Yasuzumi Fujimori

Effect of towing speed and net mouth size on catch efficiency in framed midwater trawls

To investigate fish density using rigid-mouth, towed net gear, such as a tramed midwater trawl (FMT), it is necessary to know the catch efficiency. The catch efficiency with an FMT is described as the rate of fish entering the net (entering rate) versus the rate of fish retention, i. e. the mesh selectivity. This study investigated the change in the catch efficiency under several towing conditions using three types of FMT of different net mouth areas (4.0, 12.3, and 16.0 m2) with the same mesh size. The catch efficeency of the FMT was evaluated by comparing catch per unit effort obtained from each towing condition. The catch efficiency of FMT varied with towing speed and net mouth area. Particularly, the effect of net mouth area on the catch efficiency was greater than that of towing speed. Additionally, under the same towing condition, the catch efficiency for large length fish was lower than that for small length fish.

The influence of warp length on trawl dimension and catch of walleye pollock Theragra chalcogramma in a bottom trawl survey

Variations in trawl dimension, bottom contact, and catch with differing warp lengths during walleye pollock surveys conducted by the Fisheries Research Agency were examined. The ratio of warp length to fishing depth (scope ratio) was set at 2.5, 3.0 and 3.5 at depths of 110 m. At tact the bottom. Footgear contact was complete when the scope ratio was 3.0 or more. Walleye pollock, sculpins and flatfishes were the main catch in all tows, and the catch increased with scope ratio. There was no difference in the length frequency of walleye pollock (n=300) captured at 2.5 and 3.0 scope ratio. However, the length frequency at 3.5 was significantly different from that of other scope ratios. These results suggest that at a scope ratio of 3.0 or more, the trawl door will not leave the bottom at any depths. To complimentecho surveys for walleye pollock, a 3.0 scope ratio would be optimal, since the catch data for a 3.5 scope ratio was dissimilar from that of scope ratios.

Trawling experiment in a circular water tank to assess the effects of towing speed, light intensity, and mesh shape on active escape of undersized fish

To assess the effect of towing speed and light intensity on the active escape of undersized fish through diamond and square mesh panels, a trawling experiment was simulated in a circular water tank. Juveniles of Japanese dace Tribolodon hakonensis (13-cm length class) were used as experimental fish. They were forced to swim inside a closed framed net with either diamond or square mesh (65-mm mesh size) that was moved using a speed-controllable motor. A submersible infrared CCD camera was used to observe and record the behavior of fish inside the net when it passed in front of the camera. Results indicated significant effects of towing speeds and light intensities (P<0.05) on the escape of fish through the diamond and square meshes. Increase in light intensity enhanced the ability of fish to escape at lower towing speeds. At higher towing speeds, few fish could escape under light conditions. A strong negative correlation was found between towing speed and the frequency of fish escape (diamond mesh R2=0.99, square mesh R2=0.96). There was no significant difference, between the numbers of fish escaping through the diamond and square meshes. These results suggest that the square-mesh panel may not be effective in the trawl cod end under dark or very low light intensity at high towing speed.

Simultaneous observations of zooplankton distribution and density using net sampling, a laser‐optical plankton counter, and acoustics

Conventional plankton sampling methods are based on net tows, which are often poorly suited to capturing spatiotemporal variability. Various automatic plankton samplers based on optical principles have been developed recently. The laser‐optical plankton counter (LOPC) was developed as the next generation OPC. In addition, acoustic technology has been used extensively to survey zooplankton distribution and abundance. This study compared the density and size spectra determined using net contents, LOPC data, and acoustic data. Surveys were conducted for 3 years around Hokkaido, Japan. The sound‐scattering layer (SSL) was recorded on the echogram of an echosounder. Biological sampling was conducted using a 1‐m2 frame‐type mid‐water trawl (FMT; mesh size 1.5 mm) and a NORPAC net (mesh size 500 microns). An LOPC was mounted inside the mouth of these nets. Net sampling was carried out at dusk, when the SSL moves toward the surface (ca. 10–35 m). Acoustic data were derived using a Simrad EK60 echosounder operatin...