HU Fuxiang Tokyo University of Fisheries, Assistant Prof., 水産学部, 助手 (80293091)
東海 正 東京水産大学, 水産学部, 助教授 (30237044)
KANEHIRO Haruyuki Tokyo University of Fisheries, Professor, 水産学部, 教授 (80134857)
FUJITA Kiyoshi Tokyo University of Fisheries, Professor, 水産学部, 教授 (80017053)
TOKAI Tadahi Tokyo University of Fisheries, Associate Prof., 水産学部, 教授 (20017073)
|Budget Amount *help
¥3,900,000 (Direct Cost : ¥3,900,000)
Fiscal Year 1999 : ¥2,000,000 (Direct Cost : ¥2,000,000)
Fiscal Year 1998 : ¥1,900,000 (Direct Cost : ¥1,900,000)
In order to continuously utilize the fisheries resource effectively, it is necessary to estimate the stock abundance of the target fish species accurately. The quantitative collection of the larval and juvenile is pointed out as one of the means. In this research, the new midwater trawl (MOHT) was developed with a square fixed frame and constant depth can be towed irrespective of the change in towing speed to collect pelagic larval and juvenile fish quantitatively. The results are shown as follows.
(1) In the first year, the cambered V-type depressor was designed with the camber ratio of 15%, aspect ratio of 6.0 and dihedral angle of 20°. An experiment was carried out using 1/5 scale model in a circulating water tank. The maximum sinking force coefficient of 1.57 was obtained at the attack angle of 20°. It is larger about twice than the value of the flat shaped depressor. Next, this depressor was attached to the lower side of the net frame with four long bridles, the net depth by change
of the flow velocity was measured. As the result, the fluctuating range of the net depth was slight 4.4 cm, and it was possible to maintain the constant depth almost in 30-100cm/s wide range of flow velocity. (2) In this fiscal year, first of all field experiment was carried out with the 1/5 scale model using training ship "Hiyodori" of Tokyo University of Fisheries in the Tokyo bay. As for the MOHT model that weight in the water of the net is adjusted to zero, it was confirmed to be able almost to towing constant depth within the range of towing speed of 1.0-2.0 m/s at warp length 20, 30, 40, and 50m. (3) The full scale of 12.5m in total length was made with net-mouth 2.25m x 2.25m based on the results of the model testing. The field experiments were carried out by the boarding to "Soyo-maru" of National Research Institute of Fisheries Science and T/RV "Umitaka-maru" of Tokyo University of Fisheries in Suruga Bay and the off Hawaii. As the results, the fluctuating range of the net depth was only 2.7m when the towing speed was accelerated from 1.0 m/s to 2.3 m/s in the case of warp length 100m and weight in water of the net was 30kg. Especially, it is possible that depressor always keeps setting angle of attack as one of the features of the net developed by this study. Therefore, the net position can be always controlled to the target depth regardless of the towing speed, if the warp length is equal even in the different operation.
As mentioned above, in this research, the propose was achieved and the developed net has already been used to collect the juvenile fish with some research laboratories including National Research Institute of Fisheries Science. The spread as the standard fishing gear for resource investigation will be expected in the future.