OCEAN fishermen are constantly on the lookout for new places to ply their trade, as they exhaust the old ones. Thus, in the 1970s, Europe’s trawlermen turned to the deep seabed of the north-east Atlantic to replace the shallow continental-shelf fisheries closer to home that they had stripped near-bare.

But those replacement grounds, too, are not what they were. A study published in 2009 suggested that in all but the deepest of their waters—those with a seabed closer than 1,500 metres to the surface—yields had dropped by 70% over 25 years. Even in the abyss below that depth, the fall was 20%. To try to stem this decline the European Union, which regulates fishing in much of the area, is proposing to limit the depth at which trawling can take place. This would, in effect, create a marine reservoir below that level, a form of protection additional to the system of species-specific quotas that already exists. The question is where the line below which trawl-gear is forbidden should be drawn. And, until now, there have been few scientific data to inform that decision.

This has just changed, however, with the timely publication, in Current Biology, of a study by Jo Clarke of Glasgow University and Francis Neat of Marine Scotland Science, a government agency. Their work suggests that the appropriate cut-off would be at a depth of 600 metres—below which the ecological damage caused by trawling increases substantially.

Ms Clarke and Dr Neat derive their conclusion from data collected between 1978 and 2013 by Marine Scotland Science and the Universities of Aberdeen and St Andrews. These data record species caught, and also the depths of the trawls that caught them, which ranged from 250 to 1,500 metres.

The researchers note that biodiversity increases with depth. On average, an extra 18 fish species show up with each 100-metre increase. Many of these, though, are of little commercial value. Such so-called by-catch gets thrown back, but by then most of it is dead. And that, particularly because deep-sea species tend to grow more slowly than those which live near the surface, and have lower fecundity rates, can have profound effects on ocean ecology.

Trawls at 300 metres, Ms Clarke and Dr Neat found, have a ratio of catch to by-catch (in terms of weight) of five to one. At 600 metres the ratio is around three to one. At 800 metres, though, it is ten to nine; at 1,000 metres one to one; and at 1,200 metres, one to two.

Based on these findings, Ms Clarke and Dr Neat suggest that a trawl limit of 600 metres would be a suitable compromise between commercial reality and ecological necessity. Such a cut-and-dried value would have the bonus that any fishing boat caught in the north-east Atlantic with gear whose ropes were long enough to let it trawl more deeply than this would automatically be breaking the law, making enforcement easier.

Whether anyone in Brussels is listening remains to be seen. Fishery regulations are notoriously driven by vested interests first, and science a distant second. But the rule Ms Clarke and Dr Neat are proposing has the virtues of clarity and simplicity, as well as ecological logic, and would certainly be a good basis for negotiations.