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Environmental Effects of Prawn Trawling
Prawn trawling is a major fishing activity on the Great Barrier Reef Marine Park occurring mostly in the lagoon, which stretches from the coast to the reef, and also between the reefs. Research on the environmental effects of trawling in these areas will help ensure sustainable management of the industry.
The 350,000 km2 Great Barrier Reef Marine Park (GBRMP) is made up of coral reefs, the deeper waters between the reefs, and the lagoon. Studies have previously focussed on the coral reefs, which cover 6% of the Park. Relatively little research has been done on the inter-reef or lagoon areas, which together make up 70% of the area. Deep ocean waters outside the reefs account for the remaining 24%. The size of the Park and the lack of information on areas outside the coral reefs has made management of multiple-uses and enforcement difficult.
About 800 prawn trawlers are licensed to operate on the Queensland east coast and a large number of these trawl in the Great Barrier Reef region. Trawl grounds in the GBRMP cover 153,000 km2, however the intensity of trawling varies throughout the region. About 27% of the lagoon and inter-reef seabed is not trawled at all, and of the seabed that is trawled, more than 50% of the effort is concentrated in less than 20% of the area. About 70% of trawled grounds have been trawled less than one pass per year, each year since the fishery became established.
CSIRO and the Queensland Department of Primary Industries (QDPI) have completed five-years of research on the environmental effects of trawling on the far northern GBRMP. The study covered 10,000 km2, in an area closed to trawling since 1985 and known as the Green Zone. It is the worlds largest and most comprehensive study of the environmental effects of trawling and the first study on effects of prawn trawling in the tropics.
Funded by the Great Barrier Reef Marine Park Authority, the Fisheries Research and Development Corporation,
the Australian Fisheries Management Authority, CSIRO and QDPI, the research follows a recommendation from a scientific workshop in 1989 on the environmental effects of fishing in the GBRMP.
The project surveyed the physical and biological makeup of the study area, conducted experiments to simulate the physical impact of trawling on seabed animals and plants, compared the biology of areas open to trawling with those closed to trawling, and investigated prawn trawl bycatch.
Biology: The lagoon and inter-reef areas of the GBRMP had been thought to be flat, muddy and relatively barren of life, but the study recorded more than 1,000 seabed species revealing a high biodiversity. It showed that while there are extensive bare, muddy or sandy areas, there are also seagrass and algal meadows, diverse sponge and coral garden patches, and deeper coral reefs.
The study has also revealed strong inshore to offshore patterns in the physical habitats and marine life. Five cross-shelf zones were identified in the study area, in contrast to only three zones for the area of the Park off Townsville.
Illegal Trawling: The Green Zone was closed to trawling in 1985. The study found that up to 40-50 boats regularly trawled in the closed area. Illegal trawling in the Green Zone appears to have occurred mainly in the lagoon, in and around the navigation channel, and along the northern and southern boundaries.
Closed to Trawling (Green Zone) vs Open to Trawling: The study compared the marine life in the closed Green Zone with that of adjacent areas open to trawling. Few differences were found between the areas that were clearly a result of trawling. There are several explanations for this. The effects of earlier trawling and/or current illegal trawling may have masked any differences between open and closed areas. In addition, the open areas next to the Green Zone were not trawled heavily and some parts were not trawled at all, making these open areas quite similar to the closed area. Furthermore, the two open areas studied were found to be as, or more, different from each other as they were from the Green Zone making it difficult to detect differences due to trawling.
Physical Impact: Commercial trawling typically targets aggregations of prawns by repeatedly trawling patches of productive seabed, before moving to another aggregation. In a series of experiments to simulate commercial trawling activities, the study showed that the pass of a single trawl has less impact than previously thought. It was thought that a trawl would remove nearly everything from the seabed, but the research showed that each pass of a trawl along the seabed removes about 5-25% of seabed life. However, there is a cumulative effect, with seven trawls over the same area of seabed removing about half of the seabed life, while a total of 13 trawls removed 70-90%.
Vulnerability: The research shows that different seabed species show different levels of impact. For example, the large sponges and flower-pot corals are particularly susceptible to trawling, whereas seawhips and gorgonians are more resistant.
Recovery: Recovery rates of damaged seabed life are poorly known, but are thought to range from 1 to 20 years depending on the species. It is estimated that over the last 20 years, trawling in the GBRMP has depleted the most vulnerable fauna (those easily removed and slow to recover) in trawled areas by more than half, with the result that less vulnerable species (those difficult to remove and/or quick to recover) dominate the seabed community.
Bycatch: In the far northern GBRMP, for every tonne of prawns retained, about 6-10 tonnes of other species are discarded. On average, this is about 2 kg of discards per hectare of trawled ground per year. The study showed that about two thirds of the discards were fish and the remaining third were crustaceans.
Adults and juveniles of recreationally and commercially important fish, although present in the study area, were not caught in prawn trawls.
Most of the discards were eaten rapidly by seabirds or small sharks and fish. It was concluded that in most trawl grounds on the GBRMP, discarding bycatch is unlikely to be having a serious impact on populations of scavenging species (with the exception of increased numbers of crested terns).
Conclusion: There is the potential for environmentally sustainable management of prawn trawling in the GBRMP, although there are important information gaps that need to be filled:
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© Copyright CSIRO Australia, 2005