5.3 GENERAL ENVIRONMENT
5.3.1. Impacts from removal or damage to the environment
5.3.1.2 Impact to sand/shell habitat
Prawn trawling occurs mostly over the sand and shell habitats of Shark Bay. When trawling, ground chains and otter boards make contact with the sea bottom, disrupting organisms within the habitat.
Evidence from video footage of trawled areas of Shark Bay suggests that trawling over sand has the effect of flattening this otherwise rippled and three-dimensional substrate. This may also indirectly affect the species that inhabit this area by changing the nature of their habitat.
ERA Risk Rating: Impact on habitat ecology and structure (C2 L5 MODERATE)
The potential impact on the sand and shell habitat, as a result of the prawn trawling operations was considered to have only a ‘moderate’ consequence due to the following:
• The total area open to trawling is approximately 40% of the area of Shark Bay Region.
• Of the area that is legally permitted to be trawled, less than 20% of this is trawled (due to targeting of known favourable grounds) (Figure 7).
• Studies of actual impacts from prawn trawling suggest only minimal impacts to infaunal communities.
It was considered ‘likely’ that this level of impact would result since fishing on sand habitats in Shark Bay has occurred consistently for 8 months of the year for over 40 years. Consequently, the level of risk was moderate, and needs to be managed within these acceptable levels.
Operational Objective
To maintain an acceptable level of impact on the shell/sand habitat in Shark Bay.
Justification
It is understood that prawns are predominantly targeted over sandy substrate. It should be noted that the area occupied by adult prawns is strongly related to the location of where nursery areas are.
Sandy habitats harbour many infaunal and epibenthic assemblages. In particular, the diverse invertebrate fauna that likely resides within the sandy areas of Shark Bay is considered an important value of Shark Bay (Fisheries WA, 199) and should be protected. Although, trawl gear makes physical contact with the seabed, the regulations governing ground chains (max. 10mm) minimises impacts but may result in interactions with species using the sand habitat and any impact needs to be kept to an acceptable level.
Indicator
The percentage of the sand/shell habitat of the Shark Bay region that is trawled.
Performance Measure
Area of sand/shell habitat available for trawling needs to be kept to no greater than 40% of the total sand/shell habitat in Shark Bay.
Justification
The extensively trawled areas of Shark Bay are estimated to be less than 20% of the total waters in Shark Bay (13,000 km) and at least 25% of the total sand habitat (at least 9000 km) in Shark Bay (Fisheries WA, 199). Based on the average number of trawl shots and number of vessels the estimate of the amount of ground covered would be around 300km, if no overlap occurred. However, due to the concentration of fishing effort and resultant overlap, the actual swept area is estimated to be around 10km of the seabed within the bay (Department of Fisheries, in prep.). This is a relatively small percentage of the total area of bay and would provide around 75% refuge even if that area trawled were extensively impacted.
It should be noted, however, that there are a number of studies, which have shown that even in habitats that prawn trawling occurs it does not cause significant effects to the infaunal community. A meta- analysis of fishing impacts by Collie et al., (000) found that otter trawling had the least impact of all forms of trawling. Specifically, Kaiser and Spencer (199) found no detectable difference between trawled and untrawled areas (beam trawl) within mobile sediment (sand) regions. Van Dolah et al., (1991) studied changes in infaunal communities over months for areas closed to shrimp trawling.
They concluded that the seasonal reductions in abundance and number of species sampled had a much greater effect than fishing. Finally, Jennings and Kaiser (199) suggest that light shrimp trawls do not cause significant disturbance to communities in poorly sorted sediments in shallow water.
In Australia, Gibbs et al., (190) found minimal impacts on the benthic communities in sandy areas resulting from prawn trawling in Botany Bay, NSW. In southwest WA, Laurenson et al., (1993) compared
the dominant fauna of each area (sand bottom) showed marked similarities, although each group had a different group of less abundant species. The difference was attributed to the fact that the untrawled area was small and encroached in all directions by seagrass. Underwater video observation of both areas before and after the completion of the depletion experiment failed to detect any visual impact on the substrate or habitat. Extrapolating this study to Shark Bay, which is a similar habitat would indicate that trawling causes only minor and short-lived impact to sandy habitats.
Consequently, a performance measure of 40% is considered precautionary, while allowing for flexibility of the fleet for economic efficiency.
Data Requirements for Indicator
Data required would be:• Knowledge of spatial distribution of trawled and untrawled areas.
• Knowledge of spatial distribution of sand habitats within Shark Bay region.
Data Availability (past - current – future)
Knowledge of spatial distribution of trawled and untrawled areas.
Based on log book data a good record exists of the location of trawled and untrawled areas. The recent introduction of a VMS to this fishery will provide more detailed information of the trawled areas of Shark Bay.
Knowledge of spatial distribution of sand habitats within Shark Bay region.
The distribution of habitats within Shark Bay is well understood from the extensive sedimentation studies done by University of Western Australia (Logan and Cebulski, 1970). In addition, data exists in GIS format, which was largely generated by the CALM.
Evaluation
Currently, trawling is estimated to be occurring in approximately 25% of the sand habitat within Shark Bay and is therefore meeting the objective.
Robustness
HighThe indicator is considered robust as:
• Current trawling is only allowed within specific areas to provide protection to nursery areas and different habitats.
• Regulations do not allow fishing to occur in more than 40% of this habitat.
• Compliance policing includes the use of VMS, which logs the positions of vessels throughout the fishing season.
Fisheries Management Response
Current: Trawling is only allowed within specified areas to provide protection for sensitive nursery areas and seagrass habitats, which also affords protection to other habitats including the sand habitats.
Vessels are now required to have a VMS operating during the season that logs the positions of vessels to ensure that trawling does not occur outside permitted areas.
Future: Fishing effort and distribution will be monitored to ensure that no more than 40% of the available sand habitat is trawled.
Actions if Performance Limit is Exceeded: Not applicable.
Comments and Action
The regulations do not allow for fishing to occur in more than 40% of the area. Therefore, the management of this issue is really assurance and compliance.