6. Stock Status
6.1 Current Stock Status
MSC Principle 1
MSC Principle 1 (P1) focuses on maintaining, indefinitely, fishing activity at a level that is sustainable for the targeted populations (MSC 2013).
Since the start of the reference period for sea mullet (2000 – 2011), the standardised annual catch rate of this species in the WCEMF Area 2 has remained within the target range of 2.2 – 4.6 kg / 100 m netting hour for all years except in 2013, when the indicator measured 6.2 kg / 100 m netting hour (Figure 6.1). Although this shows a substantial increase from previous years, it should be noted that the 95 % confidence interval around this estimate is relatively large (4.2 – 9.0 kg / 100 m netting hour (see also Section 7.1.2.2 on catch rate analysis). Taking into account this uncertainty, the standardised catch rate is still well above the limit reference level of 1.6 kg / 100 m netting hour. The level of sea mullet catch in the WCEMF Area 2 in 2012 and 2013 (56 and 68 t, respectively) is also within the target range (i.e. between the lower and upper threshold levels) of 46 – 70 t (Figure 6.2).
The above information provides evidence that the sea mullet stock is currently exploited at a level below the maximum sustainable yield (MSY), i.e. the stock is highly likely to be above the point at which recruitment may be impaired.
Figure 6.1. Annual standardised commercial catch rate (kg / 100 m netting hour) of sea mullet in the WCEMF Area 2 relative to the associated reference points. The shaded green area reflects the reference period (2000 – 2011), from which reference points have been calculated.
Figure 6.2. Annual commercial catch (tonnes) of sea mullet in the WCEMF Area 2 relative to the associated reference points. The shaded green area reflects the reference period (2000 – 2011), from which reference points have been calculated.
Blue Swimmer Crab 6.1.2
Blue swimmer crab is a highly fecund species with a short life span (Section 2.2) and is therefore generally considered to have a low inherent vulnerability to fishing. A 2014 risk assessment, which considered the productivity of blue swimmer crabs and its susceptibility to each of the fisheries targeting the overall stock of this species in south-west WA, determined the risk to the stock as low (see Section 7.1.3.4). However, as recruitment of this species can be significantly influenced by changes in environmental conditions (Section 2.2.3.4), blue swimmer crab catches can fluctuate between years as a consequence of impacts on the stock that may not necessarily be related to fishing pressure.
The WCEMF Area 2 is currently the largest commercial fishery targeting the stock of blue swimmer crabs in south-west WA in terms of both tonnage and fishing effort (see Section 2.2.2), significant catches are also retained by recreational fishers (Section 3.2.3).
Commercial catches of blue swimmer crabs in the PHE have fluctuated at between 45 and 104 t since fishers converted using traps for targeting this species in 2000.
Since 2000/01, annual (by fishing season; 1 November – 31 October) standardised commercial catch rate of blue swimmer crabs in the WCEMF Area 2 has fluctuated within the target range of 0.7 – 1.4 kg / traplift, but has generally remained above 1 kg / traplift. The standardised trap catch rate for the 2013/14 fishing season was 1.14 kg / traplift, which is well above the limit level of 0.5 kg / traplift (Figure 6.3). Annual catches of blue swimmer crabs in the WCEMF Area 2 have remained high in recent years, with a total retained catch in the 2013/14 financial year of 104 t (from 1717 fisher days), which is equal to the upper catch
threshold for this fishery (Figure 6.4). Although total catch has fluctuated, primarily due to environmental influences, the stable time series of annual standardised trap catch rates of blue swimmer crabs in the fishery since the time of this conversion provides evidence that the current level of exploitation of the stock can be sustained and is below MSY.
In response to the standardised catch rate being just above the upper threshold level in 2012/13 (Figure 6.3), data from commercial monitoring and fishery-independent research sampling (Section 8.4.2.1.2 and 8.4.2.3.2) have been examined (see Appendix A for a summary of these data). Comparisons between current and historical commercial monitoring and fishery-independent data sets provide further support that blue swimmer crab stock abundance has been relatively stable over the past decade, with natural protection of the spawning stock occurring due to their exit out of the estuary over winter to spawn in oceanic waters where fishing effort is currently low. The observed increase in the proportion of pre- spawned females in the catches of crabs over the past few years in the PHE (Appendix A), will continue to be closely monitored, along with the level of fishing effort in the oceanic waters, to ensure that fishing pressure does not compromise the breeding stock. Long-term climate changes such as declining rainfall could significantly influence the movement patterns of crabs in the PHE (Section 5.3.2), which may result in more mated, pre-spawned females remaining inside the estuary and being available for capture during the winter months.
Figure 6.3 Annual standardised commercial catch rate (kg / traplift) of blue swimmer crabs in the WCEMF Area 2 relative to the associated reference points. The shaded green area reflects the reference period (2000/01 – 2011/12), from which reference points have been calculated. The start of the reference period corresponds to the time when the fishery converted to using traps for targeting blue swimmer crabs, as denoted by the vertical dashed line. Fishing season is defined as 1 November to 31 August.
Figure 6.4. Annual commercial catch (tonnes) of blue swimmer crabs in the WCEMF Area 2 relative to the associated reference points. The shaded green area reflects the reference period (2000/01 – 2011/12), from which the reference points have been calculated. The start of the reference period corresponds to the time when the fishery converted to using traps for targeting blue swimmer crabs, as denoted by the vertical dashed line.