Design and operation methods of a mid-water trawl for quantitative sampling of a surface pelagic fish, Pacific saury ( Cololabis saira)

Abstract

To establish a quantitative sampling method using mid-water trawls for estimating the stock abundance of Pacific saury ( Cololabis saira), we carried out experimental fishing surveys using two types of mid-water trawl (SM-trawl: 4 panels, trawler 397 GRT and KM-trawl: 6 panels, trawler 2630 GRT). To confirm the occurrence of saury in the survey areas, we simultaneously conducted experimental fishing of M-driftnets (driftnets with multiple mesh-sized panels), which have been the traditional sampling gear for saury stock assessment research. The results of the experimental fishing revealed that the SM-trawl was effective for sampling saury when the length of the towing cable was shortened to 200 m. The KM-trawl was not effective in quantitatively sampling saury. Comparisons of the structure between the KM-trawl and the SM-trawl showed that the essential conditions on the design of the mid-water trawl for sampling saury are the following: (1) the whole head rope including both the wing tips should be floated up to the surface, (2) the width of the upper panel of the trawl at the mouth should be much longer than the maximum width of the trawler. It is required about 30 m, when the maximum width of the trawler is 9 m. The conclusion is that the SM-trawl is suitable for sampling saury quantitatively. Moreover, we made some suggestions for precisely determining the catch rate of the SM-trawl for estimating saury stock abundance.