2013 Volume 60 Issue 1 Pages 11-17
Various types of opening/closing plankton nets have been devised for depth-specific sampling. While electrically-operated multiple layer opening/closing nets are commonly used to collect samples from deep layers from well-equipped research vessels, messenger-operated opening/closing nets are useful for sampling from small boats at shallow depths. The latter nets were originally designed for horizontal or oblique tows, whereas a closing (non-opening) net is still often used for a vertical haul. However a closing net inevitably suffers from contamination when a net is allowed to obliquely descend due to ship drift caused by the wind or water currents. Previous messenger-operated opening/closing nets have also had disadvantages when they are lowered vertically, i.e. contamination with surface plankton due to incomplete closing of the net mouth and sampling failure due to tangling of the slackened bridle and/or choker ropes. To overcome these disadvantages, a new messenger-operated opening/closing net was designed for vertical hauls. The frame of the net consists of a base, two mouth rings (45 cm diameter) hinged to the base, and a same-sized center ring. The mouth rings are each equipped with a cylindrical-conical net (0.10 mm mesh), with the cod end being clamped to a rope suspending the weight lead, and is equipped with a flow meter. The operating lines consist of a “sampling wire” connected to the center ring, a “first closing wire” connected to one of the mouth rings, and a “second closing rope” connected to the double mouth rings. While the net is lowered, the first closing wire is kept under tension by gravity acting on the net frame and the weight lead tightly shuts the mouth rings like a closed clamshell. Upon deployment of the first messenger, the first closing wire is released, the sampling wire is brought under tension, and the mouth rings are opened with the aid of tension springs and wires for synchronous opening. The second messenger, deployed after sampling, causes the sampling wire to be released and the mouth rings to close again through the tension of the second closing rope. The new design is almost completely free from contamination because how tightly closed the mouths are while descending, decreases in sampling failures caused by tangling of the bridle and closing rope, and gives correct counts from flow meters because the meters are stopped from working while the mouths are closed. In addition, a high sampling efficiency is expected because of its bongo-net design when the mouths are opened. An easily-operated double release mechanism was also designed for this net. Previous mechanisms for opening/closing nets are deployed with latches along the lateral side. In such a design, a mechanism inclines against the towing line by a force driven on lateral latches. Especially when vertical hauling, the inclination sometimes does not make the messenger successfully hit the trigger. Latches of the present mechanism are placed at its bottom and their releases are turned by a spring-tensioned, swinging rod connected to the trigger. A square-shaped frame is employed for the trigger, by which a swivel can be set between the towing line and the mechanism. The mechanism can be used for multi-layer sampling if a wire stop is welded on the base plate. Using the present net and double release system, the author has collected depth-specific plankton samples monthly from 70 and 200 m depths from a small boat (five tons) for the last nine months without any significant troubles.
