Marine Biology (2+1)

Operational details of Closing net

This net is provided with an opening and closing mechanism and hence, the collection of plankton between the surface and bottom of any desired depth in the vertical profile is made possible . The net is closed at any required depth by means of a drop weight (messenger) which releases the throttle nose. If, for example, only the plankton between 10 and 20 m depth is to be sampled, the closing net is sent down to 20 m depth, drawn up  slowly and at about 10 m, the drop weight is sent down and this closes the net at about 10 m. The net must be drawn out immediately without any interruption. As no more plankton enters the closed net during the have the plankton caught belong to the depth of 10-20m.

The closing net resembles the normal plankton net in many respects and differs form the latter mainly in the presence of a closing mechanism. The bag of the net in its lower part has a fine plankton filtering portion, above which, is a strip of coarse khaki cloth. Between this and net ring a coarse net is sewn. To the net ring is attached as in a typical plankton net, three suspension ropes all tied to a small brass ring. This brass ring is further attached above the closing mechanism to the two ropes, viz. the tow and closing ropes. The net is sent down with a weight at the bottom and is pulled up by the tow rope. The closing rope is led quite loose  round the part of the net made of coarse cloth by means of rings sewn in. One end of it is firmly tied to one of the rings and the other is attached to the release mechanism. When the messenger sent through the tow rope, strikes the release mechanism, the net is then freed from the tow rope and now it rests slowly on the closing rope which fastens like a ring round the bag of the net when it is drawn up and this closes the net. Since the upper part of the net is tilted sideways, no more plankton get into the net.