Marine Biology (2+1)

Plankton nets

A typical plankton net usable in the surface layers is conical in shape and has the following constituents. A net ring made of stainless steel and wrapped and sealed with polythene tubing is present anteriorly. To this, a non-filtering portion made of a coarse khaki cloth is attached using button and hole system. The filtering portion is made of monofilament nylon material as described earlier and is followed by again a non-filtering portion of khaki cloth. To the latter, a metal net bucket provided with a stop cock is tied with a strong twine. Instead of stop cock, a net bucket of a GI sheet cylinder which is open at both ends may also be attached to the caudal khaki strip. In that case, one end of the cylinder is inserted into the tail end of the non-filtering portion and tied and the other end of the cylinder is covered by tying with a piece of filtering cloth, the quality of which is similar to that of filtering portion.

It is reported that variations in the volume filtered in a series of oblique tows is usually much less than that it is in vertical tows. The net is towed from a boat for 10-15 minutes using three suspension units of bridle which are tied to the welded holes of the net ring located equidistance from each other. It is essential to maintain the speed of the boat at 5 nautical miles/hr.

Cutting methods for stitching plankton nets:

The monofilament nylon material is generally very costly and further it is not easily available. Hence while stitching the plankton nets, the filtering has to be cut in such a way that no wastage occurs. To achieve this, the following three ways have to be adopted.