A 160-cm Opening-Closing Plankton Net

II. Some Notes on the Towing Behavior of the Net

抄録

The towing behavior of three Ocean Research Institute nets, ORI-200, ORI-33, and ORI-C, is considered from the results of their performance tests and the routine samplings. The following facts are observed:
1. It seems very difficult to tow the net straightly in oblique tow. The experiment shows that the net speed and the volume of water filtered can be kept almost constantly in descent, while they are increased and greatly varied in ascent.
2. In horizontal tow, the net depth is decreased rapidly at a moment when the net is opened on a throttling system. The time required for the stabilization of the net and the vertical thickness of the towing layer increase with increasing of the length of wire out.
3. The determination of the towing depth by the measurement of wire angle is only reliable for samplings in the water shallower than 500 m.
4. The filtration ratio for each of the ORI nets is approximately 0.95.
5. When catches taken by the three nets are compared, the catches do not greatly depend upon the mesh size for the animals whose maximum width is larger than the coarsest mesh aperture.
6. The effective cross-section all area of the ORI-C net in regard to small euphausiids which pass through the coarse-mesh portion of the net is 0.63 m₂.