DISC 



of 



silk mcsl 



Fig. 5. The 'Hardy' Plankton Indicator, as used to sanipk" plankton by fishing vessels working 



on the hcrrint; cirounds. 



Hardy of Oxford) started an occanographic department whose main function 

 was to sample the plankton over as broad an area as possible using commercial 

 vessels, either fishing vessels or the larger ships used on the normal steam- 

 ship lines. Two different types of apparatus were developed. The first of 

 these, called the Plankton Indicator, is a high speed net reduced to its simplest 

 terms (Fig. 5), being merely a metal tube with a constricted opening, a 

 fixed diving plane instead of a weight or detachable depressor, and stabilizing 

 fins. The net is merely a disc of 60 mesh silk attached to a ring placed inside 

 near the tail. It was designed for easy handing aboard herring fishing vessels, 

 and to be towed when the skipper was near the grounds chosen for his 

 night's fishing. If the disc showed plenty of herring food his chances of a 

 good catch of herring were increased, as discussed in more detail in Chapter 

 ID. With the modern development of the echo sounder a more positive 

 indication of the presence of herring is given and the 'Indicator' has gone 



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