Lit fishing nets avoid catching wrong fish

Overfishing has had a major impact on our marine ecosystems. Apart from depleting fishing stocks, catching unwanted fish, known as bycatch, can disrupt food chains, damage the natural world and needlessly further endanger rare species. However, reducing the amount of bycatch while still maintaining profitable fishing enterprises has been difficult. But now, new research is shedding some light on a solution. Robert Spencer asked marine biologist Charlotte Birkmanis, from the University of Western Australia and who wasn’t involved in the study, to take a look at the idea…

Charlotte - A lot of small scale fisheries use gillnets, which are basically a net that is a curtain that is suspended in the water column, and it catches a lot of fish. It especially catches a lot of larger animals that can't swim through the gaps in the net. If you have a boat and a couple of people to help you, you can catch a lot of fish with minimal effort. Like everything, if something is too efficient, if you are catching too many fish, there's going to be implications to the ecosystem.

Robert - How much of a problem is bycatch? Is it a large amount that these nets in particular catch, or is it just every now and then they find something they didn't intend to get?

Charlotte - I think a lot of the time they catch something that they don't intend to get. In fact, a study came out in 2020 and they estimated that 9.1 million tons are discarded annually. That is almost 11% of the global catch. So, they are targeting tuna or something like that, that has a high commercial value, and they actually get these other animals caught in the net as well. It depends on the species: it can be sharks, it can be other animals that they're definitely not targeting like dolphins, like turtles. A lot of fishermen I do believe make a great effort to get them back in the water alive, but there's a lot of them that are already dead when they're pulled aboard.

Robert - Can you describe what the study in 'Current Biology' did in order to try and reduce the problem?

Charlotte - This was actually a very elegant study. This study got green LED lights, and it fixed them to gillnets at every 10 meters on the net. They also had other nets in the water which served as controls, so they put other nets in the water at the same time and in the same area to compare the two, to see what's actually happening with these LED lights. They found that the illuminated nets actually caught 63% less bycatch in total. That's over 80% fewer squid and 95% fewer sharks and rays by weight. So, that is very effective in eliminating bycatch, but the species that they were targeting, they were still capturing. It was a great study that showed that not only can we help conservation and the environment, but we can also ensure that these fishers catch the fish that they want.

Robert - That's incredible that you managed to select out over 90% of certain bycatch species, but not really reduce your target. Do we have any idea how that works? Why do the bycatch fish avoid a lit net and not the target catch?

Charlotte - That is the question and that's what needs more research. We know that elasmobranchs, which are sharks and rays, this group do have highly developed visual systems, and this is what they noted in the study. It's unclear if they were actually attracted to the net or deterred by the illumination but it was obviously some sort of a visual cue for them. The squid that were deterred as well, we know that they actually have enlarged eyes because they're predators. That's something that we need to look at in further depth.

Robert - How do you power these lights underwater? Is it something that you run cables back to land or are they battery operated?

Charlotte - I believe these ones were battery operated, and each one of these LED lights costs approximately eight US dollars. So, if you are in a fishery that can afford it, that's great, but, for a lot of these smaller fisheries, that price could be an issue. So, the researchers are also looking into having these solar powered lights. That would definitely make these smaller subsistence fisheries able to use this method as well.

Robert - Adding lights to your netting is presumably a time intensive and labour intensive process - is it worth it for these fisheries?

Charlotte - As you say, there will be a cost for having these LED lights, but there's also a benefit. It also helps them by eliminating the time consuming and cumbersome task of actually cleaning and untangling these nets and repairing them. If you get a big shark in the nets, it can actually rip your net, which is expensive and you've got to clean it. And it's also dangerous handling these large animals because they're not too happy being caught in this net either. So, there possibly is an increased expense in having these lights, but perhaps it'll be a greater benefit in general of having them.