No abstract

SUMMARY: For design or improvement of fishing nets, a model test based on Tauti's law or Froude's law is well executed. However, because these modeling rules are based on the proportion of the drag of the net to the square of water velocity, the converted drag of the net from the result of model testing is considerably large compared with the observed value. In this study, Tauti's law and Froude's law were corrected considering the drag coefficient of the net to the Reynolds number based on the twine diameter used experimentally. Three different scale models: 1/12, 1/20 and 1/50 of a midwater trawl net of 61 m in total length were made according to the modified Tauti's law and Froude's law, and the full‐scale and model experiment were carried out. Consequently, the difference between the converted drags of full‐scale by three scale models and measured values was within 10%. Each of these modified modeling rules is effective for the model test of trawl nets.

No abstract

The experimental operations concerning the catching efficiency and the fish selectivity for bottom gillnet and trammel net were examined with 3 different mesh sizes and 3 different slacknesses (the ratio of the height of inner net to that of outer net) in the Tokyo Port during 1981 to 1982.The results obtained are summarized as follows:1) The catch pattern of gizzard shad Clupanodon punctatus, sardine Sardinops melanostictus, and big eyed herring Herklotsichthys zunasi changes due to difference of mesh size of inner net.2) The slackness of trammel net seems to affect to some degree the catches, while in the case where the mesh size of inner net is suitable for the size of fish, the slackness of inner net does not affect the catches of trammel net so much.3) In the case where both the mesh size of inner net of trammel net and the mesh size of bottom gillnet are suitable for the size of fish, it seems that the catching efficiency of gizzard shad by bottom gillnet exceeds that by trammel net.4) It follows from the selection curves of trammel net and bottom gillnet obtained by this experiment that when the slackness is 1.1, the sharpness of selection curve for the trammel net has the same tendency as that of the bottom gillnet. In the case where the slackness is more than 1.5, it is expected that the larger size of fish is caught more easily by trammel net than by bottom gillnet.5) The selection curve of the trammel net has the same character as that of bottom gillnet. This means that the smaller size of fish is selected severely by the mesh sizes.(Tokyo University of Fisheries, Konan, Minato, Tokyo 108, Japan).

No abstract