A new research project will map how much marine litter is floating around in the water column of the North Sea, how is it distributed and what type it is.

Bastian Huwer, a researcher at DTU Aqua in Denmark, has received a research grant from the VELUX Foundation to record and analyse litter items which have been caught as bycatch in annual herring larvae surveys carried out by DTU Aqua in collaboration with most other countries around the North Sea.

Speaking about the ‘MARLINS—Marine Litter in the Water Column of the North Sea’ project, which starts this month, Huwer stated: “Now I have the opportunity to dig into the details and analyse all samples very precisely, and it will be interesting to see the big picture in the entire North Sea.”

Almost all plastic

The Danish sampling area in the North Sea is a broad "belt" between Denmark and England and preliminary patterns show that almost all litter is plastic.

A large part of the litter near the coasts are blue or orange plastic threads, which are likely to originate from so-called ‘dolly ropes’, particularly common in beam trawl fishery, which mainly takes place in the southern part of the North Sea.

The research will look at whether the litter originates from dolly ropes or other fishing gear, plus where the plastic threads floating along the shores of Denmark come from.

Plankton net sampling

A fine-meshed plankton net has been used for the herring larvae surveys. This collects litter in the entire water column, compared to other marine litter studies which use bottom trawls or look at litter washed up on beaches.

The net is dragged in a V shape from the surface to 3m above the sea floor and up again. There are also small flow meters inside the plankton net. “It is a small propeller gadget that counts how much water we have filtered and allows us to quantify the amount of litter per cubic metre of water," explained Huwer.