Fisheries and dolly rope

Dolly rope is the name for blue or orange rope that are used to protect bottom trawling nets against wear and tear. During fishing operations or maintenance work on the net, threads or bundles of dolly rope threads may end up in the sea.

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. 

Dolly rope consists of around 30 strings, each string has around 25 threads. The rope is prepared in such a way that it can then be attached to the underside chafer. Chafer is a piece of netting underneath the cod-end of the net, which is used to protect it against wear and tear. Before use, the dolly rope is split into pieces of a fixed length (usually around 1,5-2 meters in length). These pieces are then folded and attached to the chafer, much like a swallow-tail, each around 60-100cm long. 

When in use, the smaller threads within the dolly rope will spread out, creating a cushion like cover and protecting the net from wear and tear by acting as a protective buffer between the seafloor and the net. 

Depending on the type of fishery, the type of seabed (sand/clay/stones/rock) and the size of the net, bottom trawling fishing vessel owners purchase between 325 and 3500 kilograms annually. 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. 

Usually, around 10–25% of newly applied dolly rope ends up tearing off during the first few weeks of use, depending on the seafloor conditions. After that period, the strands of dolly rope threads usually start to tangle into one another. From that moment onwards, the loss of individual pieces of dolly rope due to wear and tear decreases, but at the same time the dolly rope loses its flexibility. The entangled threads cause large amounts of sand and grit to get caught up in the rope, at which point the fishermen often replace the strands of rope with new ones, some of which may also end up in the sea. 

Fisheries and dolly rope. Reference: WJ Strietman