MORETON BAY BUGS
Thenus parindicus, Thenus australiensis, Thenus spp.
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Stock Status Overview
|Commonwealth||Northern Prawn Fishery||NPF||Sustainable||Catch|
|Commonwealth||Torres Strait Prawn Fishery||TSPF||Sustainable||Catch|
- Northern Prawn Fishery (CTH)
- Torres Strait Prawn Fishery (CTH)
Reef Bug (Thenus australiensis) and Mud Bug (T. parindicus) are known collectively as ‘Moreton Bay Bugs’. Moreton Bay Bugs are distributed along the tropical and subtropical coast of Australia from northern New South Wales to Shark Bay in Western Australia2. No studies have been carried out on the biological stock structure of Australian Moreton Bay Bugs. The two species have overlapping distributions; may be trawled together; are undifferentiated in the catch; and are assessed together.
Given the uncertainty in biological stock structure, here assessment of stock status is presented at the management unit level—Queensland and Commonwealth; and the jurisdictional level—Western Australia.
Northern Prawn Fishery
Northern Prawn Fishery (Commonwealth) trawl surveys were used to estimate the biomass of Moreton Bay Bugs in the Gulf of Carpentaria, from which an estimate of acceptable biological catch was derived3. This assessment estimated the annual acceptable biological catch for Moreton Bay Bugs in the fishery at 1887 tonnes (t) (95 per cent confidence interval 1716–2057 t). Annual commercial catches have remained well below this (catch peaked at 120 t in 1998). Catches were 59 t in 2014 and 77 t in 2015. The above evidence indicates the biomass of this stock is unlikely to be recruitment overfished.
Fishing mortality has been low in recent years, and ecological risk assessments4 have indicated that the risk of stock depletion of Moreton Bay Bugs is low. A trigger catch limit of 100 t is also in place. If this limit is reached then additional analysis will be conducted to ensure that there are no sustainability concerns with the harvest level. Fishing mortality of juveniles is also reduced by regulating the size at which Moreton Bay Bugs may be retained, and spawning potential is protected through prohibiting retention of egg bearing females. Given the low level of catch in recent years compared to estimates of acceptable biological catch, the stock is unlikely to become recruitment overfished.
On the basis of the evidence provided above, the Northern Prawn Fishery (Commonwealth) management unit is classified as a sustainable stock.
Torres Strait Prawn Fishery
No formal stock assessment exists for Moreton Bay Bugs in the Torres Strait Prawn Fishery (Commonwealth) (TSPF) management unit. Assessment of seabed and associated biodiversity in the Torres Strait5,6 estimated the 2007 Moreton Bay (Reef) Bug biomass at 124 t, only 19 per cent of which was located within the area exposed to prawn trawling (based on the 2005 footprint of the fishery using vessel monitoring system data). The biomass of Mud Bugs was estimated to be 151 t. Only 18 per cent of biomass was located in areas exposed to prawn trawling. With the decline in fishing effort in recent years, fishing mortality is also likely to have declined. Fishing mortality of juveniles is also reduced by regulating the size at which Moreton Bay Bugs may be retained, and spawning potential is protected through prohibiting retention of egg bearing females. Research has found that Mud Bug egg production is maintained at the minimum size limit of 75 mm CW7.The above evidence indicates the biomass of this stock is unlikely to be recruitment overfished.
The Torres Strait assessment of seabed and associated biodiversity5 indicated that Moreton Bay Bugs are unlikely to have been exposed to high levels of fishing pressure in the Torres Strait Protected Zone. In 2013–15 annual catch of Moreton Bay Bugs averaged 23 t, which is estimated to be less than 10 per cent of available biomass, most of which inhabits extensive areas outside of fished areas. Trawl operations in the TSPF cover only a small proportion—approximately 20 per cent6—of the Torres Strait Protected Zone. Lower fishing effort has resulted in reduced spatial coverage of the fishery in recent years. 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 Torres Strait Prawn Fishery (Commonwealth) management unit is classified as a sustainable stock.
Moreton Bay Bug biology16,17
|Species||Longevity / Maximum Size||Maturity (50 per cent)|
|MORETON BAY BUGS||~7 years T. australiensis: Males 106 mm CW , Females 124 mm CW T. parindicus: Males 87 mm CW, Females 103 mm CW||T. australiensis (Female): 82 mm CW T. parindicus (Female): 75 mm CW|
Distribution of reported commercial catch of Moreton Bay Bugs
|Retention of females with eggs prohibited|
|51 in NPF, 23 in TSPF|
- Northern Prawn Fishery (CTH)
- Torres Strait Prawn Fishery (CTH)
|Commercial||77.63t in NPF, 27.88t in TSPF|
- Northern Prawn Fishery (CTH)
- Torres Strait Prawn Fishery (CTH)
a Commonwealth – Recreational The Commonwealth Government does not manage recreational fishing. Recreational fishing in Commonwealth waters is managed by the states or territory immediately adjacent to those waters, under their management regulations.
b Commonwealth – Indigenous The Commonwealth Government does not manage non-commercial Indigenous fishing (with the exception of the Torres Strait). In general, non-commercial Indigenous fishing in Commonwealth waters is managed by the states or territory immediately adjacent to those waters. In the Torres Strait, both commercial and non-commercial Indigenous fishing is managed by the Torres Strait Protected Zone Joint Authority (PZJA) through the Australian Fisheries Management Authority (Commonwealth), Department of Agriculture Fisheries and Forestry (Queensland) and the Torres Strait Regional Authority. The PZJA also manages non-Indigenous commercial fishing in the Torres Strait.
c Queensland – Indigenous In Queensland, under the Fisheries Act 1994, Indigenous fishers 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 can be obtained through permits.
Commercial catch of Moreton Bay Bugs - note confidential catch not shown
Effects of fishing on the marine environment
- The impact of trawling on habitats is managed in the Queensland and Western Australian fisheries which harvest Moreton Bay Bugs. In Queensland, the Great Barrier Reef Marine Park (GBRMP) occupies 63 per cent of the East Coast Otter Trawl Fishery18, 34 per cent of which is open to trawling5, but effort is highly aggregated, occurring within only a small fraction (six per cent) of the open area9. South of the GBRMP, the fishery operates in only 10 per cent of the area open to trawling19. In Western Australia, extensive permanent and temporary closures result in the fleet operating in only seven per cent of the Shark Bay region, less than 30 per cent of the Exmouth Gulf, and less than three per cent of the north coast region. Fishing operations are restricted to areas of sand and mud, where trawling has minimal long-term physical impact20–23. The Northern Prawn Fishery (Commonwealth) (NPF) also uses a system of closures (spatial and seasonal) to manage the fishery, as well as other input controls (for example limited entry, gear restrictions). A total of 2.1 per cent of the total managed area of the fishery is subject to permanent closures, while 8.3 per cent is subject to seasonal closures24. The Torres Strait Prawn Fishery (Commonwealth) (TSPF) also employs spatial and temporal closures which are used to protect small prawns, but also important habitat areas like breeding populations of turtles25.
- Although the incidental capture of byproduct and bycatch species by trawling can lead to a range of indirect ecosystem effects26, studies in Queensland and Western Australia, found no significant difference in biodiversity or overall distribution patterns of seabed biota between trawled and non-trawled areas12,22. An assessment of trawl-related risk in the GBRMP found that the East Coast Otter Trawl Fishery (Queensland) (Qld ECOTF), posed no more than an intermediate risk of overfishing species assemblages exposed to trawling9. The spatial contraction and/or temporal reduction in effort in these jurisdictions (see above) are likely to have mitigated somewhat the ecosystem impacts of trawling. Similarly, the Commonwealth fisheries have undergone varying levels of ecological risk assessment. In the NPF, the ecological risk management report identifies priority species at high risk. However, no target or protected species have been assessed as high risk because of the fishery27. An ecological risk assessment has also been conducted for the TSPF and identified nine species where trawling has had a negative impact on their biomass28.
- The use of bycatch reduction devices (BRDs) in trawling can significantly reduce bycatch—by more than 50 per cent by weight in some fisheries29. In the Qld ECOTF the mandatory use of BRDs from 1999 and of turtle excluder devices (TEDs) from 2001, largely eliminated capture of most large bycatch species, including turtles, sharks and rays30. BRDs and TEDs became mandatory in NPF in 2001. Use of TEDs in the NPF reduced turtle bycatch from 5 700 individuals per year (before 2001) to approximately 30 per year (after 2001)4. The introduction of TEDs in the Western Australian trawl fisheries in 2003 reduced turtle bycatch by at least 95 per cent31. BRDs and TEDs have been mandatory in the Shark Bay and Exmouth Gulf prawn fisheries since 2003 and in all northern Western Australian prawn fisheries since 2005. All prawn trawlers operating in Western Australia now must use BRDs, including TEDs, secondary fish exclusion devices and hoppers to increase survival of returned fish. Commitment to continuous improvement in bycatch mitigation has facilitated increased use of best practice TEDs and BRDs in the Qld ECOTF since 2008. Reduced impact of trawling and a general absence of high risk of overfishing bycatch species has been acknowledged in recent ecological risk assessments of the fishery9,10.
Environmental effects on MORETON BAY BUGS
- There are suggestions that ocean acidification, changes in ocean current patterns (for example, strengthening of the East Australian Current), and increased intensity of tropical storms associated with climate change may affect food availability, larval survival, dispersion and settlement patterns, abundance of Moreton Bay Bugs, and the distribution and level of catches in the East Coast Otter Trawl Fishery (Queensland)32. Increased rainfall and sea level rise have been identified33 as key impacts of climate change in the region of the Northern Prawn Fishery (Commonwealth). These impacts have the potential to modify the geographical distribution of Moreton Bay Bug stocks.
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- 10 Queensland Department of Agriculture and Fisheries 2016, unpublished, An ecological risk assessment of the East Coast Trawl Fishery in Southern Queensland Including the River and Inshore Beam Trawl Fishery, Queensland Government, Department of Agriculture, Fisheries and Forestry, Brisbane.
- 11 Queensland Department of Agriculture and Fisheries, unpublished, Queensland Stock Status Assessment Workshop, 14–15 June 2016, Species Summary Pages, Brisbane.
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