Seabed impacts of trawling

CSIRO scientists are changing the way we think about the affects of trawling. A report completed by CSIRO scientists titled, Predicting benthic impacts & recovery to support biodiversity management in the South-east Marine Region has found that trawling only touches 6% of the south east seafloor and has not had a major effect on groups of invertebrates (animals without backbones such as corals, sponges and sea-squirts). In fact, these invertebrate groups are now recovering, due to positive actions of industry and management.

The south-east Australia supports Australia’s second largest trawl fishery. The fleet of approximately 30 trawl vessels catches 10-12,000 tonnes of fish worth $50m at the port. The fishery is the largest supplier of fresh and healthy Australian seafood like ling, flathead and blue grenadier to the citizens of Melbourne and Sydney.

The South East is also home to the world’s largest network of continental shelf and deepwater marine parks. The 14 parks in the 388,464km2 South East Marine Reserve Network are about the same size as the landmass of the state of South Australia.

During this work CSIRO scientists working within the National Environmental Research Program Marine Biodiversity Hub integrated many data sets to produce three types of maps for the south-east:

1) Maps of where habitat forming bottom invertebrate (animals without backbones) groups occur based on previous surveys and computer modelling with depth, sediments, currents, temperature, nutrients and others.

2) A map of 15 different seafloor habitat types, or assemblages, obtained from existing survey data for all fishes, using invertebrates as indicators of different types of habitat and by again using computer modelling of things like depth.

3) A map of the trawl fishery’s swept area. Deepwater trawl fishing vessels log their position and width of their gear each time they shoot and haul their net and record the width of their fishing gear.

Existing research about the damage that trawl fishing gear causes to these invertebrate groups, and their recovery afterwards, was also considered.

CSIRO research calculated the effect of various management interventions including the removal of 86 trawl licences, establishing the South East Commonwealth Marine Reserve and other fishery area closures.

Key findings of this work were:

1) Only 6% of the seafloor is currently trawled annually because trawling is aggregated into a small area.

2) Some assemblages of invertebrates are not ever trawled because the area of seafloor where they live is either closed by fishery closures, protected by marine parks or is not trawled because it is unproductive or too rough. Large areas (44%) of the region are closed through fishery closures (39%) and marine parks (9%) with some overlap.

3) All 10 groups of seafloor invertebrates declined after trawling started to a low point around the year 2005 when these invertebrates reached 80-93% of their un-trawled abundance.

4) Closures, marine parks and licence buy-backs contributed to the recovery of invertebrates by 1-3% to 82-94% of their un-trawled abundance. Licence buybacks improved the status of all groups and closures contributed for most groups. However, most fishery closures and marine parks had little detectable influence on the abundance of invertebrates.

100 years of the south-east providing healthy seafood in relation to the loss of 6-18% of the SE region’s invertebrate abundance contrasts favourably to the loss of 40% of Australia’s forests and 50% of its wetlands . The trawl fishing of 6% annually of the south-east seafloor compares very favourably with the farming of 26% of the Australian landmass.

The South East Trawl Fishing Industry Association’s (SETFIA) Chairman, Mr Tom Bibby, explained,

“The fishing industry has always known that there are very few trawl vessels operating in a very limited area and that our footprint would be minimal. This research proves that. However, internationally funded green groups must try to convince the Australian community that trawl fishing is harmful in order to drive continued donations. Industry is not lobbying for the removal of marine parks but the industry operates under the constant threat of more closures; this work shows that they are not needed. Invertebrate communities never reached low levels and are rebuilding strongly”.

Research about the impact of trawling on global continental shelves.