Video Evidence of Flatfish Herding by NWFSC Groundfish Survey Trawl Sweeps

The Northwest Fisheries Science Center (NWFSC) combined shelf-slope trawl survey has occurred annually since 2003 and data from the survey have been used in 2007 and 2009 stock assessments. The estimate of q (the scalar that relates the magnitude of a survey index to the predicted size of a population) was high in the 2009 petrale sole stock assessment because the magnitude of the relative index of abundance from the NWFSC survey was larger than the population estimates from the stock assessment model. One factor contributing to the high estimate of q could be that the area-swept calculation was an underestimate of the actual area swept by the trawl because of the herding of petrale sole by the gear in front of the net.  It is widely known that flatfish are commonly herded by trawl nets, but the extent of herding of flatfish by the NWFSC survey trawl was unknown. To address this, in August 2009 we undertook a pilot project using an underwater video camera to determine if the sweeps, or the cable gear running between the trawl doors and the footrope, on survey trawls were herding fish towards the net, resulting in a larger effective area swept than what was calculated in the assessment using the distance between the wings. At first view, over 91% of flatfish showed an orientation relative to the sweeps that was indicative of herding; this rose to over 93% at final view.  Those that were not herded either escaped over or under the mud gear or were stationary during the time they were observed.  Taken together, these results provide direct evidence that flatfish are herded by the sweeps at the front of the NWFSC survey trawl.