Table 1: Stock status determination for Blue Grenadier
Jurisdiction |
Commonwealth |
Stock |
CTS |
GABTS |
Stock status |
|
|
Sustainable |
Sustainable |
Indicators |
Spawning stock biomass, fishing mortality |
Low current and historical fishing pressure |
CTS = Commonwealth Trawl Sector; GABTS = Great Australian Bight Trawl Sector (Commonwealth)
Data from otolith studies provide support for two separate biological stocks of Blue Grenadier: one in the region of the Great Australian Bight Trawl Sector (Commonwealth) and the other in the region of the Commonwealth Trawl Sector1.
Commonwealth Trawl Sector biological stock
Blue Grenadier in the Commonwealth Trawl Sector and the Gillnet, Hook and Trap Sector (Commonwealth) is managed under a multiyear total allowable catch (TAC). The Commonwealth Trawl Sector accounts for the vast majority of the catch from these two sectors (99.9 per cent in the 2010 fishing season). Given the limited catch from the Gillnet Hook and Trap Sector, the biological stock is referred to in this report as the Commonwealth Trawl Sector biological stockb. The most recent assessment2 includes estimates of spawning biomass from acoustic surveys in 2003–10 and egg survey estimates of female spawning biomass in 1994. The assessment estimated the female spawning stock biomass in 2010 to be 87 per cent of the unfished level. The biological stock is not considered to be recruitment overfished.
On the basis of the 2008 assessment3, a long-term recommended biological catch of 4700 tonnes (t) per year was established for 2009 to 20114. The 2010 TAC was set at 4700 t. Uncaught quota in 2009 resulted in an adjusted TAC for the 2010 fishing season of 5088 t5. Total commercial catch in the 2010 fishing season was below this TAC, at 4031 t. This level of fishing mortality is unlikely to cause the biological stock to become recruitment overfished.
On the basis of the evidence provided above, the biological stock is classified as a sustainable stock.
Great Australian Bight Trawl Sector (Commonwealth) biological stock
There have been no stock assessments for the Great Australian Bight Trawl Sector (Commonwealth) biological stock of Blue Grenadier, and no estimates of fishing mortality or biomass have been made. A catch trigger of 400 t is in place, above which data collection and the development of an assessment plan are required. A cease-to-fish catch trigger of 500 t also applies6.
Blue Grenadier from this biological stock are targeted on the upper continental slope (around 200–700 m depth). Fishing effort on the Great Australian Bight continental slope has been decreasing since 2005 due to targeting of shelf species, and accounted for 3 per cent of total effort in the fishery in 20107. There is substantial slope habitat across the Great Australian Bight and Western Australia, with fishing generally limited to a small area. It is therefore likely that parts of this biological stock remain unfished. Commercial catches of Blue Grenadier are typically low in the Great Australian Bight Trawl Fishery (9 t in 2010), with a peak commercial catch in 2005 of 422 t. This evidence suggests that the current level of fishing mortality is unlikely to cause the biological stock to become recruitment overfished, and that the biological stock is unlikely to be recruitment overfished.
On the basis of the evidence provided above, the biological stock is classified as a sustainable stock.
Table 2: Blue Grenadier biology1
Longevity and maximum size |
25 years; ~110 cm TL, weight 6 kg |
Maturity (50%) |
4–5 years; females 64 cm TL, males 57 cm TL |
TL = total length
Figure 1: Distribution of reported commercial catch of Blue Grenadier in Australia waters, 2010
- Demersal otter fish trawl is used in both the Commonwealth Trawl Sector and the Great Australian Bight Trawl Sector (Commonwealth). In addition, midwater fish trawl is used to target spawning Blue Grenadier in the Commonwealth Trawl Sector. Hook and line (specifically dropline and demersal longline) methods take an incidental amount of catch in the Gillnet, Hook and Trap Sector (Commonwealth).
- Both input and output controls are applied to the Blue Grenadier biological stocks:
- Input controls in both sectors include limited entry and spatial closures.
- Output controls include a commercial TAC and individual transferable quotas in the Commonwealth Trawl Sector and trigger limits in the Great Australian Bight Trawl Sector (Commonwealth).
- In the 2010 fishing season, Blue Grenadier catch was recorded for 26 Commonwealth Trawl Sector vessels (5 of which reported more than 100 t of catch); 11 Gillnet, Hook and Trap Sector (Commonwealth) vessels; and 3 Great Australian Bight Trawl Sector (Commonwealth) vessels.
- The total amount of Blue Grenadier caught commercially in Australia in 2010 was 4040 t, comprising 4025 t in the Commonwealth Trawl Sector; 6 t in the Gillnet, Hook and Trap Sector (Commonwealth); and 9 t in the Great Australian Bight Trawl Sector (Commonwealth). This species is not believed to be targeted by either recreational or Indigenous fishers.
a)
b)
Figure 2: a) Commercial catch of Blue Grenadier in Australian waters, 1979 to 2010 (calendar year);
b) percentage of unfished biomass for the Commonwealth Trawl Sector biological stock, 1979 to 2010
In the Commonwealth Trawl Sector, commercial catch increased from 1999 to 2004, with a corresponding increase in effort. Commercial catch and catch per unit effort have been relatively constant over recent years.
Commercial catch of Blue Grenadier in the Great Australian Bight Trawl Sector (Commonwealth) has historically been low, only exceeding 50 t from 2004 to 2007, as a result of increasing fishing effort on the continental slope over this period. Fishing effort on the slope has dramatically decreased since this period, with catches not exceeding 10 t since 2008.
- There can be a substantial level of bycatch in the fish trawl sector. In 2006, mandatory gear requirements were introduced for otter trawls to reduce the catch of small species and juvenile fish.
- Interactions occur with animals protected under the Environment Protection and Biodiversity Conservation Act 1999. Interactions are known to occur with marine mammals (dolphins, seals and sea lions), seabirds, some shark species, and seahorses and pipefish (syngnathids).
- Seal interactions have been observed in the winter factory vessel trawl fishery for Blue Grenadier off western Tasmania (in the Commonwealth Trawl Sector). Seal excluder devices have been compulsory in this component of the Southern and Eastern Scalefish and Shark Fishery (Commonwealth) since 2005, with the aim of reducing seal mortalities. Operations have developed further mitigation protocols, including using breakaway ties that keep the net closed until it is below depths that seals regularly inhabit, adopting techniques to close the trawl opening during recovery to minimise opportunities for seals to enter the net, switching off gantry lights that are not required during night trawling to avoid attracting bait species and seals, and dumping offal only when the boat is not engaged in deploying or hauling gear8.
- Changes in ecosystem structure and function due to changes in climate may affect larval recruitment of Blue Grenadier9.
a Australian Bureau of Agricultural and Resource Economics and Sciences
b The fishery includes two components, a winter fishery
targeting spawning aggregations on the west coast of Tasmania, and a
non-spawning fishery, which is less targeted.