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Excluder Research Aims to Reduce Bycatch of Pollock Fishing


One of the inherent difficulties in fishing is bycatch, when unwanted or protected species are swept up in nets.  To counter this, fishermen may sail to different waters or use devices called excluders to let specific types of fish and marine life escape.  KDLG’s Chase Cavanaugh has more on a device that would reduce certain bycatch in the Bering Sea.

Earlier this year, the North Pacific Research Board held its annual Alaska Marine Science Symposium.  The event featured research on many different topics, including bycatch.  This is the term for unwanted or protected species that are swept up along with a fisherman’s regular catch.  Areas such as the Bering Sea have hard limits on the amount of bycatch fishermen may collect, and if they reach the limit, they can’t continue fishing there.

Enter John Gauvin, the Fisheries Science Director of the Alaska Seafood Cooperative.  He delivered a keynote address at the Symposium about his work on excluders, devices fitted to a net that let certain animals escape, while keeping target species trapped.  Gauvin’s work centered on testing designs that would catch pollock in the Bering Sea while allowing king salmon to escape.  He says an accurate test requires being able to count both types of fish. 

“We came up with a secondary net to append to the trawl that’s flown on water kites, and this allows us to retain what effectively is supposed to be escapement. We can look at the number of salmon and pollock that are retained in this secondary net compared to the amount in a haul of pollock and salmon that we count and weigh.”

The excluders go by a variety of names, such as tunnel, flapper, and over-under.

“We’re pretty bad at naming things in the fishing industry as you can tell. (laughs) Flapper, I don’t even know where that came from.”

Each name refers to the way an excluder design operates.  The tunnel excluders, first researched in 2003, use square mesh panels to change the flow of water in the middle of a net.  With slower water flow in front of the mesh, salmon could escape through the top or sides and out the back, while pollock were pinned to the mesh.  This worked in theory, but in field testing, researchers found that the panels cause nets to bunch up when towed behind boats, and when they catch a lot of pollock, even rip.  So Gauvin says research switched to a new flapper design.

"We have video that says some of these fish moving back, the salmon moving back, actually just won’t move back. They’re able to stay in this part of the net, they just don’t want to keep moving back to the caught end, and so we’d invented a trap door. We took a piece of webbing and some escapement holes and the flow of water keeps that thing completely closed while you’re towing. The fisherman then slows down, the weight that we put on there drops that panel down, opens up the trap door, the salmon swim forward and out.”

While initial trials showed 12-20% escapement of king salmon, the slowdown required to open and close the trapdoor brought the net into a higher level of water, which typically contained even more salmon, cancelling out the excluder’s effects.  Gauvin says for the subsequent flapper design, they had to make some modifications.

"The concept had some redeemable qualities if you will. You can take this sheet of webbing and weight it enough in this front section that it will trail back straight. Like that tunnel idea, where everything has to come to the back and then come out, we’re just going to put some weight here. The advantage to this is that if you do get a lot of pollock coming down your net, there’s no square mesh in here to start to pin fish. This panel will just lift up. Even if it does lift up, we create a hood up here with some flotation to allow more room so even when it pushes up, a salmon can still come out."

This new design allowed 25-34% king escapement and less than 1% of pollock escaped, so it was widely adopted by fishermen, with each tuning the weight based on the power of their boat and size of their net.   However, the design didn’t allow chum salmon to escape.  To remedy this, Gauvin’s team designed an excluder with two trapdoors. 

“An over and under excluder allows escapement both out the top and out the bottom. This is done by a weighted panel on the top and a floated panel on the bottom. The water flow keeps those in the center. You have a scoop on each side that you create out of webbing, effectively allowing a double pathway, we’re thinking chinooks will go out the top and chums will go out the bottom.”

Testing of the over-under excluder was still ongoing when Gauvin gave his presentation at the Science Symposium, but in a recent interview, he said results are promising.

“This spring, we had two more Gulf of Alaska Pollock boats testing the over and under excluder and both had approximately 40% Chinook escapement with somewhere under 5% loss of pollock. This pretty much matched the earlier tests we’d had that I reported on at the science symposium from 2013.”

Overall, Gauvin says fisherman willingly make use of the excluders, as it allows them to fish longer and reduce their level of bycatch.

“I think they view it as a tool in the toolbox that’s important. I haven’t heard any fisherman that we’ve worked with think that losing some pollock was not justified as long as they can keep the pollock loss rate manageable so they’re not just out there burning fuel and towing longer.”

Gauvin and his research team will return to the field in mid-September to continue testing of the over-under excluder.  After finishing their work in the Gulf of Alaska, they will test the design in the Bering Sea.  Gauvin hopes the private fishing industry can make use of his research to produce more effective and economical excluder designs, as well as devise other ways to reduce bycatch.