Skepper, C, Thomson, A and Wallis, D 2015, North Coast Nearshore and Estuarine Fishery Status Report, in Fletcher, WJ and Santoro, K (eds.), State of the fisheries and aquatic resources report 2010/11, Western Australian Department of Fisheries, Perth, 182–188.
  4. 4 Saunders,T 2015, King Threadfin Polydactylus macrochir, in Northern Territory Government 2015, Status of Key Northern Territory Fish Stocks Report 2013. Northern Territory Government. Department of Primary Industry and Fisheries. Fishery Report No. 114. 57–61.
  5. 5 Welch, D, Gribble, N and Garrett, R, 2002, Assessment of the Threadfin Salmon Fishery in Queensland – 2002. Department of Primary Industries, Brisbane.
  6. 6 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.
  7. 7 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.
  8. 8 Department of Agriculture and Fisheries 2016, Queensland stock status assessment workshop 2016, 14–15 June 2016, Brisbane, DAF, Brisbane.
  9. 9 Moore, BR 2011, Movement, connectivity and population structure of a large, non-diadromous tropical estuarine teleost. PhD thesis, James Cook University.
  10. 10 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.
  11. 11 Bibby, JM, RN Garrett, CP Keenan, GR McPherson and LE Williams 1997, Biology and Harvest of Tropical Fishes in the Queensland Gulf of Carpentaria Gillnet Fishery, Department of Primary Industries, Brisbane.
  12. 12 Webley, J, McInnes, K, Teixeira, D, Lawson, A and Quinn, R 2015, Statewide Recreational Fishing Survey 2013-14. Department of Agriculture and Fisheries, Brisbane.
  13. 13 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.
  14. 14 Halliday, IA, Ley, JA, Tobin, A, Garrett, R, Gribble, NA and Mayer, DG 2001, The effects of net fishing: addressing biodiversity and bycatch issues in Queensland inshore waters, Fisheries Research and Development Corporation project 97/206, Queensland Department of Primary Industries, Brisbane.
  15. 15 West, LD, Lyle, JM, Matthews, SR, Stark, KE and Steffe, AS 2012, Survey of Recreational Fishing in the Northern Territory, 200910, Fishery Report No. 109, Northern Territory Government, Darwin.
  16. 16 Fukuda, YP, Webb, G, Manolis, C, Delaney, R, Letnic, M, Lidner, G, and Whitehead, P 2011, Recovery of Saltwater Crocodiles following unregulated hunting in tidal rivers of the Northern Territory, Australia. The Journal of Wildlife Management, 75: 1253–1266.
  17. 17 Halliday, IA, Saunders, T, Sellin, M, Allsop, Q, Robins, JB, McLennan, M and Kurnoth, P 2012, Flow impacts on estuarine finfish fisheries of the Gulf of Carpentaria, Fisheries Research and Development Corporation project 2007/002, Queensland Department of Agriculture, Fisheries and Forestry, Brisbane.
  18. 18 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.