By Wouter Jan Strietman on 02 August 2018
‘Dolly rope’. Not many people know this name, but some may recognize what it is. All around the North Sea coast, when strolling along the shoreline, chances are that you will notice blue or orange plastic threads sticking out of the sand. These threads may look familiar because they resemble fishing line, but they are actually dolly rope threads.
Dolly rope is the name for orange or blue plastic threads that are used to protect (the cod-end) of bottom trawling nets against wear and tear. Every year, thousands of kilos of dolly rope end up in the sea. Within the DollyRopeFree project, we are working on solutions to prevent this from happening.
What is dolly rope and where is it being used?
Dolly rope is a piece of rope that consists of dozens of smaller twisted separate pieces of polyethylene threads. Before use, dolly rope is split into pieces of a fixed length (usually around 2 meters in length). These pieces are then folded and attached to the net, like a swallow-tail. When in use, the smaller threads within the dolly rope will spread out, creating a cushion like cover and protecting the net.
The use of dolly rope is common amongst different types of demersal fisheries all over the world, but mainly in Europe around the North Sea, Channel, Irish Sea and the Bay of Biscay. It is also being used by pelagic trawlers in the Barents Sea around Svalbard.
Usually, more dolly rope is being applied in fishing areas with a rocky or stony seafloor, whereas less dolly rope is usually used in fishing areas with a sandy or clay seafloor
How does dolly rope end up in the sea?
During fishing operations, the end of a net usually floats freely in the water. But when stones, sand and/or other material ends up in it, the net usually sinks and scrapes along the seafloor. In these situations, Dolly rope acts as a protective buffer between the seafloor and the net. But the effect is that threads or bundles of threads get torn off and end up in the sea.
Dolly rope frays easily and 10–25% of dolly rope ends up tearing off. In this way, annually, at the (very) least 25.000 kilograms of dolly rope threads end up in the North Sea alone, worldwide much more. Besides dolly rope tearing off, potentially a similar amount is cast overboard during maintenance work.
As a result of this, you may find over 100 pieces of dolly rope on a 100 meter stretch of beach in Belgium, The Netherlands, Denmark, Germany or southern Norway.
What are the environmental effects?
Dolly rope is made out of polyethylene, a type of plastic that is not biodegradable. Polyethylene particles will merely break down into small fragments, but never disappear. These tiny left-over particles are transported by marine currents and easily spread throughout the marine food chains. Fish, but also birds such as gannets, seagulls and fulmars are likely to mistake small pieces of plastic for something edible, and can get entangled in the dolly rope.
Working on solutions: the DollyropeFree project
In 2014, VISNED, the Dutch fishermen’s organization, the North Sea Foundation (an environmental NGO) and the Dutch government expressed their concerns about the issue of dolly rope. Coordinated by Wageningen Economic Research and Wing, these organizations teamed up with material specialists and scientists to develop solutions.
Within the Dolly ropeFree project, the approach is to developing solutions for the three main reasons why dolly rope ends up in the sea:
- Developing alternative materials and net protection methods. The challenge here is to find alternatives which are much less prone to wear and tear and/or biodegradable, and/or are designed in a different way than threads, but in such a way that they also protect the net. The last couple of years, we have tested tens of different materials and variations in design both in controlled conditions and out at sea. Currently, three alternative materials and designs stand out: strips made out of yak leather, in seawater biodegradable plastic threads and a net protection method that is also being used in France, but adapted for use in Dutch waters. These alternatives are in the process of being tested by several ships to see how they perform in practice, if used on a large scale. The results of which will be published later this year.
- Developing alternative net designs (lifting the cod-end of the net). In this approach, the focus is on improvements in the design of the cod-end to prevent it from scraping along the seafloor. Together with industrial designers, fishermen and other experts, alternative net designs are currently being developed in Germany by the Thünen Institute.
- Improving litter management onboard fishing vessels. The focus in this approach is on engaging with fishing vessel crews to raise awareness regarding litter management onboard (including dolly rope) and engage with the relevant stakeholders to develop practical solutions improve litter collection at fishing harbours. Currently, as part of the ‘Green Deal Fisheries for a Clean Sea’ in The Netherlands, stakeholders related to fisheries litter have committed themselves to improve waste management practices on board fishing vessels and fishing harbours.