After fishing of the orange roughy, a deepwater fish, started, populations collapsed within 15 years (Image: Kim Westerskov/Getty)

In an autumnal ritual as unvarying as migrating geese, European Union officials head back to Brussels next week – to fight over fish. But this time they might protect Europe’s richest and most threatened ecosystem, the deep seabed.

They will discuss proposals to ban deep-sea trawling. Now in the first study of its kind, researchers have discovered just how deep the trawling should go.


Joanne Clarke of the University of Glasgow and her colleagues say that if trawling should go deeper than 600 metres, the percentage of a catch that is commercially valuable plummets, as deepwater species increase. If deeper trawling was banned, biodiversity would be protected with minimal loss to fisheries, they say. Existing EU proposals, backed by France, allow trawling to 800 metres, however.

Most fishing takes place on the relatively shallow continental shelves. But fishers have increasingly targeted the deep sea, the world’s largest ecosystem and one of its most biodiverse.

Fish at this depth can live more than a century, and breed very slowly, making them vulnerable to overfishing. Scientists set catch quotas to prevent overfishing, but these have sometimes been too high even for fast-breeding species like cod and have failed for deepwater fish.

For example, New Zealand started fishing for one deepwater fish – known as the slimehead but renamed orange roughy for the trade – in the 1980s. By the 1990s, populations had collapsed.

Undaunted, Europe started fishing orange roughy off Ireland in the 1990s. The fishery collapsed by 2002, says Matt Gianni of the Deep Sea Conservation Coalition in Amsterdam.

Vessels also catch and discard many commercially worthless fish at this depth, including threatened species of shark and ray. Heavy nets hauled across sea beds destroy fragile ecosystems including corals – which are among the planet’s oldest creatures.

The EU bans trawling in “vulnerable” areas, mostly seamounts dominated by corals. But researchers at the University of Plymouth calculated in 2013 that this protects less than half the corals and even less of other communities such as deepwater sponges.

In 2012 the European Commission proposed more comprehensive limits on deep sea catches. Spain, France and Scotland have opposed them.

Clarke’s team looked at data from scientific trawl surveys from 1978 to 2013. They found that biodiversity in the nets shot up as they went below 400 metres, with 18 new species every 100 metres.

But the percentage of the catch discarded climbed too – from a quarter at 600 metres to nearly two-thirds at 1300 metres. Even commercial fish in the catch were often too small to keep. The percentage of sharks and rays, known to be highly threatened, also shot up below 600 metres.

Taken together, says Clarke, the evidence shows that collateral ecological damage rises sharply as trawls go deeper than 600 metres. “Meanwhile there’s less commercial gain per unit effort and per area fished.”

That might be why relatively few vessels fish that deep, landing just 1 per cent of Europe’s catch – and why, other research has shown, they probably wouldn’t bother without subsidies.

Moreover, says Gianni, banning trawling under 600 metres would protect 85 per cent of corals, and all of the other fragile ecosystems. The question now is whether Europe’s fisheries ministers – who have a track record of disregarding scientific advice – will accept the evidence.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2015.07.070