sites and possibly reflects a seasonal variation in abundance and / or movement between trawled and untrawled areas. Invertebrate species abundance was only observed to decline significantly between February and June in both trawled and untrawled sites. This was attributed to the very abundant species: A. balloti, P. latisulcatus, P. rubromarginatus and H. pallida, with the first two being commercially-harvested species. As with the fish species, these four dominant invertebrate species drove the abundance patterns (Kangas & Morrison 2013).
The depletion experiment clearly indicated that some species were much more vulnerable to trawling, with catchability depending on their mobility, pattern of movement, size, physical form, burying ability, feeding mode and whether they are attracted to disturbed substrates.
Susceptible species were considered to have catchability coefficients greater than 0.6 and included the invertebrates L. maculata (sea star) and the phylum Porifera (sponges) and the fish P. octolineatus, Parupeneus chrysopleuron, L. genivittatus, S. sageneus, P. vitta, C. cephalotes and S. robusta (Kangas & Morrison 2013).
Despite a clear abundance decline in some species, no obvious impact from trawling could be discerned from abundance and distribution patterns of these species, even those with high catchability coefficients. All species recorded on trawl grounds were also present in untrawled areas. Many of the moderate- to high-catchability species that are also widespread are good candidate indicator species for trend analysis (Kangas & Morrison 2013).
10. Retained (Non-Target) Species
Bugs 0.0 0.0 0.1 1.5 0.7 1.5 2.2 13.2 0.9 2.5 3.2
Octopus 0.3 0.4 0.4 0.7 0.5 0.2 0.5 1.0 0.5 0.2 0.6
Although there is no formal stock assessment process for non-target retained species, total catch is used to assess the annual level of exploitation of each species’ stock. For each species, the acceptable catch ranges have been set to reflect the historical catches of these species as follows:
• Minor prawn species (i.e. coral prawns): annual acceptable catch levels based on historical catches during the period 1989 – 1998;
• Blue swimmer crabs: as per the Shark Bay blue swimmer crab resource annual TACC; and
• All other retained non-target species: annual acceptable retention levels based on historical landings during the period 1990 – 2010. Note that retention levels for finfish species have only been identified for the current most-commonly retained species groups, i.e. whiting, flathead, mulloway and flounder, and these acceptable retention levels are still being developed. Industry has indicated that retention of non-target catch may increase in coming seasons as part of its commitment to minimising bycatch discards.
10.1.1 Risk Assessment Outcomes Endeavour Prawns
10.1.1.1
2010 Risk Rating: Impact to breeding populations (C2 L5) Low 2014 PSA assessment: (2.01) Low
Although endeavour prawns are not targeted by the SBPMF, they are caught in reasonable numbers in some years (see Table 10.1).
Endeavour prawns (Metapenaeus spp.) are restricted to northern Australian waters between northern New South Wales and the Gascoyne coast of WA (Grey et al. 1983) and are found in coastal waters down to approximately 50 m in muddy or sand / mud sediment substrates.
Within Shark Bay, they are generally found in the southern end of the South CPL (Kangas et al. 2006) and are thus protected from trawling for a large part of the year.
Additionally, endeavour prawns are considered to be more resilient to fishing pressure due to their smaller size (and thus lower catchability) and the lower level of targeting compared to brown tiger and western king prawns (Kangas et al. 2006).
Given no change in effort, catches of endeavour prawns should remain within the acceptable catch range of 1 – 30 t. This low level of catch should ensure that there is sufficient breeding stock to continue recruitment at levels that will replenish what is taken by fishing, predation and other environmental factors (Kangas et al. 2006).
Coral Prawns 10.1.1.2
2010 Risk Rating: Impact to breeding populations (C2 L4) Moderate 2014 PSA assessment: (2.01) Low
Coral prawns consist of a number of smaller prawn species, but in Shark Bay a large proportion of are Metapenaeopsis crassissima. M. crassissima is restricted to Australia and has been reported from South Australia, WA and north to Darwin in the Northern Territory.
They most commonly occur down to 30 m over soft muddy bottoms (Grey et al. 1983). Little is known about the life history of M. crassissima; in Shark Bay, it appears that spawning may occur throughout the year, and it is assumed that nursery areas are inshore sheltered habitats similar to those preferred by western king and brown tiger prawn juveniles. M. crassissima is considered to be one of the largest species of this genus (Racek & Dall 1965), but it is still much smaller than both the western king and brown tiger prawns. Because of their small size, many pass through the cod end mesh and are therefore not captured by the fishing gear.
Coral prawn landings are highly variable due to their low value and therefore, lack of targeting by the fleet and generally low rate of retention. Landings of coral prawns tend to supplement the catch when the target species are in low abundance (Kangas et al. 2006).
Given no change in effort, catches of coral prawns should remain within the acceptable catch range of 80 – 280 t. This level of catch should ensure that there is sufficient breeding stock to continue recruitment at levels that will replenish what is taken by fishing, predation and other environmental factors (Kangas et al. 2006).
Blue Swimmer Crabs 10.1.1.3
2010 Risk Rating: Impact to breeding populations (C1 L4) Moderate 2014 PSA assessment: (2.01) Low
In WA, blue swimmer crabs are distributed from Albany to the Northern Territory border.
They inhabit a wide range of inshore and continental shelf areas, from the intertidal to at least 50 m depth.
There is a comparatively limited area where blue swimmer crabs are caught by prawn trawlers in Shark Bay, and extensive refuge areas exist both north and south along the coast and in deeper waters of the continental shelf that are not generally fished.
Fishers retain crabs at a voluntary minimum size of approx. 135 mm carapace width (CW;
spine to spine). This is well above the size at maturity (90 – 110 mm CW) and larger than the legislated minimum size of 127 mm CW. Fishers are also not allowed to retain egg-bearing females.
The SBPMF has historically taken only a small proportion of the total catch of blue swimmer crabs within Shark Bay; however, there has been increased retention of blue swimmer crabs in recent years. Both the trawl and trap industries voluntarily stopped fishing for blue
caused by adverse environmental conditions (see Section 5.3.2 for more details on the Shark Bay blue swimmer crab resource). Following partial stock recovery, the fishing season reopened in September 2013 with a TACC of 400 t (264 t for the trap sector and 135.2 t for the trawl sector) for the 2013/14 season.
The Shark Bay blue swimmer crab stock continues to recover from environmental impacts on recruitment as indicated by high biomass levels in February 2014 and sustained biomass levels in April 2014 (DoF unpublished data). Recovery of the stock in the deeper grounds to the west of CPL has been observed for the first time since the stock decline in late 2011. This recovery on the main trawl fishing grounds is supported by the intensity of commercial fishing in these grounds by the both trap and trawl sectors since the resumption of fishing in 2013.
Bugs1 10.1.1.4
2010 Risk Rating: Impact to breeding populations (C0 L5) Negligible 2014 PSA assessment: (2.01) Low
Bugs have an extensive distribution and wide geographical range and are generally caught in very low numbers by the SBPMF (e.g. 3.2 t in 2013). Under current arrangements, the fishery is considered to have only a remote likelihood of having even a minor impact on this stock resulting in a negligible risk to the stock, thus no further targeted management is required (Kangas et al. 2006).
Cephalopods 10.1.1.5
2010 Risk Rating: Impact to breeding populations (C0 L5) Negligible 2014 PSA assessment: (2.23) Low
Squid and cuttlefish are retained in moderate amounts in the SBPMF, while octopus are retained in very low amounts (generally less than one tonne annually).
Four species of cuttlefish occur in Shark Bay. All are geographically-widespread species at or near the end of their ranges in Shark Bay. Two species (Sepia apama and S. novaehollandiae) are temperate species near the northern limit of their ranges, while the other two (S. cultrata and S. pharonis) are widespread tropical species near the southern limit of their ranges.
Therefore, the populations of the cuttlefish species in Shark Bay make up only a small proportion of the total ranges of these species.
Additionally, cuttlefish are most common where there are rock outcrops, seagrass beds, and other areas, which provide habitat diversity and protection. As a result, a significant proportion of the populations in Shark Bay would be unavailable to be caught by trawling operations as they do not occur in trawling areas (Dr Fred Wells [WA Museum] pers. comm.;
Kangas et al. 2006).
1
Given the biological characteristics (i.e. short life span, fast growth and high fecundity), population size, wide distribution, and the current catch levels of these species, the SBPMF is considered to have only a remote chance of even having a minor impact on these species’
stocks resulting in a negligible risk (Kangas et al. 2006).
Mixed Finfish 10.1.1.6
2010 Risk Rating: Impact to breeding populations (C0 L5) Negligible 2014 PSA assessment: Whiting Sillago spp. (2.01) Low
Flounder (Family Bothidae) (2.09) Low Flathead (Family Platycephalidae) (2.28) Low Mulloway Argryosomus japonicas (2.82) Medium
Mixed finfish species are generally caught in very low amounts (< 1 – 15 t), with 13 t retained in 2013. This level of harvest is highly likely to be minor in comparison to the likely stock sizes of finfish species across the WA coast (Kangas et al. 2006). Further harvest control rules have been established for some mixed finfish species that are commonly retained (i.e. whiting, flounder, flathead and mulloway) (see SBPMF Harvest Strategy 2014 – 2019 [DoF 2014a]).