Moreton Bay Bug ​Thenus australiensis^a, T. parindicus

East Coast Otter Trawl Fishery (Queensland) management unit

Moreton Bay Bugs are targeted in the East Coast Otter Trawl Fishery (Queensland) management unit. Although no formal stock assessment exists, the 2009 ecological risk assessment reported a low risk of the species being recruitment overfished in the Great Barrier Reef Marine Park (GBRMP)³, where harvesting pressure is greatest; the GBRMP catch of Moreton Bay Bugs averages 90 per cent of the East Coast Otter Trawl Fishery catch. An intermediate risk of the stock being recruitment overfished exists south of the GBRMP⁴, where about 10 per cent of the Moreton Bay Bug catch is taken. From 2009 to 2013, nominal fishing effort declined by 23 per cent in the GBRMP and 21 per cent in the East Coast Otter Trawl Fishery, implying that the risk of the stock being recruitment overfished has not increased. Long-term trends indicate decreasing fishing effort, increasing catch rate in areas where Moreton Bay Bug is in high abundance, and generally stable annual catch levels⁵. Nominal catch rate has been increasing since 2001 and is at historically high levels⁵. Since 2010, retention of berried female bugs has been allowed, which has probably contributed to the higher catch rates. The above evidence indicates that the biomass of the management unit is unlikely to be recruitment overfished.

Permanent closures in the GBRMP protect significant portions of the biomass in eastern Queensland. Research has estimated that closures included 54 per cent of the estimated GBRMP biomass of Reef Bug and 45 per cent of the estimated GBRMP biomass of Mud Bug in 2005⁶. Without evidence to the contrary, similar levels of protection from fishing mortality are assumed to have been maintained since then. In addition, a minimum legal size limit of 7.5 cm carapace width (CW), based on yield-per-recruit analysis, allows Mud Bug the opportunity to spawn before entering the fishery⁷. Individuals below this size are discarded from the retained catch. Research has found that post-capture survival is high among Moreton Bay Bugs⁸, turtle excluder devices (TEDs) lower catch rates of legal-sized Moreton Bay Bugs (greater than 7.5 cm CW),⁹ and 10 cm square-mesh codend bycatch reduction devices (BRDs) greatly lower incidental capture of undersized Reef Bug⁹ in the East Coast Otter Trawl Fishery (Queensland) management unit. In 2013, about one-third of East Coast Otter Trawl Fishery vessels used square-mesh codends. The above evidence indicates that the current level of fishing pressure is unlikely to cause the management unit to become recruitment overfished.

On the basis of the evidence provided above, the management unit is classified as a sustainable stock.

Western Australia

No formal stock assessment exists for Moreton Bay Bugs in Western Australia. Moreton Bay Bugs are not specifically targeted in Western Australia, and current commercial catch levels are low—only 6 tonnes (t) was reported as landed in 2013 for all Western Australian fisheries combined. Insufficient information is available to confidently classify the status of this stock.

On the basis of the evidence provided above, Moreton Bay Bug in Western Australia is classified as an undefined stock.

Northern Prawn Fishery (Commonwealth) management unit

An assessment of byproduct species in the Gulf of Carpentaria within the Northern Prawn Fishery (Commonwealth) was conducted in 2010¹⁰. This assessment estimated the annual acceptable biological catch for Moreton Bay Bugs in the fishery at 1887 t (95 per cent confidence interval: 1716 to 2057 t). Historical annual commercial catches are well below this; catch peaked at 120 t in 1998. The above evidence indicates that the biomass of the management unit is unlikely to be recruitment overfished.

Fishing mortality has been low in recent years, and ecological risk assessments¹¹ have suggested 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, additional analysis will be conducted to ensure that there are no sustainability concerns with the harvest level. Following the 2013 catch of 109 t, resources have been redirected to the collection of size-frequency distribution data for bugs for additional analysis. Fishing mortality is also reduced among the immature biomass by regulating the size at which Moreton Bay Bugs may be retained, and among the spawning biomass through prohibiting retention of egg-bearing females. Research has found that Mud Bug egg production is maintained when the minimum size limit is 7.5 cm CW⁷. Given the low level of catch in recent years, the management unit is unlikely to become recruitment overfished.

On the basis of the evidence provided above, the management unit is classified as a sustainable stock.

Torres Strait Prawn Fishery (Commonwealth) management unit

No formal stock assessment exists for Moreton Bay Bugs in the Torres Strait Prawn Fishery (Commonwealth) management unit. A recent assessment of seabed and associated biodiversity in the Torres Strait^12,13 estimated that, in 2007, biomass of Reef Bug was 124 t, only 19 per cent of which was located within the area exposed to prawn trawling (from the 2005 footprint of the fishery, using vessel monitoring system data). The biomass of Mud Bug was estimated to be 151 t. Again, a low percentage (18 per cent) of biomass was located in areas exposed to prawn trawling. With a decline in fishing effort in recent years, fishing mortality is also likely to have declined. Fishing mortality is also reduced among the immature biomass by regulating the size at which Moreton Bay Bugs may be retained, and among the spawning biomass through prohibiting retention of egg-bearing females. Research has found that Mud Bug egg production is maintained when the minimum size limit is 7.5 cm CW⁴.The above evidence indicates the biomass of the management unit is unlikely to be recruitment overfished.

The Torres Strait assessment of seabed and associated biodiversity¹² 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, a Moreton Bay Bug catch of 22 t was reported from areas open to fishing. This is less than 10 per cent of available biomass, most of which inhabits extensive areas outside the areas subject to fishing pressure. Trawl operations in the Torres Strait Prawn Fishery (Commonwealth) cover only a small proportion—approximately 20 per cent¹³—of the Torres Strait Protected Zone. Lower catch of Moreton Bay Bugs in recent years is likely a result of declining effort, accompanied by reduced spatial coverage of the fishery. The above evidence indicates that the current level of fishing pressure is unlikely to cause the management unit to become recruitment overfished.

On the basis of the evidence provided above, the management unit is classified as a sustainable stock.