Science - July 31, 2017

Yak leather as substitute for dolly rope

Didi de Vries

Plastic thread hanging in bunches off fishing nets is a familiar sight. This ‘dolly rope’ is effective at preventing the wear of fishing nets, but also pollutes the seas. Wouter Jan Strietman, researcher at Wageningen Economic Research, is involved in the search for alternative materials, such as yak leather.

Assessing alternatives for dolly rope. © Wouter Jan Strietman

When fishing for plaice and sole, fishing nets drag along the bottom of the sea, causing them to wear at a faster pace or get stuck and tear. That is why bunches of orange and blue thread hang off the nets. This dolly rope is attached to the parts that are most exposed to wear and tear and functions as a buffer between the bottom and the net. The Dutch fishing industry uses several dozen tonnes of dolly rope each year. About 25 percent of that wears down within two months, with all associated consequences.

‘Everything that is lost here in the North Sea follows the stream to the north and washes up on the beaches’, Strietman says. There is an average of a hundred pieces of dolly rope on each one hundred metres of beach. On the small German archipelago of Heligoland, the northern gannets use the dolly rope to build their nests. Birds sometimes also get entangled.

Within project VisPluisVrij (DollyRopeFree), fishermen actively seek an alternative material for dolly rope. ‘We are focusing on cheap materials that are strong, flexible, durable, recyclable and degradable in sea water’, Strietman explains. Many materials seem suitable at first, but turn out not to be good enough in practice. Natural fibres such as sisal and hemp were found not to be able to withstand the large forces exerted on the material. ‘Those fibres were worn after a fishing trip of just several hours.’

Natural fibres were worn after a fishing trip of just several hours.
Wouter Jan Strietman

In the past years, dozens of options have been tested at sea by fishermen. This summer, the most successful materials so far are tested as long as possible. The most promising alternatives are biodegradable plastic and yak leather.

Yak leather
According to Strietman, the switch to yak leather is not that farfetched. In the old days, dolly rope used to be made from cow leather. However, the ships and nets were smaller then, and cow leather was strong enough. ‘Yak leather is quite a bit thicker and works just as well as plastic dolly rope in the short term’, Strietman explains. The upcoming research should reveal whether the leather can be used in the long term as well.

Additional reading:

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  • P.

    Alsof dat zo een vriendelijke oplossing is voor het mileu...

  • Robertson

    Plastic oceaanafval is problematisch, dat ongetwijfeld. Toch wordt achter het net gevist in de zoektocht naar een duurzame oplossing. Kijk naar de impact die visserij op het milieu heeft: een zee zonder leven zorgt voor een serieus probleem, zowel voor planeet als mensheid. Om over het verlies van extra yakkenlevens nog maar te zwijgen.