Science - April 6, 2006

Dolphin escape route in nets

Deep-freeze trawlers fishing in open seas can halve their unwanted by-catches of big fish, marine mammals and sea turtles by over half by using special nets, according to research by the Wageningen marine institute Imares. The nets have special ‘dolphin exits’ through which large marine wildlife can escape.

Trawling off the coast of Mauritania for sardines and mackerel is a contentious activity because of the relatively large by-catches of large marine animals such as sharks, manta rays, sea turtles and dolphins. The highly productive waters around north-west Africa are regularly fished by between forty and seventy foreign trawlers. The ships include a number of Dutch deep-freeze trawlers, that are among the largest fishing ships in the world. The fishing takes place under international agreements between Mauritania and the European Union.
Marine biologists at Imares examined the by-catches of these trawlers and ways of reducing them through at the request of the Dutch ministry of agriculture and the fishing companies association. ‘The financiers of the research have shown that they are aware of the problem, and this research has given them an opportunity to contribute towards sustainable entrepreneurship in the fisheries sector,’ says Dr Jaap Jan Zeeberg, who heads the project for Imares.

By-catch
The research off the coast of Mauritania started in 2002 and has now finished. The most important results will be published in May in the journal Fisheries Research, and are already available online. The total European fishing fleet is responsible for somewhere between one and two thousand sharks being caught in nets. The marine biologists call these figures ‘high and unsustainable’.
The experimental nets, developed by the Dutch company Maritiem, can reduce by-catches considerably. Working together with the researchers, the developer of the net built in a ‘marine mammal exit’ in the top of the trawling nets, and along part of the lower length of the net there is a filter and an escape tunnel for large fish. Zeeberg: ‘Dolphins are claustrophobic and therefore swim at the front of the net. The exit enables them to get out of the net. The other fish are forced to the back and the coarser filter enables large fish to escape on the underside. The smaller fish, the object of the catch, stay behind at the back of the net.’
The experiments showed that one hundred percent of the sea turtles and rays escaped from the nets. For the manta rays, hammerhead sharks and marlins the escape rates were 75, 55 and 40 percent respectively. The results for the sunfish (between 5 and 20 percent) were disappointing. On the basis of underwater video images, Zeeberg suspects that this is because the sunfish is a very passive swimmer. The fish gets forced against the net by the prevailing water current. An important factor in the acceptance of this innovation by fishers is that the adjustments do not result in lower catches of commercial fish, according to Zeeberg. For this reason the filter has been kept relatively coarse. ‘It cannot prevent all by-catches, but it is the largest types that are most vulnerable.’

Compulsory
The Dutch fishers have already indicated that they will use the new net. Zeeberg: ‘It is a very progressive development in which the fishers approached the researchers to improve their fishing activities. Now it’s up to the European governments to make something of the new opportunity.’ Carel Drijver, head of oceans and costs at the conservation organisation WWF, is impressed by the research. ‘It is very praiseworthy that the Dutch fishers plan to continue using the new nets. But they have done this on a voluntary basis and only have ten ships in the area. We are now going to try and convince the Mauritanian government and the European Union that these escape nets should become compulsory on all ships. / GvM

Re:act