THE CASPIAN SEA 605 



migration of Eurytemora grimmi in the Caspian Sea. This crustacean never 

 forms a maximum in the surface layer. It starts its upward movement from 

 the depths in the afternoon. At midnight it begins to sink again. This is clearly 

 pronounced in the early morning hours, and by 8 o'clock in the morning 

 almost all the Eurytemora grimmi are already at a depth of 50 to 83 m, where 

 they remain till their next ascent. The nature of the changes is shown in 

 Table 252, which includes the data for all the stages (in number of specimens 



Table 252 



per 1 m 3 ). No significant changes in the process of migration for different 

 ages have been recorded for E. grimmi, but such changes were noted for the 

 Northern Sea Calanus finmarchicus by A. Nicholls (1953). The rate of upward 

 movement of these small Copepoda is about 2 cm/sec (72 m/h) ; their descent 

 is almost as rapid. 



Borgoov has given an interesting estimation of the biological significance 

 of the'vertical migration of E. grimmi. An average of 7 mg/m 3 of living matter 

 is transferred during one day. It is understandable that the feeding significance 

 of plankton in different layers changes sharply in connection with these 

 movements. A certain layer can contain very different amounts of food- 

 forms at various times of the day. Bogorov has established the feeding value 

 of a given layer (the product of mean biomass by the number of hours, corres- 

 ponding to the given state of the biomass) and the feeding intensity of a given 

 layer (quotient of mean biomass divided by the number of hours for a given 

 biomass). For E. grimmi, one of the most important food-plankton of the 

 Caspian Sea, these values are given in Table 253. 



Table 253 



Depth Feeding Feeding 



m value intensity 



The 10 to 25 m and 50 to 83 m layers have the highest feeding value, while 

 the highest feeding intensity is found in the to 10 m and 50 to 83 m layers, 

 since a huge number of organisms is gathered there for a short time. Using 



