The SBPMF boundaries and fishing areas overlap fishing areas of the Shark Bay Scallop Managed Fishery (SBSMF; Figure 9.4), and some prawn vessels catch scallops during the scallop season under licences as part of the SBSMF. The interaction of the different fishing gear configurations and fishing dynamics with the benthic habitat and biota means there is potential for the activities of one industry to influence the other in the areas of stock overlap (Kangas et al. 2012).
Figure 9.4. General map of Shark Bay with key management lines and spatial overlap (dark purple) in fishing grounds of the scallop (light purple) and prawn (blue) trawl fleets in 2009. TPSA: Tiger Prawn Spawning Area (now referred to as North CPL); ENA:
Extended Nursery Area (now referred to as South CPL) (Source: Kangas et al.
2012)
Unlike prawns, juvenile scallops recruit directly onto the main fishing grounds where adults occur and become vulnerable to gear impacts from both the scallop and prawn fleets.
Scallops can tolerate air exposure for longer periods than many bycatch species, thus
discarded scallops are more likely to survive to be recaptured later; however, the overlapping fishing grounds where both fleets operate are of high trawl intensity, where the rate of repeated scallop discarding is likely to be high and their long-term survival is unknown (Kangas et al. 2012).
Historically, the prawn fleet commenced fishing before the scallop fleet on the main prawn fishing grounds, but they were not permitted to retain any scallops until the scallop season commenced (generally April / May). As such, there was a period of several weeks when scallops were being continuously captured and discarded, which resulted in some trawl- induced morality and unavoidable damage to prawn nets from the accumulation of large amounts of scallops within the nets (Kangas et al. 2012).
In 2004, this system was abolished, and both fleets began fishing on the same date (except in the Denham Sound fishing grounds, which has its own opening and closure dates based on meat size and quality). The change in regulation to simultaneous openings for both fleets has had positive outcomes for scallop harvesting, as most scallops that are caught in summer are retained. Scallops of non-market sizes (< 85 mm), however, still continue to be discarded by both fleets during this time. In order to reduce the amount of discarding over this period, industry has recently trialled square mesh cod ends instead of the traditional diamond mesh cod ends in order to improve gear selectively to reduce the capture of sub-legal scallops.
Additionally, the forward shift in scallop season commencement in 2004 meant that the scallop fleet ceased fishing before the peak scallop spawning period began, which resulted in an overall reduction in fishing intensity during the key spawning months. The prawn fleet, however, continues fishing operations during this period (June – August) but are required to discard all scallops caught in their nets during this time to maintain scallop spawning abundance (Kangas et al. 2012).
The need to develop an understanding of the impact from gear interactions between the scallop and prawn fisheries led to an FRDC-funded project (no. 2007/051), which included research to:
1. Determine size-specific recapture mortality rates of scallops (Amusium balloti) as a result of repeated capture and release experiments and gear impacts on newly recruited juvenile scallops;
2. Investigate if small-scale spatial closures assist recruitment of A. balloti by reducing gear impacts and capture mortality but without affecting prawn catches; and
3. Examine whether existing hydrodynamic models can guide the selection of spatial closures and investigate the larval transport mechanisms of both prawns and scallop larvae in Shark Bay (Kangas et al. 2012).
The spatial and temporal differences in the survival of discarded scallops were investigated under different post-capture treatments from field experiment simulating commercial trawl activities using multiple mark-recapture trials. Tag-recapture experiments were conducted at
sites of moderate scallop abundance (~ 3000 – 5000 scallops / trawl) based on annual scallop survey data collected in 2007 / 2008 (Figure 9.5). Scallop tagging and recapture experiments were conducted in winter (September 2008), when most scallops were in post-spawning phase, and in summer (February 2009), when scallops were in pre-spawning phase (Kangas et al. 2012).
Experimental sites were trawled to capture approximately 2000 scallops for marking on the tagging night, with scallops separated into an air exposure treatment (exposed for approx.
40 minutes), a hopper treatment or a control treatment. Experimental sites were trawled the following four nights (winter) and three nights (summer) after the tagging night to recapture tagged scallops (Kangas et al. 2012). Direct damage and injury to scallops were visually assessed from a sub-sample of 300 scallops (tagged and untagged) collected across all trawl sites during the summer experiments using a damage scale of 0 (i.e. no damage) to 5 (i.e.
dead scallop; Kangas et al. 2012).
Figure 9.5. Experimental tagging sites in the central bay region of Shark Bay in summer (West Shark Bay, East Shark Bay and Short Recovery Experimental [SRE] site) and lower bay region (Upper Denham Sound, Lower Denham Sound, SRE site). Red dot indicates location of scallop recaptures by commercial fishers during the scallop fishing season in 2008 (prawn boats) and 2009 (prawn and scallop boats). (Source:
Survival estimates of discarded scallops were found to be significantly higher in winter (post- spawning) than during summer (pre-spawning), but there were no large differences in survival between fishing grounds or between post-capture treatment groups (air exposed or hopper). This suggests that thermal stress from large differences in seasonal temperatures is more critical to scallop survival than differences in scallop reproductive condition. Cooler conditions during the spawning months would favour greater survival of the discarded spawning scallops, while reproductive energy diverted to spawning is likely to prolong their recovery period, thus decreasing their catchability by trawl nets (Kangas et al. 2012).
The majority of scallops sampled indicated a Level 1 injury (i.e. minor chipping to the edges of valves), and 94 % of scallops were below a Level 3 damage grading. Level 3 damage (i.e.
extensive chipping of valves exposing soft tissue) was seen in 4 % of scallops; 1.1 % had Level 4 damage (i.e. proportions of valves missing, visible injury to soft tissue but scallop alive) and mortality was seen in 1.4 % of scallop assessed (Kangas et al. 2012). These results are similar to A. balloti harvested in the Queensland fishery, where estimates of dead scallops with crushed or cracked valves were very low (1 %), while the majority incurred chipping to the outer edges of the valves (Campbell et al. 2010).
The impact of trawl effort intensity and distribution on scallop recruitment in Shark Bay was also investigated as part of the FRDC project in order to explore potential benefits of spatial closures of areas within key scallop grounds in Shark Bay. This included evaluating the impacts of management actions (i.e. the introduction of the CPL in 1991) on historical spatial effort distribution of the prawn fleet; evaluating historical and current spatial effort distribution of the scallop fleet; and evaluating temporal effort distribution by both fleets on the central Shark Bay fishing grounds (Kangas et al. 2012).
In evaluating both the historical and recent catch and effort data of the prawn and scallop fisheries, it was apparent that changes in trawl effort distribution were not a major driver of the scallop recruitment in Shark Bay. Other factors, such as environmental factors, or a combination of effort and these external factors are more likely to drive recruitment success (Kangas et al. 2012).
Despite the potential impact of changing environmental conditions on the variability in annual scallop recruitment, there is a lack of understanding of the detailed hydrodynamic processes that are required to interpret the recruitment dynamics in the Shark Bay region.
Using a combination of field measurements and numerical modelling Kangas et al. (2012) examined the dynamics of circulation throughout the scallop trawl grounds during the scallop spawning season with the aim of establishing source-sink relationships for larvae.
The hydrodynamic modelling indicated limited larval connectivity between Denham Sound and northern fishing grounds, and it appeared that the key grounds were largely self-seeding.
Northern Red Cliff and Denham Sound had a higher likelihood of larval loss (flushing) out of Shark Bay under certain environmental conditions. The management implications from these results are that it is essential to retain spawning stock in each fishing ground in order to
replenish stocks on each fishing ground. The current management strategy of fishing scallops to a catch rate threshold to ensure carryover of stock is therefore appropriate, however, the implementation of spatial closures may still be a reasonable strategy to protect spawning stock and newly settled scallops due to the lack of connectivity between fishing grounds (Kangas et al. 2012).