MORETON BAY BUGS (2020)
Thenus parindicus, Thenus australiensis, Thenus spp.
Date Published: June 2021
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Reef Bug and Mud Bug, collectively known as Moreton Bay Bugs, are sustainable species distributed along the tropical and subtropical coast of Australia.
Stock Status Overview
|Commonwealth||Northern Prawn Fishery||Sustainable||
|Commonwealth||Torres Strait Prawn Fishery||Sustainable||Catch|
Reef Bug (Thenus australiensis) and Mud Bug (Thenus 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 Australia [George and Griffin 1972]. 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—Northern Prawn Fishery, Torres Strait Prawn Fishery (Commonwealth) and East Coast Otter Trawl Fishery (Queensland); 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 derived [Milton et al. 2010]. This assessment estimated the annual sustainable biological catch for Moreton Bay Bugs in the fishery at 1 887 tonnes (t) (95 per cent confidence interval 1 716–2 057 t). Annual commercial catches have remained well below this (catch peaked at 120 t in 1998). Catches were 35 t in 2018 and 36 t in 2019. The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired.
Fishing mortality has been low in recent years, and ecological risk assessments [Griffiths et al. 2007] 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 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. Catches have been low in recent years compared to estimates of acceptable biological catch. The above evidence indicates that the current level of fishing mortality is unlikely to cause the stock to become recruitment impaired.
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 (TSPF) management unit. Assessment of seabed and associated biodiversity in the Torres Strait [Pitcher et al. 2007b, Turnbull and Rose 2007] 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 with only 18 per cent of biomass being 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 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 carapace width [Courtney 2002].The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired.
The Torres Strait assessment of seabed and associated biodiversity [Pitcher et al. 2007b] indicated that Moreton Bay Bugs are unlikely to have been exposed to high levels of fishing pressure in the Torres Strait Protected Zone. In 2017–19 the annual catch of Moreton Bay Bugs averaged 12 t, which is estimated to be less than 6 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 cent [Turnbull and Rose 2007]—of the Torres Strait Protected Zone. Lower fishing effort has resulted in further reduction in spatial coverage of the fishery in recent years. The above evidence indicates that the current level of fishing mortality is unlikely to cause the stock to become recruitment impaired.
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 biology [Courtney 1997, Jones 1988]
|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|
|Retention of females with eggs prohibited|
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.
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.
Queensland – Indigenous (management methods) for more information see https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing
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- QFish, Department of Agriculture and Fisheries, www.qfish.gov.au
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