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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.

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.

East Coast Otter Trawl Fishery

Moreton Bay Bugs are targeted in the East Coast Otter Trawl Fishery (Queensland) (Qld ECOTF) management unit. They are taken predominantly by two fleets, one fleet north of 22­° S trawling for prawns and one fleet south of 22° S trawling for scallops. No formal stock assessment has been conducted. The 2014–19 catch shows a slight increasing trend [QFISH 2020]. Standardised catch rates in both fleets increased between 2002 and 2013 and then declined before stabilising in recent years (2015 to 2019) [Helidoniotis 2020]. After 2014 the trend between the fleets diverged and the catch rate from the northern fleet declined while the southern fleet catch rate increased, suggesting that targeting of Moreton Bay Bug was more pronounced in the southern fleet [Helidoniotis 2020]. Since 2010, retention of berried female bugs has been allowed, which has probably contributed to the higher subsequent catch rates. On average 90 per cent of the east coast Moreton Bay Bug catch is taken from areas open to trawling in the Great Barrier Reef Marine Park (GBRMP) [Zeller et al. 2014]. Biophysical modelling estimated that in 2005 significant parts of the biomass (54 per cent of T. australiensis; and 45 per cent of T. parindicus) were within GBRMP trawl closures [Pitcher et al. 2007a]. Moreton Bay Bugs have a 45 day larval phase [Jones 1988]. It is likely that spawning adults in areas closed to fishing within GBRMP closures contribute recruits to other areas open to trawling. Recent ecological risk assessments have reported a low risk of the stock being recruitment overfished in the GBRMP [Pears et al. 2012], where harvesting pressure on the stock is greatest, and an intermediate risk of being classified as recruitment overfished south of the GBRMP [Jacobsen et al. 2018]. The above evidence indicates that the biomass of this stock is unlikely to be depleted and that recruitment is unlikely to be impaired.

The number of days on which Moreton Bay Bugs were caught and the number of boats catching Moreton Bay Bugs declined from 2000 to 2010, and have been relatively stable since [QFISH 2020]. Stable effort in high abundance grids since 2010 indicates that fishing mortality in high biomass areas has not increased substantially. Extensive trawl closures in the GBRMP ensure that a significant proportion of the biomass is not subject to fishing mortality. Based on yield-per-recruit analysis, capture at ≥ 75 mm CW allows Moreton Bay Bugs to spawn before they enter the fishery [Courtney 1997]. Square-mesh cod end bycatch reduction devices (BRDs) were made mandatory in the Scallop Fishery in 2015, allowing juvenile Moreton Bay Bugs to escape trawl capture and reducing incidental fishing mortality [Courtney et al. 2008]. Moreton Bay Bugs are also known to survive discarding well [Hill et al. 1998]. 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 East Coast Otter Trawl Fishery (Queensland) management unit is classified as a sustainable stock.

Moreton Bay Bug biology [Courtney 1997, Jones 1988]

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

1. Courtney, AJ 1997, A study of the biological parameters associated with yield optimisation of Moreton Bay Bugs, Thenus spp., final report (project 92/102), Queensland Department of Primary Industries, Brisbane.
2. Courtney, AJ 2002, The status of Queensland’s Moreton Bay Bug (Thenus spp.) and Balmain Bug (Ibacus spp.) stocks, Queensland Government, Department of Primary Industries, Brisbane.
3. Courtney, AJ, Campbell, MJ, Roy, DP, Tonks, ML, Chillcott, KE and Kyne, PM 2008, Round scallops and square meshes: a comparison of four codend types on the catch rates of target species and by-catch in the Queensland (Australia) saucer scallop (Amusium balloti) trawl fishery, Marine and Freshwater Research, 59: 849–864.
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