Trawling is a contentious form of fishing, often surrounded by a highly charged debate.
CSIRO has led a recent study into trawling around the world. The study assessed the status of seabed communities for 24 large marine regions where trawling occurs. It found that when trawling is managed sustainably its impact on the life of the seabed floor is also low.
This study is the product of an international research collaboration called the Trawl Best Practice[Link will open in a new window] (TBP) project. The TBP project has been working since 2012. It's aim is to generate scientific information to understand and effectively manage the environmental impact of trawling.
University of Washington fisheries scientist Professor Ray Hilborn is one of the project co-leaders.
“This paper addresses the fundamental question of the consequence of trawling on the seabed in different regions, by synthesising the trawl footprints with impact and recovery rates,” he said.
What is trawling?
Trawling is basically a type of fishing that involves pulling a net through the water behind one or more boats. The exact method and gear may vary.
The study looked specifically at the impact of different types of bottom trawling, in which parts of the gear make contact with the seabed.
Dr Roland Pitcher from CSIRO is the lead author of the paper. He said that the study set out to understand the relationship between the distribution and intensity of trawling and the status of the ecological communities that live in and on the seabed. The paper builds on previous work done through the TBP.
“The first phase of the project was to gather detailed spatial mapping data for trawling effort and calculate trawling footprints for the different regions,” he said.
“The second phase was to collate and analyse all available published studies of the impacts of trawling on benthic (seabed) communities that live on or in the sediments, as well as studies of recovery.”
Assigning an impact status to trawled areas
Bringing these metrics together, the study generated a relative benthic status (RBS) for each marine region assessed.
The RBS indicates how different the collection of plants and animals on the seafloor is to what it would be if there had been no trawling. It uses a scale between 1 and 0. A status of 1 indicates no change or impact and 0 indicates full modification of the pre-trawling ecological communities.
Dr Pitcher said that a low status indicates a disturbance rather than a lack of life. So while the most sensitive organisms may have disappeared due to intensive trawling, more robust ones may have persisted or even increased in number.
“Lower status of a region’s seabed could occur in a few different ways,” he explained.
“Either more area of seabed was trawled or the intensity of trawling was high. In regions with the lowest seabed status, both occurred.”
Of the 24 regions, 15 had an RBS of over 0.9, indicating a lower level of overall impact. However, 1.5% of the total area studied had a status of 0. The study also identified a number of regions with large areas that have been so intensively trawled as to have their character totally changed. These included the Adriatic Sea and a number of other regions in Europe.
Five Australian regions were included in the study, which comprised a total area of 2.62 million km2. Over 2.2 million km2 had a status of 1, about 24,000 km2 had a status less than 0.8, and less than 1,000 km2 was assessed as 0.
“The most intensively trawled areas in Australia are in southern Queensland, including the southern Great Barrier Reef[Link will open in a new window], and into northern New South Wales, where there is prawn trawl fishing, as well as parts of south-eastern Australia where there is fish trawling.”
A case for effective fisheries management
Typically trawling is managed by fisheries managers in consultation with the fishing industry and the community. Fisheries managers juggle economic, social, political and environmental factors. But a key function of fisheries management is to ensure the population health of the fish species being targeted.
The study was able to establish a clear relationship between effective fisheries management and the impact of trawling. It found that where trawling is effectively managed for the sustainability of fish stocks, the seabed RBS will also be high.
“Across a variety of target species types and habitats the study indicated that if fisheries management was effectively implemented, then the rest of the environment would also benefit,” Dr Pitcher said.
Way forward to research trawling on data poor areas
The 24 large marine regions assessed include Australia, New Zealand, and parts of Europe, North America and South America. The study sought to go as wide as possible. However, a lack of data meant that some of the world’s most highly trawled areas were not included.
“We did not set out to exclude any countries. But for many, detailed data about trawling simply doesn’t exist, or was unavailable,” Dr Pitcher said.
However, having established a relationship between a region’s total amount of trawling activity and the condition of the seabed, the study has enabled a means to estimate the impact of trawling where there is a low level of data.
Professor Hilborn said the TBP group has a future global study in its sights, which would build on the results and relationships already established. The study would include deep ocean areas where trawling occurs, and areas of sensitive habitat.
The recent study used a global database of seabed sediment types, maintained at the University of Colorado. This database indicates that most areas where trawling occurs are mud, gravel or sand. However, on a world-wide scale there is a lack of data on the distribution of relatively rare but sensitive environments. These environments include deep-water coral reefs and cold-water sponge habitats. Dr Pitcher says mapping these areas should be a priority for future research.
“What is left to do, is to find out where these sensitive habitats occur and to ensure they are not being inappropriately trawled. This paper was not able to delve into that level of detail,” Dr Pitcher said.
Trawl impacts on the relative status of biotic communities of seabed sedimentary habitats in 24 regions worldwide was recently published in Proceedings of the National Academy of Science.