128 THE SEAS 



off our coasts it is necessary in the daytime to fish at a depth 

 of ten to fifteen fathoms in order to get the largest catches. 

 In the open ocean the depths at which the greatest assem- 

 blages of plankton animals are found are generally consider- 

 ably deeper. Now, as has been said, this is the case in the 

 daytime ; but it is a remarkable fact that at night matters 

 are quite different, and a curious change comes over the 

 plankton distribution. On the approach of dusk, just after 

 the sun has set, all the animals begin to swim in an upward 

 direction, so that by about nine or ten o'clock, in the summer 

 months, the surface layers, so barren in the daytime, have 

 been enriched by animals that have swum up from the 

 deeper levels. In Plate 49 is shown the actual results of 

 collecting at different depths in daylight and at dusk. It 

 can be clearly seen that whereas in the daytime the layers 

 down to about five fathoms are very poor in plankton 

 compared with the deeper levels, at dusk the two upper 

 collections are quite as large as the deeper ones. It will be 

 noticed that the bottom catch at dusk is also greater than 

 that taken in the daytime, this is because of the addition of 

 large quantities of animals that have come up from deeper 

 levels quite near the sea bottom ; in fact, there are certain 

 small creatures that live actually on the bottom itself in the 

 daytime which move upwards towards the surface at 

 night ; they may never actually have time to reach the 

 surface itself, because at dawn all the animals begin to move 

 downwards again to take up their abodes at their usual day 

 levels (Plate 48) . We all know that the herring fishermen only 

 shoot their drift nets at night. This is because at night, the 

 herring, like the plankton animals, also come to the surface. 



This phenomenon of upward movement at night, or 

 vertical migration, throws considerable light on why the 

 different animals prefer certain depths in the daytime. 



It seems almost certain that the causes of the up and 



