Saucer Scallop ​Amusium balloti

Mervi  Kangasa and Brad  Zellerb


Saucer Scallop

Table 1: Stock status determination for Saucer Scallop

Jurisdiction

Western Australia

Queensland

Stock

Western Australian
(AIMWTMF, SBSMF, SCTF, SWTF)

ECTF

Stock status

 

 

Sustainable

Sustainable

Indicators

Recruitment surveys, catch rate, catch

Catch, catch rate

AIMWTMF = Abrolhos Islands and Mid West Trawl Managed Fishery (Western Australia); ECTF = East Coast Trawl Fishery (Queensland);
SBSMF = Shark Bay Scallop Managed Fishery (Western Australia); SCTF = South Coast Trawl Fishery (Western Australia); SW TF = South West
Trawl Fishery (Western Australia)


Stock Structure

Saucer Scallop is distributed from the New South Wales south coast to the Northern Territory and down the Western Australia coast to Esperance1. There are two genetically separate biological stocks of Saucer Scallop that are separated across northern Australian waters: the western Australian biological stock and the eastern Australian biological stock. The western Australian biological stock spans most of the Western Australia coast; it is divided into a number of functionally independent management units that reflect the geography and differences in larval life history. As a result, the Shark Bay, Abrolhos Islands, and South West and South Coast trawl fisheries' biological sub-stocks need to be managed and monitored separately. Stock classification for the western Australian biological stock is based on evidence from each of the management units. The eastern Australian biological stock occurs from Innisfail (Queensland) to Jervis Bay (New South Wales). No commercial fishery for Saucer Scallop exists in New South Wales waters, and hence biological stock classification is based on the commercial fishery in central and southern Queensland (22–27ºS).


Stock Status

Western Australian biological stock

The western Australian Saucer Scallop biological stock is taken commercially from four management units, and stock assessments have been undertaken at the fishery level. Information from each of the assessments is combined here to determine the status of the biological stock.

The Shark Bay Scallop Managed Fishery (Western Australia) management unit is managed under a harvest strategy based on a constant escapement policy, to ensure adequate spawning biomass. Commercial catch rate threshold levels are used (400–450 kg meat weight/day) to stop fishing, to maintain breeding stock during the key spawning period (April–June); closure can be triggered when this level is reached, or on a date in May, irrespective of catch rate2. The current threshold levels are deemed to leave an adequate level of spawning stock to provide recruitment in the acceptable range, given 'normal' environmental conditions. A part of the biological stock is also carried over to the following year to provide a buffer for the spawning stock in case of low recruitment. Recruitment surveys are carried out before each season3–4 to determine a catch projection and whether the fishery should open. The commercial catch projection for 2010 was 1475 tonnes (t) (whole weight), and a total of 1592 t was taken2, which is within the acceptable catch range (1250–3000 t). This management strategy ensures that the biomass of this part of the biological stock is unlikely to be recruitment overfished. As well, fishing mortality is set at a level that is commensurate with the biomass and therefore will not cause this part of the biological stock to become recruitment overfished.

The Abrolhos Islands and Mid West Trawl Managed Fishery (Western Australia) management unit is also managed under a harvest strategy based on a constant escapement policy, to ensure that adequate spawning biomass is available during each spawning season. Breeding stock levels are maintained by fishing after the majority of the mature scallops spawn, setting the fishing period according to the catch prediction (based on recruitment surveys), and closing the fishery at a threshold catch-rate level (250 kg meat weight/day), or not opening sections of the fishery if the abundance is not considered sufficient2. The commercial catch projection for 2010 was 880–1320 t (whole weight), and a total of 806 t was taken2. Two factors contributed to the level of catch being below the target catch range: the Wallabi area was not fished to its potential catch, and fishing ceased early because the catch-rate threshold was met. This management strategy ensures that the biomass of this part of the biological stock is unlikely to be recruitment overfished, and fishing mortality will not be at a level that will cause this part of the biological stock to become recruitment overfished.

The South West Trawl Fishery (Western Australia) and South Coast Trawl Fishery (Western Australia) management units are small, low-effort fisheries. The effort is related to the abundance of Saucer Scallop in any one year, which can be highly variable. The level of effort (limited by the fact that only a few vessels are licensed in these fisheries) and the geographic extent of fishing in comparison with the biological stock distribution are not expected to adversely impact these parts of the biological stock2.

On the basis of the evidence provided above, the entire western Australian biological stock is classified as a sustainable stock.

East Coast Trawl Fishery (Queensland) biological stock

Saucer Scallop from eastern Australia is only taken commercially in the East Coast Trawl Fishery (Queensland). Stock assessments from this fishery form the basis of the biological stock status classification in this report.

Recent stock modelling5– 6 indicates that recent levels of Saucer Scallop fishing effort and harvest have been within sustainable limits. The East Coast Trawl Fishery ecological risk assessment (DEEDI, pers. comm. 2012) found that there is not more than an intermediate risk of Saucer Scallops being overfished at 2010 effort levels outside the Great Barrier Reef Marine Park (GBRMP) and a low risk of Saucer Scallops being overfished at 2010 effort levels within the GBRMP. The low risk within the GBRMP is due to a comprehensive network of permanent fishing closures, which protect a high proportion of the Saucer Scallop biomass—the most recent (and only) estimate of Saucer Scallop biomass within the GBRMP closures was 45 per cent of the total biomass in 20057. This evidence indicates that the current level of fishing mortality is unlikely to cause the biological stock to become recruitment overfished.

Quantitative modelling6 has found that recent harvesting levels (411 t meat weight) are less than the estimated range for maximum sustainable yield (500–800 t meat weight). To protect the biological stock from depletion, temporal closures are in place. The first is an annual southern closure to all trawling in Queensland waters south of latitude 22ºS from 20 September to 31 October. This closure protects post-spawning scallops. The second is a rotational temporal closure of spatially fixed high-density harvesting areas (Scallop Replenishment Areas), the locations of which have been identified through fisher experience and confirmatory fishery-independent catch-rate monitoring. Staggered opening of Scallop Replenishment Areas for 9 months and closure for 15 months provides alternating periods of access to the mature resource, interspersed with harvest-free periods that allow sub-adults to mature and spawn. Other management settings include a 9 cm minimum legal size from 1 November to the following 30 April, and 9.5 cm from 1 May to the end of the southern closure, and a total ban on daylight trawling5. This evidence indicates that the biomass of 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: Saucer Scallop biology1

Longevity and maximum size

2–3 years, maximum 4 years; 11.5–14.0 cm SH

Maturity (50%)

1 year; 8.5–9.0 cm SH

SH = shell height


Figure 1: Distribution of reported commercial catch of Saucer Scallop in Australian waters, 2010

Figure 1: Distribution of reported commercial catch of Saucer Scallop in Australian waters, 2010


Main features and statistics for Saucer Scallop stocks/fisheries in Australia in 2010
  • Commercial catch of Saucer Scallops is predominantly taken using commercially low-opening demersal otter trawls.
  • A range of input and output controls are applied to the two Saucer Scallop stocks in Australia:
    • Input controls include limited entry, mesh and net size regulations, spatial closures, temporal closures and catch-rate thresholds.
    • Output controls include catch limits and size limits.
  • The number of commercial vessels that caught Saucer Scallops in 2010 included 32 in the Shark Bay Scallop Managed Fishery (Western Australia), 15 in the Abrolhos Islands and Mid West Trawl Managed Fishery (Western Australia), 3 in the South West Trawl Fishery (Western Australia), 3 in the South Coast Trawl Fishery (Western Australia) and 131 in the East Coast Trawl Fishery (Queensland).
  • The total commercial catch of Saucer Scallop in Australia in 2010 was 4340 t (949 t meat weight), comprising 1592 t (318 t meat weight) in the Shark Bay Scallop Managed Fishery (Western Australia), 807 t (161 t meat weight) in the Abrolhos Islands and Mid West Trawl Managed Fishery (Western Australia), 185 t (37 t meat weight) in the South West Trawl Fishery (Western Australia), 112 t (22 t meat weight) in the South Coast Trawl Fishery (Western Australia) and 1644 t (411 t meat weight) in the East Coast Trawl Fishery (Queensland). There is no recreational or Indigenous take.

Figure 2: Commercial catch of Saucer Scallop in Australia waters, 2000–10 (calendar year)

Figure 2: Commercial catch of Saucer Scallop in Australia waters, 2000–10 (calendar year)


Catch Explanation

The commercial catch of Saucer Scallop in all locations is always highly variable, since it depends on sporadic recruitment, which appears to be strongly influenced by local and regional environmental conditions. The Shark Bay Scallop Managed Fishery (Western Australia) generally contributes the highest commercial catch of Saucer Scallop in Western Australia; the good recruitment is usually associated with a weak Leeuwin Current that occurs during El Niño events. Record Saucer Scallop commercial catches were reported in the Abrolhos Islands and Mid West Trawl Managed Fishery (Western Australia) in 2003 and 2005. Since then, catches have been highly variable, including in 2009, when the fishery was not opened due to extremely low Saucer Scallop abundance. A high catch of Saucer Scallop was observed in the South Coast Trawl Fishery (Western Australia) in 2000; this was attributed to La Niña events, with a very strong Leeuwin Current and above-average temperatures in that year. Commercial catches in the South West Trawl Fishery (Western Australia) are generally low, with few operators and low fishing effort; however, in 2010, Saucer Scallop landings were the highest they had been in the past 20 years, almost equalling the 1990 catch of 220 t whole weight (44 t meat weight)2.

Record high Saucer Scallop commercial catch rates in Queensland during the 1980s were followed by very low catch rates in the early to mid-1990s, when overfishing was likely to be occurring. Biological stock biomass appears to have increased since 1997, when rotational closures were put in place and catch rates began to recover. From 2001 to 2010, commercial landings and catch rates have been variable. However, continued lower catch rates compared with historical levels indicate problems with stock production and possible habitat degradation. Since 2000, when the Fisheries (East Coast Trawl) Management Plan was introduced, effort has decreased significantly. Catch rates have increased, particularly throughout the summer months, to levels similar to those experienced in the late 1990s. The increases coincide with the opening of previously closed areas, which attract high effort and result in high catches8.


Effects of fishing on the marine environment
  • Habitat effects in Western Australia are considered low risk, with fishers generally operating over a small proportion of the licensed area. Therefore, the total area impacted by trawling is small. The areas associated with scallops are sandy habitats, and trawling activity does not impact these significantly9.
  • Food-chain effects are considered low risk in Western Australia because the total biomass taken by these fisheries is small. Moreover, because of the high natural variability of scallop stock abundance6, it is unlikely that any predators are highly dependent on this species9.
  • Bycatch reduction devices are mandatory in the Shark Bay Scallop Managed Fishery (Western Australia) and Abrolhos Islands and Mid West Trawl Managed Fishery (Western Australia). Turtle excluder devices are mandatory in the East Coast Trawl Fishery (Queensland) and are effective in removing larger bycatch species (e.g. turtles, sharks and rays). Compliance monitoring of bycatch reduction devices and vessel activities (through vessel monitoring systems) occurs in each management area. Since 2009, the Queensland Government has provided material and financial incentives to assist the widespread uptake and effective use of square-mesh codend bycatch reduction devices, which reduce the catch of smaller bycatch species10.

Environmental effects on Saucer Scallop
  • In Western Australia, strong La Niña conditions have generally resulted in below-average Saucer Scallop recruitment in Shark Bay11–13. There were two significant flooding events and record high water temperatures from December 2010 to February 2011 that appear to be linked with the very strong La Niña event14. These appear to have had a significant negative influence on the Saucer Scallop fisheries. This La Niña event may also have negatively impacted scallop stocks in the Abrolhos Islands.
  • An investigation into the effect of environmental variables on the Queensland Saucer Scallop stock has recently been proposed.


a Department of Fisheries, Western Australia
b Department of Agriculture, Fisheries and Forestry, Queensland