Northern Prawn Fishery (Commonwealth) Brown Tiger Prawn management unit
Brown and Grooved Tiger Prawn stocks are assessed as part of an integrated bioeconomic model undertaken for the Northern Prawn Fishery (Commonwealth) 2. The base-case estimate of the size of the Brown Tiger Prawn spawner stock at the end of 2012 as a percentage of spawner stock size (S) at maximum sustainable yield (MSY; S2012/SMSY) was 104.1 per cent2. On this basis, the management unit is not considered to be recruitment overfished3.
In 2010, fishing effort (E) was well below the level that would achieve MSY (E2012/EMSY was 28.8 per cent)2. This level of effort is unlikely to cause the management unit to become recruitment overfished3.
On the basis of the evidence provided above, the management unit is classified as a sustainable stock.
Northern Prawn Fishery (Commonwealth) Grooved Tiger Prawn management unit
The base-case estimate of the size of the Grooved Tiger Prawn spawner stock at the end of 2012 as a percentage of spawner stock size at MSY (S2012/SMSY) was 108.6 per cent2. On this basis, the management unit is not considered to be recruitment overfished 3.
In 2010, fishing effort on Grooved Tiger Prawns was well below the level that would achieve MSY (E2012/EMSY was 38.8 per cent) 2. This level of effort is unlikely to cause the management unit to become recruitment overfished3.
On the basis of the evidence provided above, the management unit is classified as a sustainable stock.
Torres Strait Prawn Fishery (Commonwealth) Brown Tiger Prawn management unit
The most recent assessment of Brown Tiger Prawn in the Torres Strait Prawn Fishery (Commonwealth) used two separate modelling approaches, producing two separate estimates of MSY and effort at MSY4. Commercial catch of this stock has been below the mean estimates of MSY (606 tonnes [t] and 676 t) for the past eight seasons (2006 to 2013), and effort has been below the estimates of EMSY (8245 and 9197 fishing nights) for the past 10 seasons (2004 to 2013). This level of catch and effort is unlikely to cause the management unit to become recruitment overfished5.
Some components of the assessment were updated in 20076; this indicated that biomass in 2006 was 60–80 per cent of the unfished (1980) level. This was considerably higher than biomass that supports MSY (BMSY), estimated to be around 28–38 per cent of the unfished level4,6. As a result of the 2006 biomass estimate, and low levels of catch and effort since that estimate, the management unit is not considered to be recruitment overfished5.
On the basis of the evidence provided above, the management unit is classified as a sustainable stock.
East Coast Otter Trawl Fishery (Queensland) Brown and Grooved Tiger Prawn management unit
Recent modelling of the combined Brown and Grooved Tiger Prawn resource in the East Coast Otter Trawl Fishery (Queensland) suggests that fishing mortality may have been excessive before 2000, causing the spawning stock to be depleted to less than the level that would achieve MSY (SMSY)7. The spawning stock has since recovered to a level well above SMSY7. Regional nominal catch rates have all increased and are at historical high levels8,9. A key indicator of risk from trawling is the percentage of combined Brown and Grooved Tiger Prawn biomass in areas closed to trawling. Research estimated that, in 2005, 38 per cent of the Brown Tiger Prawn biomass and 26 per cent of the Grooved Tiger Prawn biomass within the Great Barrier Reef Marine Park (GBRMP) was unavailable to trawl capture as a result of permanent closures10. These closures remain in place. The above evidence indicates that the biomass of the management unit is unlikely to be recruitment overfished.
From 2000 to 2013, total catch showed a decreasing trend, but this coincided with a significant decline in fishing effort, currently at a historical low level9. At 966 t, the 2013 catch is well below the MSY estimate of 1239 t11 and the more recent initial estimate of 1706–1907 t 7. The most recent assessment of combined Brown and Grooved Tiger Prawn fishing effort11 in 2004 estimated EMSY at 19 618 fishing days. From 2007 to 2013, effort has been well below EMSY9. Recent semiquantitative risk assessment of individual Tiger Prawn species found that the risk of their becoming recruitment overfished at the level of fishing pressure exerted on the management unit in 2009 was low for Brown Tiger Prawn12,13, low for Grooved Tiger Prawn south of the GBRMP13 and intermediate for Grooved Tiger Prawn within the GBRMP 12. Although fishing power for Tiger Prawn is increasing at a rate of 1.5–3 per cent per year7, since 2009 the number of trawl days in the GBRMP has declined by 23 per cent9. Effective fishing effort in the GBRMP is therefore unlikely to be increasing. 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.
Shark Bay Prawn Managed Fishery (Western Australia) Brown Tiger Prawn management unit
Standardised catch-per-unit-effort data (also referred to as catch rate) are used as an indicator of abundance in the Shark Bay Prawn Managed Fishery (Western Australia) Brown Tiger Prawn management unit, and can be used to monitor changes in stock levels from year to year. The average commercial catch and catch rate are compared with a 10-year (1989 to 1998) reference point14.
This management unit is also assessed each year using fishery-independent recruitment and spawning stock surveys. These methods are the primary means for assessing stock status. Recruitment surveys provide an index of annual recruitment and are also the basis of an annual Brown Tiger Prawn catch prediction.
A spawning stock–recruitment relationship exists for Brown Tiger Prawns15–17, and the maintenance of adequate spawning stock (using a target catch rate) is the key management objective. Brown Tiger Prawns are managed to reference levels (catch rates) and accompanying control rules. A mandatory closure of the Brown Tiger Prawn spawning area is enforced, either to a catch-rate target or on a set date (around June–July), whichever is sooner, to protect the spawning stock. The fishery operates on a real-time management basis: commercial catch rates are monitored nightly to ensure that the Brown Tiger Prawn spawning areas are closed at the appropriate time. As fishing ceases in an area, fishery-independent surveys are conducted to verify catch rates in the spawning area.
The July 2013 spawning stock survey showed a mean standardised catch rate of 20.2 kg/hour in the Brown Tiger Prawn spawning area. This is below the target level of 25 kg/hour18, but above the threshold level (10 kg/hour). It is therefore considered that the biomass of this management unit is unlikely to be recruitment overfished, and it appears that, overall, there was an adequate spawning stock during the key spawning period in 2013.
With respect to fishing mortality, the assessment sets an annual target catch range for the 2013 season of 400–700 t. In addition, the Brown Tiger Prawn catch prediction (based on fishery-independent recruitment surveys) was 400–600 t. The total catch (660 t) was slightly above the prediction, but within the target catch range and above the average catch over the past 15 years (504 t)18. The level of fishing effort since 2007, when all boats adopted quad gear (four standardised nets), has remained between 33 000 and 41 000 trawl hours (standardised to twin nets); fishing effort in 2013 was 36 000 trawl hours. This evidence indicates that the current level of fishing mortality 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.
Exmouth Gulf Prawn Managed Fishery (Western Australia) Brown Tiger Prawn management unit
Stock assessments for the Exmouth Gulf Prawn Managed Fishery (Western Australia) Brown Tiger Prawn management unit are undertaken using similar methods to those used in the Shark Bay Prawn Managed Fishery (Western Australia). The management objective is to maintain the spawning biomass above the historically determined biological reference points; the current target is 25 kg/hour, with a limit of 10 kg/hour, in the spawning stock surveys. Three standardised Brown Tiger Prawn spawning stock surveys were carried out from August to October 2013, with an average catch rate of 22.6 kg/hour, slightly below the target level but above the limit reference point. The above evidence indicates that the biomass of this stock is unlikely to be recruitment overfished.
The stock in 2012 had been reduced to a level at which there was a significant risk of recruitment failure. This was despite the spawning stock abundance in the previous year (2011) being as high as ever recorded (60–100 kg/hour) throughout the key spawning period, and well above the target of 25 kg/hour. The 2013 recruitment surveys indicated continued low recruitment levels (32 kg/hour, which was above the limit but below the recruitment survey target of 40 kg/hour), but an improvement on 2012 recruitment (21.5 kg/hour). This was also reflected in slightly higher catches in the fishery. The recruitment surveys are used to predict the annual catch. The predicted low annual landings for both years resulted in modifications to the harvest strategy, including a later season opening, conservative fishing on Brown Tiger Prawns (47 t landed in 2012 and 95 t landed in 201318, compared with the long-term target catch range of 250–550 t) and daily monitoring of catch rates to ensure cessation of fishing at an adequate catch rate. This evidence indicates that the current level of fishing mortality is unlikely to cause the management unit to become recruitment overfished.
There is evidence of a link between declines in Tiger Prawn biomass and a heatwave in 2010–11 that was followed by continued higher-than-average water temperatures in the Exmouth Gulf in the summers of 2011–12 and 2012–1319. The declines appear to be partly associated with the loss of structured habitat (as a result of continued higher-than-average temperatures) in inshore Tiger Prawn nursery areas, which could impact survival of juveniles20 and cause low recruitment. Given the monitoring and management frameworks in place to control fishing pressure, it is unlikely that the low levels of recruitment have been due to overfishing or lack of appropriate fisheries management. There is confidence that recruitment levels will build under existing management, assuming that the habitat improves and no major negative environmental perturbations occur.
On the basis of the evidence provided above, the management unit is classified as a sustainable stock.
North Coast Prawn Managed Fisheries (Western Australia) Brown Tiger Prawn management unit
Small quantities of Brown Tiger Prawns have been landed from the North Coast Prawn Managed Fisheries (Western Australia) in recent years, with Brown Tiger Prawn being a key target species in only the Onslow Prawn Managed Fishery. These fisheries use annual catch in reference to a target catch range as an indicator of acceptable performance and whether the stock is subject to overfishing. If the annual catch falls outside the range, this needs to be adequately explained or additional investigations undertaken. In 2013, the Kimberley Prawn Managed Fishery landed 6 t18 (target catch range 15–60 t), and the Onslow (target catch range 10–120 t) and Nickol Bay (target catch range 2–40 t) Prawn Managed Fisheries each reported landings of less than 0.5 t. These low landings reflect the very low effort expended in these fisheries, particularly for Brown Tiger Prawn. Only one boat operated, for four nights in total, in the Onslow Prawn Managed Fishery in 2013. The fishing effort in the Kimberley Prawn Managed Fishery in 2013 was the second lowest on record (effort was lower in 2011). The fishing effort in the Nickol Bay Prawn Managed Fishery is primarily directed at White Banana Prawns, and hence overall effort is related to White Banana Prawn abundance. However, the overall annual mean fleet effort in this fishery has decreased since 2007 to around 160 boats-days, compared with 700 boat-days between 1990 and 2005. Therefore, the current level of fishing mortality is unlikely to cause the management unit to become recruitment overfished, and, given the recent history of low annual catches, the biomass of this stock is unlikely to be recruitment overfished.
On the basis of the evidence provided above, the management unit is classified as a sustainable stock.