5.2 NON-RETAINED SPECIES
5.3.1 Impacts from removal or damage to the environment
5.3.1.4 Impact to the mud/sand habitat
Prawn trawling in Exmouth Gulf occurs predominantly over mud and sand habitats. 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.
While mud/sand habitat structure and ecology were originally identified as separate issues, it is not possible to separate them in this assessment process because in real terms physical change to the bottom structure is largely irrelevant unless it impacts on the surrounding ecology.
ERA Risk Rating: Impact to habitat ecology and structure (C2 L2 LOW)
The potential impact on the mud and sand habitat on Exmouth Gulf, as a result of the prawn trawling operations was considered unlikely to have even a minor consequence (which provides a low risk) due to the following:
• Of the area that is permitted to be trawled, only around 35% of this is actually trawled (due to targeting of known favourable grounds).
• Furthermore, 28% of the area is permanently closed to trawling.
• Studies of actual impacts from prawn trawling suggest only minimal impacts to infaunal communities.
It should also be noted that the mud substrate in Exmouth Gulf is generally comprised of coarser and heavier sediments and is therefore thought to be more ‘resistant’ to disturbance by trawling activities.
Moreover, such ‘exposed’ seabeds are naturally dynamic as a result of environmental influences.
Whilst the level of risk was considered LOW, the operations of the fishery need to be managed to keep it within these acceptable levels.
Operational objective
To maintain an acceptable level of impact on the mud/sand habitat in Exmouth Gulf.
Justification
Prawns are predominantly targeted over mud/sand substrate in Exmouth Gulf. Mud/shell habitat harbors many infaunal and epifaunal assemblages.
Indicator
The percentage of the mud/sand habitat of the Exmouth Gulf region that is trawled.
Performance measure
Area of mud/sand habitat available for trawling needs to be kept to no greater than 40% of the total mud/sand habitat in Exmouth Gulf.
Justification
The extensively trawled areas of Exmouth Gulf are estimated to be about % of the total waters in Exmouth Gulf (1. km). This is a relatively small percentage of the total area of bay and would provide a substantial amount of refuge even if the area trawled was extensively impacted.
However, it should be noted that there are a number of studies, which have shown that no significant effects to the infaunal community are caused in areas of similar habitat where trawling occurs. 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
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ESD Report Series No. 1 – Exmouth Gulf Prawn Fishery
trawled and untrawled areas (beam trawl) within mobile sediment 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) suggested that light shrimp trawls do not cause significant disturbance to communities in shallow water with poorly sorted sediments.
In Australia, Gibbs et al. (190) found only minimal impacts on the benthic communities resulting from prawn trawling in Botany Bay. In southwest Western Australia, Laurenson et al. (199) compared trawled and untrawled areas using trawl samples and underwater video. Their study concluded that the dominant fauna of each area 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 Exmouth Gulf would indicate that trawling causes only minor and short-lived impact to mud/sand habitats.
Consequently, a performance measure of 0% 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 mud/sand habitats within Exmouth Gulf 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 upcoming introduction of a VMS to this fishery will provide more accurate information of the physically trawled areas of Exmouth Gulf.
Knowledge of spatial distribution of mud/sand habitats within the Exmouth Gulf region.
The distribution of habitats within Exmouth Gulf is relatively well understood. This data exists in GIS format, largely generated by the Department of Conservation and Land Management.
Evaluation
Currently, trawling is estimated to occur over 35% of the mud/sand habitat within Exmouth Gulf.
Robustness
Robustness is considered relatively high given the long-term log book data available and the introduction of the VMS in 2002. Furthermore, the distribution of different types of habitats in Exmouth Gulf is relatively well understood.
Fisheries management response
Current: Trawling is only allowed within restricted areas to provide protection for sensitive nursery areas, which coincidentally also affords protection to other habitats including mud/sand habitats. In 2002, vessels will be 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 total available mud/sand habitat is trawled.
Actions if Performance Limit is Exceeded: If monitoring reveals that greater than 40% of the mud/
sand habitat is being trawled, the legal trawl boundaries will be amended to regulate for the 60%
mud/sand refuge area. This action will be initiated within a season or prior to the beginning of the next season.
Comments and action
As the introduction of VMS occurs, the management of this issue will be refined.
External driver checklist
Natural events and prevailing environmental conditions also have the potential to impact the structure and ecology of mud/sand habitats in Exmouth Gulf. For example, the rainfall run-off generated by cyclones that frequent Exmouth Gulf has the potential to alter the bottom topography of substrates like mud/sand habitats. Similarly, wind generated water movements have the potential to change bottom features especially in relatively shallow waters. The ‘greenhouse effect’ may also impact on the structure and potentially the ecology of the mud/sand habitats in Exmouth Gulf in the long-term as global weather patterns change the prevailing oceanographic currents.