Trawling the bottom of the ocean, dragging heavy metal equipment along the seabed at high speed, is the most destructive form of deep-sea fishing in the world. The fishing industry loves it because it is very effective. But it is indiscriminate and leaves behind a trail of devastation.

This month, the European Union (EU) is scheduled to vote on a proposed ban on deep-sea bottom trawling. If passed, the ban would be the first of its kind, although it would build on existing prohibitions on trawling in shallower water. It could give the seas some breathing space and fish stocks a chance to recover.

There has been intense lobbying against the proposed ban by a fishing industry that has strong ties with the governments of several countries and support from Ifremer, the predominant French fisheries-research organization. Lobbying groups have threatened legal action against scientists for publishing data deemed to be critical of the industry. The EU Fisheries Committee includes Members of the European Parliament from French, UK and Spanish areas in which deep-sea fishing vessels are docked. These ties have slowed the committee's consideration of the proposed ban to a crawl: it has postponed its vote by several months from the originally scheduled date. By contrast, the EU environment committee, which is not so dominated by members from fishing ports, voted 58 to 1 in favour of the ban in March.

In the run-up to the Fisheries Committee vote, now set for 18 September, science has been dragged into the political fight. The fishing lobby has published a series of influential pamphlets that start with the famous phrase “the inconvenient truth”. In essence, the pamphlets suggest that it is possible to use a lighter version of trawling equipment to trawl in the deep sea without doing much damage; that stocks of the target species are not being depleted; and that non-target catch is made up of just a few abundant species that are not in any kind of trouble. Many of the 'truths' listed are quoted by European politicians. Yet they are bunk.

If the European Parliament passes the ban, it will have global force and influence on conservation. As such, responsible scientists cannot let these claims go unchallenged. Here is a rundown of why the assertions are wrong.

Claim: damage can be limited by making a lighter trawl. In fact, trawls must be heavy to reach the sea floor at 800–1,500 metres, the zone in which most deep-sea fishing occurs. They need to be strong in order not to come apart when pulled through the water at 3 knots (5.6 kilometres per hour) or more. Deep-sea organisms are known to be delicate and fragile, often consisting of as much water as tissue. By analogy, it makes no difference if you are run over by a small car that weighs one tonne or a large truck that weighs several tonnes. The flesh of the body is no match for the strength of steel, however light the equipment.

“The ban could give the seas some breathing space and fish stocks a chance to recover.”

Claim: catches of deep-sea fish species of interest are reaching a sustainable level (maximum sustainable yield; MSY). What this does not take into account is that population assessments are based on industry data for ten index species, so what we know about fish stocks we get from fisheries catches. A vessel is not likely to trawl in an area that does not produce fish, so the abundance data are biased. Yet these are the data used to determine whether the species is at MSY.

Unfortunately, all catches of deep-sea fish in Europe are now at about 20% or less of their peak levels. Because the catches have levelled out at these low values, it is claimed that the populations are at MSY. However, it is well known that the French catch, for example, comes from a smaller area than previously, and it is possible that, rather than 'doing fine', as the range shrinks, the measured populations experience increasing effective mortality rates. Fishing could cause a complete collapse, as happened for cod stocks off eastern Canada in the early 1990s. Furthermore, there is no information on the populations of non-index species, dozens of which are threatened and are routinely caught.

Claim: by-catch is limited and no by-catch species are in trouble. Actually, studies suggest that trawls for 3 target species can catch more than 100 non-target species. Reported catch of the non-commercially valuable Baird's slickhead (Alepocephalus bairdii), for example, which accounts for more than one-third of by-catch by weight, has declined precipitously, and is now at about 6% of its 2002 high. This decline would be enough for the fish to be classed as endangered by the International Union for Conservation of Nature.

The final claim in the published fisheries propaganda is economic, not scientific. But let us examine it.

Claim: the French deep-sea fishing sector is operating sustainably and is an important employer. In fact, it has benefited from large subsidies, from both the EU and the French government. Since 2004, the three companies involved — Scapêche, Euronor and Dhellemmes — have received about €15 million (US$20 million) but showed more than €11 million in losses. The three firms employ a relatively small number of people on their boats: about 112 in total, or 0.5% of the entire French fisheries-vessels crew.

The vote this month is a chance to reverse much of the damage done to fisheries. Deep-water trawling should be consigned to history.