Fig. 37. Diagrammatic representation of the diurnal vertical migration of three common 

 species in the plankton; a copepod, a jellyfish and a fairy shrimp. After Russell. 



at 50 feet per hour and sink at three times that speed. The larger euphausid 

 Mcgmiyctiphaucs can chmb at 300 feet per hour, and sink at 420 feet per hour, 

 though in short two-minute bursts it dives at speeds of 700 feet per hour. 



Another reason for diurnal migration, to be considered with light inten- 

 sity, is that ot self defence. Animals migrate to the surface at night in order 

 to feed, but as daylight approaches they sink into dark regions so as to avoid 

 being seen by predators such as fish which are dependent on sight. Herring, 

 for example, a plankton feeding fish, cannot see to feed at depths below about 

 100 metres. Associated with this, the zooplankton species which have daily 

 migrations tend to have a longer life span, one to three years as against one 

 to three months, and lay fewer eggs than those which do not migrate. To 

 produce large numbers of eggs in a short time requires a corresponding 

 increase of food intake and the non-migratory species get this by staying in 

 the region of maximum food. The survival of the race is ensured on the one 

 hand by evasion of capture, and on the other by excess egg production. 



Migratory species which are drifted by the currents to shallow waters 

 where they cannot sink into darkness during the day help to form the food 

 of cod and haddock and other fish usually less dependent on plankton than 

 are the pelagic fish such as herring. 



Migrations of this sort mean that herbivores like Calaiitis are to be found 

 during the night up near the surface amongst their food, the phytoplankton, 

 and they sink during the day where they release the products of metabolism 

 to enrich the deeper layers (p. 124). As the currents at the surface and 

 below will not be identical, the animals will not be in quite the same place 

 when they ascend the next night. Such movements are usually helpful in 



156 



