A trousers-type framed midwater trawl (T-FMT) was developed for the simultaneous sampling of plankton and micronekton, which allows the use of two kinds of mesh sizes at the same time. This paper describes the performance of T-FMT in the survey of plankton and micronekton. In addition, the estimation of size selectivity for these small creatures was conducted by using the SELECT analysis expanded for a sub-sample from the trousers trawl experiment. The experiments for plankton and juvenile walleye pollock were conducted offshore of Usujiri, Hokkaido. Three kinds of codends were prepared: two moji-nets with mesh sizes of 1.7mm (M17) and 3.1mm(M31), respectively, and a plankton mesh with a mesh size of 526μm (PL). A pair of codends, M17 vs. PL and M17 vs. M31, was tested for sampling plankton and juvenile walleye pollock, respectively. Euphausiids dominated the catches of the M17 codend, whereas copepods were dominant in the PL codend. The Iength distribution of euphausiids in the PL codend showed a bimodal shape, whereas of those in the M15 codend showed a unimodal shape. The average L_<50> and S. R. values for the estimated selectivity curve of the M17 codend for euphausiids were 7.8 and 1.0mm, respectively. For the juvenile walleye pollock caught in April (early season after spawning), the length distribution in the M17 and M31 codends showed a deviant shape, i.e. there were no fish smaller than 12mm in length. For the fish caught in June, the length distributions for the M17 and M31 codends showed a unimodal shape, and there was no difference in length distribution between the M17 and M31 codends. Based on these results, it was concluded that the PL and M17 codend pair was suitable for the simultaneous sampling not only of plankton and micronekton but also of juvenile walleye pollock in this area from April to June. In addition, the availability of T-FMT in the study of size selectivity for small creatures was confirmed.
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