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By Simon Torok 12 September 2017 4 min read

Image: Jonathan Ayres/Alamy Stock Photo

Commercial trawling for fish, prawns and scallops is the most extensive direct human pressure applied to the seabed globally.

However, the first Australia-wide assessment of seabed biodiversity’s exposure to, and protection from, trawling has found that less than 5 per cent of Australia’s entire continental Exclusive Economic Zone (EEZ) is subject to trawling.

The results, published in the journal Diversity and Distributions, show the trawling footprint in Australian waters over recent years accounts for 1.1 per cent of the total EEZ.

“This is smaller than might have been expected,” says the paper’s lead author, CSIRO’s Dr Tessa Mazor.

The study also reported that trawl fishing is prohibited in 58 per cent of Australia’s EEZ, although this may change with a current review of Commonwealth marine reserves.

“The proportion of protection is much higher than in most other countries,” says Mazor.

These protected areas are made up of State and Commonwealth marine reserves (37 per cent of Australia’s EEZ), which aim to protect representative habitats, and also fishery legislated areas (30 per cent of Australia’s EEZ) closed to trawling to protect fish stocks or nursery grounds.

“Fishery closures provide a high level of protection for seabed fauna,” says Mazor.

“Our findings show that fishery closures make an important contribution to protecting seabed biodiversity, even though marine reserves are commonly considered the main way to protect the marine environment.”

Invertebrates exposed

The potential risks of trawling not only depend on how much trawling is done, but also on the distribution of seabed life in regions where trawling occurs.

Mazor and colleagues used data from 18 surveys of seabed biota to model and predict the large-scale distributions of seabed invertebrate communities in nine continental shelf and slope regions around Australia. They combined these predictions with spatial data for marine reserves, fishery closures and trawling effort to assess the extent these invertebrate communities were exposed to, or protected from, trawling.

The researchers found that across all 134 invertebrate communities studied, an average of 7 per cent of each occurred in trawled areas, with 38 per cent in areas protected from trawling. In other words, about five times more seabed fauna is protected from trawling than exposed. The remaining majority (average 55 per cent) of each invertebrate community is not protected, but also is not trawled.

“Our results show that Australia’s seabed invertebrates such as crabs, sea urchins, starfish, and sponges have relatively low exposure to trawling and imply they may be at low potential risk from trawling,” says Mazor. “This new information will help management of sensitive sea life on the ocean floor.”

“Although we have found most marine invertebrates have a high level of protection and low exposure to trawling, this work helps identify those places and those invertebrates that are most exposed and least protected, and hence may still be at risk due to trawling,” she says. “These require detailed quantitative status assessment and, if necessary, risk management.”

Co-author of the paper, Dr Roland Pitcher, says the work is the first step towards a national assessment of the sustainability of seabed habitats and communities.

“The next step is to apply a quantitative status assessment to all regions,” he says. “We can now do this for all of Australia because we have been able to quantify the entire Australian trawling footprint for the first time by collating data from fishing vessel monitoring systems and from trawlers’ logbooks together in one place.”

“We’ll look at the areas trawled and the habitats and invertebrate communities they support to quantify their exposure and risks. We’ll move beyond calculating the percentage exposed to trawling, and apply an assessment method that estimates their status. We’ll be able to give a percent status: for example, that a habitat or community is 90 per cent of what it would be if there was no trawling.”

Pitcher says the paper builds on a wealth of CSIRO work that extends back to research starting in the 1980s on the effects of foreign trawling on Australia’s northwest shelf.

“CSIRO has worked with government regulators, commercial fisheries and community stakeholders to quantify the effects of trawling in Australia,” says Pitcher. “A variety of research approaches are required, including impact experiments, studies of bycatch, measuring recovery of trawled areas, mapping of seabed biodiversity and trawl footprints, developing and applying modelling and risk assessment methods.” “We’re currently working with international colleagues, applying methods and assessments to overseas regions on several continents world-wide.”

5 surprising statistics about trawling

  1. Less than 5 per cent of Australia’s EEZ waters are trawled
  2. 58 per cent of Australia’s EEZ is protected from trawl fishing
  3. On average 7 per cent of each invertebrate community studied are in trawled areas
  4. On average 38 per cent of each invertebrate community studied are protected from trawling
  5. On average 55 per cent of each invertebrate community studied are not trawled or protected.

(From Mazor et al. 2017)

Further reading:

CSIRO research into the effects of trawling

State of the environment, Australia 2016: Footprint of commercial demersal trawl fishing

In Methods in Ecology and Evolution: Estimating the sustainability of towed fishing-gear impacts on seabed habitats: a simple quantitative risk assessment method applicable to data-limited fisheries

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