BEHAVIOUR 



dispersion, but on the other hand in some places they seem to be essential in 

 keeping the plankton in the same place. For example, during the colder 

 weather the water surrounding an island will cool and so sink, taking the 

 plankton with it, but at night the plankton will rise to the surface again and 

 be returned to the waters converging on the island. 



Light intensity is, however, not the whole story in diurnal migrations, 

 for sometimes Calaiiiis can be found at bright noon in calm weather danc- 

 ing at the actual surface. Experiments have shown that at other times a 

 few hours of bright sunshine is lethal; although sometimes Cnhtnis re- 

 covered after four hours' exposure, they never did after eight ! 



What is an optimum light intensity for one organism is not necessarily 

 the best for others, so that as the plankton in the upper layers moves to- 

 wards the surface at night it is partly replaced by an upward movement of 

 deeper and darker-loving species, while the faster swimmers from quite 

 deep water — such as lantern fish, squids and euphausids — will swarm at 

 the actual surface. The phenomenon of diurnal migration appears in very 

 deep water also. These layers are not easy to sample in detail with closing 

 nets but the behaviour pattern can be quite readilv seen on the echo-sounder. 



The echo-sounder (Plate XL) is an instrument originally designed for 

 navigational purposes, its main function being to hnd and record the depth 

 of water under the ship. It works by sending down a sound mipulse which 

 is then echoed back from the sea floor and the echo received is electrically 

 amplified on board the ship. As the time taken for the sound impulse to 

 go to the sea floor and back is in direct relationship to the depth a very 

 accurate measurement of the depth is obtained. The electrical impulse can 

 be made to mark a slowly moving paper band so that a continuous picture 

 of the bottom is obtained. If the instrument is made sensitive enough, 

 echoes will be received from any obstruction between the ship and the 

 bottom, such as a shoal of fish. Whether or not an echo is received wiU 

 depend on the sensitivity of the instrument, and on the size and texture 

 of the obstruction. Fish with air bladders give very effective echoes, but 

 jellyfish and many other plankton organisms have a composition so nearly 

 the same as sea water that they are not nearly such good sound reflectc^rs. 

 Some of the larger plankton organisms like squids or the hard-shelled 

 prawns can give a good echo and so can very small planktonic fish if they 

 have small swim-bladders. 



The echo-sounder, then, can be used to watch diurnal migration of 

 those planktonic organisms capable of reflecting enough sound to record 

 on the sensitive paper, and we can thus stud)' their movements at sea under 

 natural conditions, instead of in the laboratory, and follow differences such as 

 on moonlight and dark nights. The echo-sounder revealed, too, that there 



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