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Summary of progress on a salmon excluder for the Bering Sea pollock fishery

(Oct 2006) Since the fall of 2004, development of a salmon excluder device has focused on designs that utilize the swimming strength of salmon relative to that of pollock. One design that is being tested includes a web funnel that reduces the diameter within the mid section of the net, known as the intermediate. This slows the water outside of the funnel and creates a turbulent area within the net where the slower water meets with the faster flow through the funnel itself. Escape portals are cut into the top and upper sides of the intermediate, large enough to allow salmon to pass. Despite the large portals, most pollock can't escape through them because they lack the swimming strength of salmon. Deck boss Kori readies an underwater camera with recorder tube for placement aft of the salmon excluder installed in the net’s intermediate.

Katy McGauley (field project manager) Steve Patterson (on right, Steve served as the gear specialist on EFP project on Northern Eagle- he works for NET Systems); and mate (left) on the Northern Eagle Brooks in the wheelhouse while the vessel was moving to anther fishing location where catch rates for chinook salmon were expected to be higher.

In our tests to date, the effectiveness of the excluder was evaluated by mounting a secondary net above and behind the escape portals that recapture any fish that escape. By comparing catches in the two nets, we can determine how many salmon escape and how much pollock is lost in the process.

The results from three trials indicate that 36 to 43 percent of chinook salmon escape through the portals while the loss of pollock was only one-half to one and one-half percent. The funnel, however, also created a problem when pollock were pinned against the sides of the net ahead of the excluder. As more and more pollock became pinned, the net bulged and, under dense fishing conditions, caused the net to tear. The latest field trial in the winter of 2006 was the most encouraging because the device worked for the first time without any such "bulging" problems.

Steve Patterson examines a salmon before the recapture net cod end was dumped into the tank, The experiment involved using the secondary “recapture net” to capture salmon that “escape” via the excluder. This allows comparison of the number of salmon in the codend (that failed to escape) to the number in the recapture nets that would be expected to escape under normal use of the excluder.

Our field work has concentrated on adjustments in the placement of the excluder to enhance salmon escapement while avoiding problems with bulging. The current placement is well within the 4 inch mesh section of the intermediate which appears to reduce bulging but we continue to experience these problems under fast fishing conditions. Deck crew hoisting the recapture net’s back end. Note a few salmon can be seen. On some of the early tows, up to 60% of the chinook salmon escaped as measured by the number in the recapture net compared to the overall number in the tow.

The bulging problem (and others reported during testing done by pollock captains) led us to take a somewhat different approach to the excluder. Noting that some escapement occurs during slowdowns when the net is being retrieved, we placed weights on top of the excluder funnel to make it sink faster out of the way during slowdowns. This helps optimize salmon escapement during slowdowns because with the excluder collapsed, salmon that swim forward are effectively ushered out the escapement portals.

Our newest design includes a weighted "sheet" of webbing to cover the escapement portals during towing. When the vessel slows, the sheet should drop to release the salmon that move forward. The advantage of this over the weighted funnel is that it does not slow water down during normal fishing operations so it shouldn't result in any bulging.

Steve Paterson looking over the recapture net prior to setting out the next haul. The wing-shaped devices on the recapture net are water kites that we developed to fly the recapture net above the intermediate of the regular net. This creates sufficient space for “escaping” salmon to slip out of the escape portals of the excluder.

Tests of this new device this fall and during the winter of 2007 will allow us to measure the escapement rates of this new approach compared to the rates achieved in previous trials with the funnel device. If this approach is effective, we may have solved a large part of the problems with the current excluder. It would, however, require fishermen to make periodic slowdowns on tows of longer duration but this may be preferable to the problems that fishermen have experienced with the current device when they find dense schools of pollock.

Patterson and deck crew sewing on the low light camera before setting the next tow. This camera was placed aft of the excluder during the test. It was used mainly to evaluate the shape and taper of the excluder while towing. In addition, it provided some important behavioral information of how pollock and salmon react to the rapid increase in water flow through the back end of the excluder. Some salmon escapes were observed with this camera but far fewer than the number of escaped salmon found in the recapture net. This is because the sometimes dense flow of pollock obscures many of the escapes during towing.

Development of the salmon excluder for the Bering Sea pollock fishery is still a work in progress. While we have made encouraging progress, the excluder device has problems when fishing in dense schools of pollock and in areas of high jellyfish abundance. From our preliminary work it is still too soon to tell whether we will be able to overcome these difficulties sufficiently to produce a practical salmon excluder device.