Marine Biology

The Clarke – Bumpus horizontal closing net

This equipment, (a rapid towing one) is mainly used for a horizontal collection of plankton belonging to a specific stratum. The net consists of a solid front section, a brass tube of 15 cm long and 12.7 cm in diameter attached to the filtering net portion by a bayonet lock. Inside the brass tube, the propeller vanes and outside a counter are also present to help registering the volume of water passing  through the net.

The brass tube is so mounted in a frame that is movable up and down, so that, in any position of the frame, the net lies horizontally. A metal bar fixed between the brass tube at the front end and the net jar at the caudal end acts as a stabilizer and the two outer vanes located on the brass tube help to keep the net in horizontal position while in operation. The frame itself is so attached to a tow rope (draw line) that it can swing freely and that the opening of the brass tube is directed forward by any movement of it. The brass tube can be opened and closed by a closing device, i.e. a pivoted metal cover (disc) activated by a drop weight. While operating, the net is sent in a closed state to the desired depth and is opened with a drop weight acting on the release mechanism; after the plankton have been collected,  it is closed again by means of a second drop weight. The volume of water passed through the net during the operation is determined based on the revolutions in the counter and the factor value, i.e. quantity of water associated with one revolution which is determined through standardization of the net as for a normal standard net described already.