At the time of writing this report, full assessments of the stocks of brown tiger prawns and western king prawns in Shark Bay were available for 2013, with some 2014 data also available for the two species. Both species are considered to have adequate spawning stock levels such that the only factor affecting recruitment is the environment.
6.1.1 Brown Tiger Prawns
Of the two target species in the SBPMF, brown tiger prawns are typically considered more vulnerable to overfishing than western king prawns because of their shorter breeding period and higher catchability (Penn & Caputi 1986). Therefore, monitoring and assessment has been largely focused on this species.
With the exception of 2012, the brown tiger prawn spawning stock index has fluctuated around the target level since 2002 (Figure 6.1). The spawning stock index in 2013 increased to be well above the limit reference point (10 kg / hr) and in 2014 the index increased to be above the target level (25 kg / hr; Figure 6.1). The reasons for the low brown tiger prawn spawning stock index (abundance) for the key northern spawning area (North CPL, formerly known as the TPSA) in 2012 is being examined (see below).
No declining trend in the fishery-independent recruitment index for brown tiger prawns is evident since 2000 (Figure 6.2), suggesting that recruitment variability in recent years has been driven largely by environmental factors given that the spawning stock is adequate. The recruitment index for this species in 2014 was within the historical levels and well above the target level of 25 kg / hr (Figure 6.2).
Figure 6.1 Mean catch rates of brown tiger prawn (quad gear equivalents) in the key northern spawning area, North CPL during July and August between 2002 and 2014 and target and limit reference points.
Figure 6.2. Mean recruitment indices for March and April surveys for brown tiger and western king prawns in Shark Bay from 2000 to 2014. The target and limit reference points are indicated.
There appears to be a poor correlation between spawning stock and recruitment levels of brown tiger prawns in Shark Bay between 2002 and 2014 (cf. Figure 6.1 and Figure 6.2; see also Figure 8.2 in Section 8.2.4), indicating that the spawning stock is being maintained well above the level that resulted in recruitment overfishing and subsequent low catches in the 1980s (Penn et al. 1995; see also Section 3.3). Very high recruitment occurred in 2011, probably due to favourable environmental conditions associated with the marine heatwave of late 2010 and early 2011 (Pearce et al. 2011).
0 5 10 15 20 25 30 35 40 45 50
2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
tiger pr spawning Index c/r (kg/hr)
July Aug Mean
Target
Limit
0 20 40 60 80 100 120 140 160
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
Mean recruitmentindex, Mar/Apr (kg/hr) K Mar_Apr
T Mar_Apr
Target Limit
Western king prawns Brown tiger prawns
Although the level of recruitment (and catch) of brown tiger prawns in Shark Bay in 2012 was well above the target reference level, the main abundance of this species in that year (as shown by commercial catch rates) occurred south of the northern spawning area. As of 2013, additional survey sites have been incorporated into spawning stock survey regime to provide a fishery-independent measure of spawning stock in the key southern spawning area (South CPL, formerly known as the ENA, and which has always closed from August onwards). This will be used in combination with the northern spawning area index in the future to provide a more robust overall spawning stock index than the northern spawning area alone. At this stage, the combined spawning stock index for 2013 and 2014 indicates that the abundance of brown tiger prawns in the southern spawning area in August / September is generally higher than in the key northern spawning area.
6.1.2 Western King Prawns
As western king prawns have a longer breeding period than brown tiger prawns and lower catchability (Penn & Caputi 1986), they have long been considered to be less vulnerable to overfishing. Although western king prawns are more widely distributed than brown tiger prawns in Shark Bay, the fishery-independent spawning stock survey undertaken for brown tiger prawns in the key northern spawning area also contains a significant proportion of western king prawns during the spawning period. It is thus considered appropriate that the catch rates of western king prawns from these surveys can be used as a fishery-independent measure of spawning stock abundance for this species (see Section 7.1.2 for more information).
Between 2002 and 2012 the western king prawn spawning stock has fluctuated around the target level of 25 kg / hr but has increased to well above this level in 2013 and 2014 (Figure 6.3). As with brown tiger prawns, there is no declining trend in recruitment evident in fishery-independent survey index for western king prawns since 2000 (see Figure 6.2 above).
The annual western king prawn recruitment index is fluctuating at a level well above the target reference level each year, indicating most of recruitment variability is driven by environmental factors. The recent high abundance of western king prawns in Shark Bay is also evident in the large catches relative to fishing effort in the fishery over the past few years (see Section 3.3).
Figure 6.3. Mean catch rates of western king prawn (quad gear equivalents) in the North CPL during June to August between 2002 and 2014 and target and limit reference points.
There is no significant correlation between spawning stock and recruitment indices for western king prawns for the range of spawning stock levels observed in Shark Bay since 2001 (see Figure 8.3 in Section 8.2.4). This is believed to reflect the maintenance of the spawning stock above the levels that would result in recruitment overfishing (noting that there has never been a collapse of the western king prawn stock in Shark Bay). As with brown tiger prawns, high recruitment of western king prawns occurred in 2011, likely due to favourable environmental conditions associated with a combination of the marine heatwave and flood events of early 2010/11 (Pearce et al. 2011). A second peak in recruitment of this species observed in 2008 was also associated with above-average water temperatures due to a strong Leeuwin Current in that year.