Finding Success in Excluding Salmon from Pollock Nets

Since 2002 NOAA scientists have been working cooperatively with members of commercial fishing industry in the Bering Sea to reduce bycatch of salmon in the pelagic trawls used by the walleye pollock (Theragra chalcogramma) fishery.  The current configuration of the salmon excluder is a diamond-mesh tunnel placed in the net section just ahead of the codend, with an escapement hole above the tunnel. Selectivity is achieved through the difference in swimming behavior between salmon and pollock wherein salmon can swim forward and exit the net through an escapement portal at a greater rate than can pollock. During sea trials, a recapture net captured fish after they had escaped in order to quantify the proportion of salmon and pollock escapement. Over the years, various designs of excluders have achieved salmon escapement rates as high 45% and as low as 3%, but high escape rates were achieved with designs vulnerable to bulging and loss of trawl door spread and sometimes even damage to the net at high catch rates. Salmon bycatch reduction has been a high priority for the pollock fishery in the Bering Sea and incentives for excluder development have recently increased with the approval of a hard cap on Chinook salmon (Oncorhynchus tshawytsch) bycatch that would trigger closure of the fishery if attained. The current excluder design, developed at the flume tank in October 2009, was tested in February 2010 on both a catcher vessel and a catch/processor vessel. The results are very promising with preliminary data showing escapement rates of between 25% and 35%, less than 1% loss of pollock and no problems of gear damage at high catch rates.