Bottom trawling for fish causing 'permanent damage' to deep sea forests

Wednesday, 6 March 2019

Trawlers hunting for fish in the dark, cold depths of the sea may be doing irreversible damage to vast coral reefs on the seafloor.

Findings from New Zealand researchers have some environmentalists pushing for a ban on bottom trawling, the primary method of catching deep sea fish, likening its impact on seabed wildlife to the destruction of kauri forests. 

An industry representative, however, said the findings were incomplete and New Zealand's deep sea fishing industry was world-leading on environmental measures.

Bottom trawling is the most common method of fishing internationally, but little is known about its long-term impact on the array of plant, fish and invertebrate life on the seabed.

**READ MORE:

* Bottom trawling for orange roughy has scientists worried

* Environment groups lambast fishing industry over bird deaths and sea floor damage

* Orange roughy stock rebound reopens southern fishery after 19 years

* Breakthrough films orange roughy 1km deep off NZ West Coast

* Chatham Rise mining refusal rocks shares**

The method involves dropping a large net on the seabed, at times more than one kilometre deep, and dragging it behind a ship, catching everything in its wake. 

While it's an efficient way to catch deep sea fish, it is destructive to the fragile biodiversity on the seafloor, which include centuries-old coral forests home to many types of fish and invertebrate species.

New Zealand researchers have shed light on the long-term impact of bottom trawling on those deep sea habitats, finding that the damage may be longer lasting than once thought.

The researchers, led by scientists from Niwa, have for 15 years been studying the Graveyard Seamounts, a group of undersea hills on the Chatham Rise, the gigantic area of raised ocean floor east of New Zealand.

Seamounts are typically underwater volcanoes that surge hundreds of metres from the seafloor.

Due to historic overfishing, three of the 28 seamounts have been closed to fishing since 2001, and some have not been fished at all. Others continue to be fished, to varying degrees of intensity.

The result is a 'natural experiment' for measuring the impact of bottom trawling through comparing the biodiversity on each seamount.

In a paper published last week, researchers found that 15 years after bottom trawling stopped, there was no evidence of recovery, suggesting that it could take decades, if not centuries, for the coral forests to return. 

A seamount which hadn't been fished since 2001 was 'essentially indistinguishable' from a seamount that continued to be fished intensively, researchers found. 

'After 15 years, we're not seeing any clear signs of recovery of the seabed fauna on the hills' said Dr Dave Bowden, a Niwa marine ecologist and a co-author of the research, in an interview.

'The message is that once these coral and sponge communities have been fished and knocked down, we can't expect to see them back again.'

Much like coral reefs in shallower seas, deep sea corals support high levels of biodiversity, he said. The trawls used were a blunt instrument that damaged virtually everything on the seabed.

'When you see the imagery of the hills that have not been fished, they're quite impressive coral reefs,' Bowden said.

'What we see on the hills that have been fished are bare rock and coral rubble.'

There were few examples globally of long-running research into the recovery of seabed biodiversity, meaning the results could be used to inform authorities of the wider impacts of bottom trawling. 

ORANGE ROUGHY

It could have implications for the deep sea fishing industry, which may be doing permanent, rather than temporary, damage.

The Graveyard Seamounts are a significant habitat for orange roughy, a long-lived, deep sea fish commonly eaten around the world.

New Zealand is the world's most prolific orange roughy catcher, responsible for around 95 per cent of the global catch.

A boom in orange roughy fishing in the 1980s and 1990s led to the near collapse of the population, which became a cautionary tale for overfishing. Catch limits were put in place to allow the population to recover, which have since been relaxed, following evidence of population recovery.

Because orange roughy have long lifespans – as long as 150 years – they take decades to reach maturity. Those long lifespans are matched by other deep sea fauna, which grow slowly and are rarely disturbed, unlike their shallower counterparts.

While the deep sea fishing industry acknowledged it had an environmental impact, it needed to be weighed up against the need to feed people, said George Clement, chief executive of the Deepwater Group, an advocacy group for deep sea fishing.

He said there was no data to show whether the seamounts had similar amounts of seabed fauna before trawling, meaning it was difficult to infer how much of the lack of coral was due to trawling.

'What we do know is that not all of these underwater hill features have coral and where corals do occur they most often are patchy in distribution and do not cover the entire area of the hill,' he said.

'Some of these hills have no coral and others have a lot – with variations often found between hills very close together.'

He acknowledged the need to account for the slow regrowth of seabed fauna, and said there needed to be a balance between protecting the environment and providing food.

New Zealand's deep sea fishing industry was world leading on environmental matters, Clement said, and large areas containing seamounts were already protected from fishing.

'We have set ourselves the goal of being trusted as the world's best managed deepwater fisheries and we have an international reputation for this,' he said.

'All food production necessitates some environmental change. We have a duty of care to ensure that we get the balance between utilisation and conservation right. 

'Neither of the two extremes - locking it all up, or not taking sufficient account of the conservation requirements - are acceptable.'

DEEP SEA KAURI

Environmentalists have raised concerns about the impact of bottom trawling and say the new research is further evidence of the long-term harm.

About 30 per cent of New Zealand's economic exclusive zone is closed to bottom trawling. Critics of deep sea fishing have called this misleading, as much of the area would not have been trawled anyway.

The damage to rare and vulnerable deep sea habitats should be seen no differently to those on land, said Geoff Keey, a strategic advisor at Forest & Bird.

'The Government would never allow this in our kauri forests, so why is it allowing bottom trawlers to completely ruin ancient undersea coral forests?' he said. 

'This research confirms what we suspected – that bottom trawling causes total and largely irreversible destruction of the seafloor. Some of these supposedly protected corals destroyed grow one millimetre a year.'

He said Morgue and Graveyard – the names of two of the seamounts studied – were appropriate, given the dearth of life on their slopes, and it was time to halt bottom trawling.

The research has also bolstered the case for opponents to seabed mining.

A long-running effort to mine the Chatham Rise for phosphate was rejected by the Environmental Protection Authority (EPA) in 2015 due to concerns about the environmental impact on the seabed.

It has been pitched as an alternative to New Zealand's existing phosphate source, which comes from a mine in a disputed region near the Sahara desert.

'This has serious implications for companies like Chatham Rock Phosphate, whose 2015 application to mine the Chatham Rise seabed failed because there wasn't enough scientific understanding of the impact,' said Cindy Baxter, chairwoman of Kiwis Against Seabed Mining.

'Now we have some - and it tells us the benthic [lowest level] community would take many decades to recover from bottom trawling, let alone the much more damaging seabed mining.'