What could be the fix?
It’s not possible to stop fishermen from accidentally losing gear. But if nets are lost, we can reduce macro pollution and most likely ghost fishing by ensuring they are biodegradable, so they break down more quickly in an environmentally friendly way.
However, to be an interesting alternative to nylon for the fishermen, the new nets must be resistant enough to last the entire fishing season without losing too many fish, but degradable enough to dissolve in the water sufficiently quickly if lost.
A good net is elastic enough to catch fish without breaking, but not so elastic that the meshes stretch, and you catch the wrong type of fish. So, you need to find biodegradable resins with the right properties that you can extrude into filaments (fibres, ed.) which you then make into nets.
Do such nets exist?
Colleagues in Korea have managed to find a mix of resins that is suitable for making biodegradable nets. They work quite well in fisheries there. So, we had some nets custom-made to the right colour and size and tested them on net fisheries for cod and plaice in Skagerrak to see if they could work in the Danish waters as temperatures and conditions differ from Asia.
Our trials showed that they started breaking and fish could push their way through just a couple of weeks into the season. In comparison, the nylon nets traditionally used here can last for six months to two years, so of course the current biodegradable material is not ready for use here.
I noticed while out fishing that a lot of the damage was happening at the knot. That makes sense because that’s where you bend and thus weaken the filament, which then enables cracks to grow faster than in other parts. So, my research focus has shifted to the knots, which is an area that’s not well studied until now. The nylon nets were working so why should we care, right?
What have your studies revealed?
Examining the nets more closely I found that—unlike nylon—when working with resin you cannot heat the material as much when making the knots to make them melt together and keep them from slipping. So, to cool the temperature down, the nets are sprayed with water, which is bad because biodegradable plastics are sensitive to water which makes the material weaker. So that is problematic for strength.
With the help of Louis Le Gué, a visiting student from France and using specialized tanks at the research institute where he is enrolled, we were able to expose the nets to different temperatures and relevant bacteria to see how much you can stretch the nets, before they break and how bacteria affected different parts. We found a particular type of bacteria species gathered at the knots, which could be responsible for the degradation of the material.
So now we are back doing basic research to understand what happens in the processing phase, both when extruding the resin into filament but also when manufacturing the nets, because this is where we lose a lot of the good properties.
This will then help us in the hunt for the right balance of resins that are suitable to Danish conditions, before we spend money and time annoying fishermen asking for them to test things that don't work. In the long-term, other plant and protein-based alternatives could also be great candidates.
Will there be a market for such nets?
The press often portrays fishermen as people that don’t care about the environment. But it’s not true. They would buy biodegradable nets if they could, but right now they simply don't have other options. I mean for me it costs probably 10 times more to have the nets made than buying standard nets, so there's no way this is commercially viable. In a way it's not their fault. Rather the responsibility lies with us researchers to understand the material science better so we can help the fishermen do better.
