Ropes and fishing gear used in the fisheries and aquaculture industries are a major source of microplastics in the ocean and littering along the coastline. A multidisciplinary international research team has now drawn up a plan that will help to reduce pollution.
You may have seen images of seabirds that have built their nests on discarded nets, lengths of rope and other plastic litter, or perhaps birds with stomachs full of microplastics. We also know there is an invisible and very harmful effect of discarded fishing gear, known as “ghost fishing”, where abandoned traps and nets are left behind to continue fishing almost indefinitely. This littering has major consequences for life in and around the ocean.
It may be invisible, but it is still a huge problem
Other fishing gear, ropes and microplastics sink to the bottom and become an invisible problem; a nylon line can last up to 600 years on the seabed. This gear is mostly made from non-degradable types of plastics, such as polyethylene, polypropylene, polyester and polyamide.
Scientists are now seeking to develop new materials which will break down faster without causing pollution.
“Fishing gear remains in situ for a long time and in practice turns the ocean into a plastic landfill site, because particles of microplastics are formed when the materials degrade in the ocean. This slow, natural decomposition starts with photodegradation (caused by light) and thermo-oxidative degradation (caused by temperature) of the polymers,” explains researcher Christian Karl from SINTEF.
The problem of lost fishing gear and equipment from the aquaculture industry is enormous. Most of it is made of plastic.
Fortunately, the Norwegian Directorate of Fisheries conducts annual voyages to search for lost nets and other fishing gear in areas where losses have been reported.
However, researchers are working hard to find and test alternatives to the plastic materials which either last “indefinitely” or break down to form micro- and nanoplastics.
In particular, the use of bottom trawl and Danish seine fishing techniques is leading to large quantities of microplastics ending up in the ocean.
Fishing gear remains in situ for a long time and in practice turns the ocean into a plastic landfill site, because particles of microplastics are formed when the materials degrade in the ocean.
“We have already come a long way in the work,” said Karl. He is a polymer chemist and specialises in material combinations in plastic, degradable alternatives and material analysis. For almost four years now, he and his colleagues both in Norway and abroad have been working to find solutions to the problem.
Among other things, Karl has been responsible for testing the biodegradability (laboratory and field testing) and environmental impact of fishing gear in the D-Solve project. In the project, biodegradation is tested in various marine habitats and climate zones, such as Skagerrak, the North Sea, the Baltic Sea, the Adriatic Sea and the Norwegian Sea.
The tests are conducted in temperatures of between 4 and 27 degrees Celsius, with the aim of comparing how biodegradable and conventional gear copes:
“The tests will continue for at least three years or until the materials have completely degraded. We will study microbiological, UV, thermal and chemical degradation in detail,” explains the researcher.
Facts about the clean-up operation
The Directorate of Fisheries has been removing plastics and fishing gear from our fishing grounds ever since 1983. In 2022 alone, they collected:
- 1,128 nets of various types
- 28,000 metres of line
- 26,600 metres of rope
- 204 traps and accessories
- 41,500 metres of dumped or abandoned Danish seine rope
- 8,500 metres of dumped trawl wire
- In addition to oilskins, gloves, plastic bags and other plastic litter that was caught in this fishing gear
Source: Directorate of Fisheries
Some facts about the degradation of plastic:
Degradation occurs because the chemical bonds in the materials eventually break down as a result of use and, eventually, wear. In addition, bacteria and microorganisms help to break down the polymers into CO2 and methane, among other things. Mechanical wear, wind and weather also contribute to the wear and degradation of ropes and gear.
The tip of an iceberg
One of the partners in the project is the Norwegian College of Fishery Science at UiT The Arctic University of Norway. Professor Roger Larsen works there. He began his professional career as a fisherman. He is now working with SINTEF to find solutions to the problem at D-Solve, where he is centre manager.
The aim of the work is to develop new degradable materials to replace the plastic materials that are currently being used in equipment in the fisheries and aquaculture industries, without turning into microplastics.
“Specialists from the Directorate of Fisheries are very knowledgeable and ensure that large quantities of discarded fishing gear are collected every year. Virtually everything is made of plastic.
“But, there are some unknown figures, too: In 2022, for example, no less than 40 kilometres of Danish seine rope was recovered. This rope was found by chance and had not been reported as lost fishing gear,” said Larsen.
The vision of the researchers is to reduce both ghost fishing and plastic pollution. This will be carried out in collaboration with industry, universities, research institutions and advocacy organisations. They have now identified what it will take.
The research has been conducted in “international waters” with our partners in Denmark, Germany, Croatia and South Korea, and has also led to field trials in USA, India, China and elsewhere.
The researchers suggest four main steps if we are to eliminate this source of pollution:
- Develop biodegradable ropes and fishing gear as an alternative to ordinary oil-based plastics.
• Develop new fishing gear with simpler designs (to simplify recycling).
• Contribute to the industrial upscaling of solutions.
• Ensure that materials can be recycled.
A complicated puzzle
The research team has conducted both mechanical tests and physical trials with the aim of studying the degradation of alternative materials.
In practice, this means that they have looked at the mechanical properties and wearing of the materials. If we are to make everlasting fishing gear and subsequent littering a thing of the past, it is important that the materials used are degradable, yet at the same time strong enough to do the job.
“An important aim is therefore to develop materials and fishing gear which are user-friendly during what we call the ‘service period”, but then degrade rapidly,” said the SINTEF researcher.
This is not an easy task, because the materials can consist of different material components. Some also have a core of steel or lead, or are impregnated with copper.
To make plastic production in the fisheries and aquaculture sector more sustainable and circular, it is important to cooperate across sectors and disciplines, according to Karl.
“In this way, we can solve global problems at a local level. Little by little, steady progress is being made in overcoming the challenges associated with the quality of recycled materials, or the removal of copper from fishing gear, to mention just a few problems.
Today, there is a problem with what are known as “mixed waste fractions”, i.e. fishing gear that consists of many different types of materials. This makes it difficult, and sometimes even impossible, to recycle materials in a way which ensures that the value of the materials in their future life remains sufficiently high.
“We are working on this in another project called SHIFT plastics,” adds the researcher.
Ready-to-use results
So far, researchers have succeeded in finding sustainable solutions for many different types of fishing gear, but not all:
“As regards degradable fishing nets, we currently have fishing gear that delivers on 70 percent of the properties,” said researcher Roger Larsen from UiT. “That’s why we’re now looking for the perfect combination of materials. Nets are perhaps the biggest challenge, because the mesh must be thin and invisible to the fish, yet also sufficiently strong and elastic so that it works almost as well as nylon in use.”
However, as regards longline fishing gear, the researchers have demonstrated that there is no significant difference in catch efficiency when nylon is used compared to a biodegradable alternative.
Modern mechanised line fishing (autoline) techniques can use long lines up to 90 km long. Such lines can have up to 70,000 hooks attached to the main line via gangions or snoods, which are short lengths of line about half a metre long.
“Thousands of gangions wear out or become cut and lost every year in line fisheries in Norway alone. Today, gangions are made from polyester or nylon, a very strong material that sinks and ends up on the seabed, where it can take a very long time to break down,” said Larsen.
“In the case of coastal line fishing, shorter line lengths are used, but even vessels 11-15 m in length can use lines with up to 30,000 hooks,” said the former fisherman.
But, there is hope:
“With regard to this, we have conducted tests that show there is little difference in catch efficiency when degradable materials are used (ref. K. Cerbule et al., 2022). We have a biodegradable alternative that will dissolve within a few years, without leaving any microplastics behind,” said Larsen.
How long the degradation process takes will depend on many factors, including temperature, light and microbial activity, to mention just a few.
“However, what we can guarantee is that the materials will have a ‘life in the ocean’ of much less than several hundred years, which is what the Norwegian Environment Agency estimates the lifespan of an ordinary nylon line to be.”
Some facts about marine littering
- From the mid-1960s onwards, synthetic plastic fibres began to be used in fishing gear. Today, almost 100 percent of such fishing gear is plastic-based.
According to the Norwegian Environment Agency, 10-15% of all marine litter will end up on our beaches. The remaining 85-90 percent of plastic litter ends up on the seabed, where it can have a “lifespan” of up to 600 years, especially in extensive basins with cold, deep water.
• Much of the beach litter in our part of the world consists of plastic from fisheries and aquaculture. These are polyethylene (PE) and polypropylene (PP) materials that float in the water and end up as beach litter, ultimately in the form of microplastics. Typically, much of our beach litter consists of nets, rope, floats, plastic containers, other packaging, etc.
• Fishing gear made from materials such as nylon (PA) and polyester (PES) sink in the ocean, but lost fishing gear such as trawls, traps and Seine nets where PE and PP are used in the mesh will often end up on the seabed because fishing gear is often made from plastic and metal.
• The loss of nets and traps in particular means that tonnes of fish and crabs are lost as a result of what is known as ‘ghost fishing’.
Source: UiT
Harmful bottom trawling
Two of the worst culprits in the marine realm of plastic pollution are bottom trawling and Danish seine fishing. The dragging of the fishing gear along the seabed causes extensive wear on the materials that are used. The nets used in wings, bags and trawls have attracted criticism – the lifespan of the fishing gear is relatively short. The discarded remains can also end up on our beaches and in the stomachs of seabirds. This is because the materials that are used float up to the surface, where their tempting colours can be confused with krill and other prey, or end up as litter or materials in the nests of birds.
In this context, too, researchers have looked at environmentally friendly alternative materials that can be broken down by nature itself.
“This is especially true as regards the wear mat which is placed beneath the trawl net itself and provides protection to prevent holes from being created by wear and tear, causing the catch to be lost,” said Larsen. The material, a bio-polyester produced by the Dutch company SENBIS, may also be suitable for use in Danish seine fishing. This particularly applies to the rope arms that guide the fish in towards the net.
However, this alternative is a relatively expensive investment in the short term:
“In this case, the problem is that it soon becomes too expensive for fishermen with small quotas, so we need to find cheaper solutions, such as wood fibre, animal hide or cotton. What we do know has worked well as slaughter mats/chafe mats in bottom trawling ever since ancient times is cowhide,” said Larsen, adding: “In the Netherlands, tests have been conducted on the hide of Yak bulls, among other things, with good results. However, it is important to find both the right supplier and the right quality before such animals can once again be used in fishing gear.”
“We know the solutions exist; we just have to crack the code at material level, based on the development of materials and properties linked to degradation,” said researcher Christian Karl at SINTEF.
The work highlighted in the article was carried out in the following projects:
DSOLVE (SFI)
SHIFT-Plastics (KSP)
You will find the articles and the research reports from the projects here: