Carcharhinus sorrah, Carcharhinus tilstoni, Carcharhinus limbatus
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Stock Status Overview
|Northern Territory||Gulf of Carpentaria||ONLF||Undefined||Catch|
|Northern Territory||North and West Coast||ONLF||Sustainable||Catch, mark recapture, CPUE , pup production|
- Offshore Net and Line Fishery (NT)
In the context of Australian fisheries, the Blacktip Shark species complex, part of the family Carcharhinidae (whaler sharks), comprises three species: Carcharhinus tilstoni (Australian Blacktip Shark), C. limbatus (Common Blacktip Shark) and C. sorrah (Spottail Shark). Whereas C. tilstoni and C. sorrah are distributed within Australian and Indo–West Pacific waters, respectively, C. limbatus is globally distributed in tropical and warm temperate waters. In Australian waters, genetic studies have identified two biological stocks of C. tilstoni (a Western stock extending from the western Northern Territory into northern Western Australia, and an Eastern stock extending from the Gulf of Carpentaria to the east coast of Queensland and New South Wales), three biological stocks of C. limbatus (one across Western Australia and the Northern Territory, one in the Gulf of Carpentaria, and one on the east coast of Queensland and New South Wales) and a single biological stock of C. sorrah across northern Australia1. Stock boundaries between the Western biological stocks of C. tilstoni and C. limbatus and those in the Gulf of Carpentaria are uncertain.
Currently, commercially caught size classes of C. limbatus and C. tilstoni can only be taxonomically differentiated by genetic analyses or precaudal vertebral counts (although recent evidence of hybridisation may affect the accuracy of these techniques2,3). Reliable species differentiation is not practical during fishing operations. Although C. sorrah can be clearly distinguished from C. tilstoni and C. limbatus, it has only relatively recently been reported separately in commercial catch records from Northern Territory, Queensland and Western Australian fisheries. Because a suite of three species is grouped together for this assessment, all with differing stock structures, stocks have been assessed on the finest known scale—using the three biological stock areas identified for C. limbatus.
Here, assessment of stock status for the Blacktip Shark multispecies group is presented at the biological stock level—North and west coast, Gulf of Carpentaria, and East coast.
North and West Coast
The North and west coast biological stock straddles two management jurisdictions: the Northern Territory, west of the Wessel Islands–Western Australian border; and Western Australia.
The most recent assessment for this biological stock utilised stock reduction analysis models, which rely on catch per unit effort data. The results from these models estimated that the current harvest rates for all species within the complex are less than 20 per cent of that required to reach maximum sustainable yield (MSY) and current pup production is approximately 80 per cent of unfished levels4. A mark-recapture study in the Northern Territory for all species of Blacktip Sharks supports the stock assessment results5.
Although there is uncertainty in the species composition and magnitude of historical catches of Blacktip Sharks from Western Australia, these species have not been harvested in this jurisdiction since April 20096, allowing the biomass to increase.
The most recent assessment4 estimates that biomass in 2011 was 80 per cent of the unfished 1970 level. The stock is not considered to be recruitment overfished.
On the basis of the evidence provided above, the North and west coast multispecies biological stock is classified as a sustainable stock.
Gulf of Carpentaria
In 2014, the Queensland Department of Agriculture and Fisheries commissioned a scientific assessment of shark stocks. This assessment was completed by fisheries scientists from Animal Science Queensland and provides MSY estimates for C. tilstoni and C. sorrah in the Gulf of Carpentaria. This assessment produced qualified MSY estimates of 95 tonnes (t) for C. tilstoni and 29.4 t for C. sorrah7. This report however, also acknowledged that there are a number of data limitations for Queensland fisheries, particularly with respect to the species identifications and the quantity and reliability of the available catch data.
In 2015, 59 t of C. tilstoni and 22 t of C. sorrah were reported from the Gulf of Carpentaria Inshore Finfish Fishery (GOCIFFF). While this is below the above estimates, species-specific data for the fishery showed that C. sorrah catch (15–34 t) exceeded MSY twice over the past 10 years with the catch of C. tilstoni (59–130 t) exceeding MSY five times over the same period. A further estimated 42–125 t (2007–15) of Blacktip Sharks was reported from the GOCIFFF each year under the ‘Blacktip Whaler Shark’ catch category that includes Graceful Sharks (C. amblyrhynchoides). At present, catch reported in the ‘Blacktip Whaler Shark’ category cannot be differentiated into individual species. If this category includes a high percentage of Blacktip Sharks, then total Blacktip Shark catches may have exceeded MSY.
Queensland introduced changes to management of the net fishery at the beginning of the 2012 season, decreasing the total length of available net to fish the stock by two-thirds, to 9 km (from 27 km) in the offshore component of the fishery. Changes to the Queensland inshore fishery (within 7 nautical miles of the coast) also reduced the capacity for boats to target Blacktip Sharks. These measures are expected to have reduced fishing mortality of Blacktip Sharks.
The inability to assign more multispecies catch records to Blacktip Shark species makes it difficult to identify catch and effort trends for this species complex. Consequently, current catch levels and their impact on the biological stock is unknown, and there is insufficient information to confidently classify the status of this stock.
On the basis of the evidence provided above, the Gulf of Carpentaria multispecies biological stock is classified as an undefined stock.
Blacktip Sharks biology2,10–12
|Species||Longevity / Maximum Size||Maturity (50 per cent)|
|BLACKTIP SHARKS||Carcharhinus tilstoni: Females 15 years, males 13 years; 2 000 mm TL Carcharhinus limbatus: Maximum age unknown, 2 500 mm TL C. sorrah: Females 14 years, males 9 years; 1 600 mm TL||C. tilstoni: 5–6 years; females 1 350–1 400 mm, males 1 200 mm TL C. limbatus: males 1 800 mm, females unknown C. sorrah: 2–3 years; both sexes 900–950 mm TL|
Distribution of reported commercial catch of Blacktip Sharks
|Hand Line, Hand Reel or Powered Reels|
|Hand Line, Hand Reel or Powered Reels|
|Total allowable catch|
|Total allowable effort|
|8 in ONLF|
- Offshore Net and Line Fishery (NT)
|Commercial||48.40t in ONLF|
- Offshore Net and Line Fishery (NT)
a Queensland – Indigenous In Queensland, under the Fisheries Act 1994 (Qld), Indigenous fishers in Queensland are able to use prescribed traditional and non-commercial fishing apparatus in waters open to fishing. Size and possession limits, and seasonal closures do not apply to Indigenous fishers. Further exemptions to fishery regulations may be applied for through permits.
b Indigenous Subject to the defence that applies under 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.
c Western Australia – Commercial (Fishing methods) No commercial fishing has occurred in the Western Australia jurisdiction since April 2009.
d Western Australia – Recreational (Management methods) A recreational fishing from boat licence is required for recreational fishing from a powered vessel in Western Australia.
e New South Wales – Indigenous Aboriginal Cultural Fishing Interim Access Arrangement - allows an Aboriginal fisher in New South Wales to take in excess of a recreational bag limit in certain circumstances, for example, if they are doing so to provide fish to other community members who cannot harvest themselves
f New South Wales – Indigenous Aboriginal Cultural Fishing Interim Access Arrangement - allows an Aboriginal fisher in New South Wales to take in excess of a recreational bag limit in certain circumstances, for example, if they are doing so to provide fish to other community members who cannot harvest themselves
Commercial catch of Blacktip Sharks
Effects of fishing on the marine environment
- Pelagic gillnets and longlines are not intended to come into contact with the seabed, and, under normal circumstances, they have no impact on benthic habitats. Demersal gillnets contact the seabed but are generally set away from any benthic structures (for example, reefs, sponges or mangroves) that could be damaged by them, or cause damage to them. The physical characteristics of these gears and the way in which they are fished are also selective for the target species and size classes.
- However, these fishing methods do interact with threatened, endangered and protected (TEP) species. Although reported interactions are low, the impact on the populations of most TEP species is either unknown13–19 or assessed as negligible to low14. Longline fishing on the east coast has been shown to have the potential to threaten the long-term viability of the east coast population of Grey Nurse Shark (Carcharias taurus)20.
Environmental effects on BLACKTIP SHARKS
- The impact of environmental factors on biological stocks of Blacktip Sharks is unknown. These species are adapted to a range of environmental conditions and are therefore likely to be resilient to environmental changes.
- 1 Ovenden, JR, Street, R, Broderick, D, Kashiwagi, T and Salini, J 2007, Genetic population structure of Black-tip Sharks ( Carcharhinus tilstoni and C. sorrah) in northern Australia, in J Salini, R McAuley, S Blaber, RC Buckworth, J Chidlow, N Gribble, JR Ovenden, S Peverell, R Pillans, JD Stevens, I Stobutzki, C Tarca and TI Walker (eds), Northern Australian sharks and rays: the sustainability of target and bycatch species, phase 2, Fisheries Research and Development Corporation, Cleveland, Queensland.
- 2 Harry, AV 2011, Life histories of commercially important tropical sharks from the Great Barrier Reef World Heritage Area, PhD thesis, James Cook University, Townsville.
- 3 Morgan, JA, Harry, AV, Welch, DJ, Street, R, White, J, Geraghty, PT, Macbeth, WG, Tobin, A, Simpfendorfer, CA and Ovenden, JR 2011, Detection of interspecies hybridisation in Chondrichthyes: hybrids and hybrid offspring between Australian (Carcharhinus tilstoni) and common (C. limbatus) Blacktip Shark found in an Australian fishery, Conservation Genetics, 13: 455–463.
- 4 Grubert, MA, Saunders, TM, Martin, JM, Lee, HS and Walters, CJ 2013, Stock assessments of selected Northern Territory fishes, Fishery report 110, Northern Territory Government, Darwin.
- 5 Bradshaw, CJA, Field, IC, McMahon, CR, Johnson, GJ, Meekan, MG and Buckworth, RC 2013, More analytical bite in estimating targets for shark harvest, Marine Ecology Progress Series, 488: 221–232.
- 6 Molony, B, McAuley, R and Rowland, F 2012, Northern shark fisheries status report: Statistics only, in WJ Fletcher and K Santoro (eds), State of the fisheries and aquatic resources report 2012/13, Western Australian Department of Fisheries, Perth, 216–217.
- 7 Leigh, GM, 2016, Stock assessment of whaler and hammerhead sharks (Carcharhinidae and Sphyrinidae) in Queensland, Agri-Science Queensland, Department of Agriculture and Fisheries, Brisbane.
- 8 Harry, AV, Tobin, AJ, Simpfendorfer, CA, Welch, DJ, Mapleston, A, White, J, Williams, AJ, Stapley, J 2011, Evaluating catch and mitigating risk in a multispecies, tropical, inshore shark fishery within the Great Barrier Reef World Heritage Area. Marine and Freshwater Research 62:710-721.
- 9 Macbeth, WG, Geraghty, PT, Peddemors, VM, and Gray, CA 2009, Observer-based study of targeted commercial fishing for large shark species in waters of New South Wales, Industry and Investment New South Wales - Fisheries Final Report Series 82.
- 10 Harry, AV, Morgan, JAT, Ovenden, JR, Tobin, A, Welch, DJ and Simpfendorfer, C 2012, Comparison of the reproductive ecology of two sympatric Blacktip Sharks (Carcharhinus limbatus and Carcharhinus tilstoni) off north-eastern Australia with species identification inferred from vertebral counts, Journal of Fish Biology, 81: 1225–1233.
- 11 Harry, AV, Tobin, AJ, Simpfendorfer, CA, Welch, DJ, Mapleston, A, White, J, Williams, AJ and Stapley, J 2011, Evaluating catch and mitigating risk in a multi-species, tropical, inshore shark fishery within the Great Barrier Reef World Heritage Area, Marine and Freshwater Research, 62: 710–721.
- 12 Last, PR and Stevens, JD 2009, Sharks and rays of Australia, CSIRO Publishing, Collingwood.
- 13 Queensland Department of Agriculture, Fisheries and Forestry 2013, Review of management arrangements for the east coast Queensland shark fishery 2013, Wildlife Trade Operation Condition 7, Queensland DAFF, Brisbane.
- 14 McAuley, R and Rowland, F 2012, Northern shark fisheries status report, in WJ Fletcher and K Santoro (eds), State of the fisheries and aquatic resources report 2011/12, Western Australian Department of Fisheries, Perth, 222–228.
- 15 Northern Territory Government 2012, Fishery status reports 2011, Fishery report 111, Department of Primary Industry and Fisheries, Darwin.
- 16 Queensland Department of Agriculture, Fisheries and Forestry 2014, Gulf of Carpentaria Inshore Fin Fish Fishery, 2012 fishing year report, Queensland DAFF, Brisbane.
Rowling, KA, Hegarty, A and Ives, M 2010, Status of fisheries resources in NSW 2008/09, Industry and Investment New South Wales, Cronulla.
- 18 Queensland Department of Employment, Economic Development and Innovation 2011, Fishery observation in the East Coast Inshore Fin Fish Fishery, Queensland DEEDI, Brisbane.
- 19 Stapley, J and Rose, C 2009, A report on data collected by fisheries observers in the Queensland Offshore Commercial Mesh Net Fishery (N9) in the Gulf of Carpentaria, 2000–2006 , Queensland Department of Primary Industries and Fisheries, Brisbane.
- 20 Bradshaw, CJA, Peddemors, VM, McAuley, RB and Harcourt, R 2008, Population viability of eastern Australia Grey Nurse Sharks under fishing mitigation and climate change, final report to the Commonwealth of Australia, Department of the Environment, Water, Heritage and the Arts; Research Institute for Climate Change and Sustainability, School of Earth and Environmental Sciences, University of Adelaide.