The Net That Lets Fishermen 'Preview' Their Catch

The cannon is ready, the ammo is loaded, and the controller blasts it into the sky. But this isn't a weapon of war: it's part of a rescue operation, and fish are the target.

When it comes to fishing, some species just aren't meant to be caught in the net, and that's where this device comes in handy. It's designed to reduce bycatch in industrial fisheries. Bycatch refers to the non-target marine animals and fish—unwanted either because they aren't the right species, or because they're too small to be legally caught—that vessels haul in unintentionally as part of their catch.

It does this by using an air-powered cannon to shoot a mini-trawl into a net, so it can collect a sample of fish. That tells fishermen whether they have the right species and size of fish, before they haul in multiple tons of the stuff.

"The main thing is to provide a tool for fishermen to target the fisheries better, so they actually get the fish they set out to catch," said Bjorn Erik ​Axelsen from the Instit​ute of Marine Research (IMR) in Norway, head of the research group that built the contraption, which they it calls "Air Powered Sampling for Purse Seine Fisheries."

The design team, which included fisheries researchers Bjørnar Isaksen and Jostein Saltskår from the IMR, and Kurt Hansen from SINT​EF Fisheries and Aquaculture, honed the technology over the course of three years and recently wo​n the World Wildlife Fund's Sma​rtGear challenge for their design.

This competition awards prizes to innovators that reduce bycatch, which this device intends to do in "purse-seine" fish​eries. This large-scale fishing method encircles schools of pelagic fish like tuna and mackerel with massive nets that can be over a kilometre long and 200 metres deep. The bigger ones are capable of reeling in 1,000 tons of fish in one go. "The net is so big that initially fish might not even know they're being caught," Axelsen said.

Of course, the sheer scale of this makes it difficult for fishermen to catch selectively, and so purse-seining perpetuates the problem of bycatch.

As they're crushed against one another, fish panic and frequently die of stress, so by the time they're pumped aboard they're beyond rescue, Axelsen said. "At that point if you don't have the target species you can no longer release a fish. It will die." This bycatch accounts for a third of the European Union's fishery—that's 1.3 million ​tonnes of fish lost each year.

But armed with their net-launching air cannon, the Norwegian designers think they can make purse-seining more sustainable. The contraption works by loading the mini-trawl into the cannon, a machine typically used to shoot rope out for docking. An air pressure pump is attached to the device to forcefully vault out the net, which flies out with metres of rope trailing behind it. Kite-like flaps at the trawl's mouth open it up as it arcs through the air, so that once it hits the water it's stretched open and ready to catch fish. With its 1.5 metre girth, the prototype net swallows roughly 50 fish at a time. Once it's full, fishermen use the rope to haul it back in.

"If you get the wrong species, you're still at the point where you can easily let the fish go," Axelsen said. Nets that return with a high percentage of unwanted species or small specimens are a cue to open the purse seine and release the catch, before hours are wasted filling it with the wrong fish. Plus, trawling takes just a few minutes, so fishermen can easily do repeat launches and increase their sample size. "We know the trawl actually fishes representatively," Axelsen added, something they established when they trialled it.

By 2019, it wi​ll be illegal for any unwanted catch in EU fisheries to be tossed overboard. Even so, the threat of bycatch still looms, because although fish won't necessarily be wasted, it still means protected species or young fish will end up dead when they shouldn't. By imposing the ban, the EU hopes it will push fishermen to try out innovations that make fisheries more selective about what they catch—like this one does.

The design team is now refining the net and its launcher to suit different kinds of purse seine fisheries. Fishing companies are interested in commercialising the canon, too. They're working with fishermen in Norway, and as far away as Peru, where they're testing it on anchoveta fisheries.

"The need is everywhere," Axelsen says. "The more information you can get about fish before you actually catch them, the better."