National Marine Week

From the 29th July – 13th August we celebrate national marine week, a time to discover and embrace our connection and relationship with the sea.

Our collections are full of maritime related treasures and so we’re focusing on an object that appears rather mysterious and unusual but that would have been a very helpful tool to our fishermen out on the seas.

And here it is……..

Plankton Indicator

(Above: GRYEH: 2006.65)

It most certainly qualifies as a ‘mystery object’ at first glance and one could hazard many guesses as to its purpose. The central section is spherical in shape with a curved fin at its top; at one end of the object is a triangular shaped tail. The opposite end has a circular filter for collecting; this tool was developed in the late 1920s from a research instrument that was used to study communities underwater in relation to distribution of another community.

For those of us who haven’t seen one before, we can reveal that it is a plankton indicator. Used by fishermen in the 1930’s, it would have helped to reveal large shoals of herring so that drifter vessels could find the best spots. If a fisherman was able to detect plankton in a certain area, he would have better chance of catching nearby herring.

Scientist, Professor Alister Hardy invented the ‘Hardy Plankton Indicator’ after finding in his research that, generally, good catches of herring were associated with areas dominated by plankton.


(Above: Professor Hardy, who at one time was a Naturalist at the Ministry of Agriculture and Fisheries laboratory in Lowestoft.)

Much like the one in our collection (pictured above), indicators were metal tube like in form and it was this that towed behind the vessel, filling up with water. After towing the plankton indicator for a few minutes the silk filtering disc would retain enough plankton to determine whether zooplankton (brown) or phytoplankton (green) was dominant. If the water turned red or pink, it showed the presence of plankton which often meant herring would be close by.

After the Second World War came, the advent of the more reliable echo sounder – a more direct means of determining the presence of herring shoals replaced indicators like this. However, it was also known for fishermen to use their own skill and intuition to detect the best areas for their catch.

Unknown Example of an echo sounder


(Above: Technology take-over as fishermen and sailors use digital devices to pick up signals and waves).


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