POPULATION DENSITY IN FISH SPECIES 273 



Table IV. The change of the egg weight oflenok in the prespawning period 

 {stage IV) in the Amur River 



Size group of fish 

 (cm) 



f ^ s 



40 — 50 — 60 — 70 — 80 



Average weight of egg (mg) . 30-0 52-6 62-1 59-9 



and in old spawning fish the fat percentage in the eggs is lower (Cheprakova, 

 i960) (Table V). 



Age changes of sperm quality (and its changes in various portions of milt) 

 are less well known. In Soviet literature the only data known to me were 

 obtained by the workers of the institute of Pond Fishery and concern the 

 sperm quality of roach at the beginning and end of spawning. These workers 

 have shown that the last portions of sperm contain less spermatozoa and 

 possess smaller fertilizing capacity. 



Table V. The amount of fat in the eggs of roach of different age {the mouth of the 

 Volga, 1957; after Cheprakova, i960) 



Age 

 (years) 



J L _______^ 



3+ 4+ 5+ 6 + 



Percentage of fat from dry weight of eggs . 6-8 8-97 8-88 8-03 



In connection with the above one can understand the adaptive importance 

 of the character of the approach to the spawning grounds by fish and the 

 order of spawning. The fish of older and middle ages are first to come to 

 spawn in most species; it is they which possess roe of high quality, and their 

 eggs are fertilized by the first portions of the sperm. 



If there are many large-sized females, males spend most of the high quahty 

 sperm to fertilize their eggs. Younger females, which approach the spawning 

 grounds later, are fertilized by the sperm of lower quahty and the percentage 

 of eggs fertilized from younger females often becomes lower. If the number 

 of older females is less, successful fertilization of younger females becomes 

 increased. Thus, individuals of younger age-groups, spawning for the first 

 time, play the role of a reserve by providing stock recruitment when the 

 population density of older fish is reduced. The problem of adaptive sig- 

 nificance of the process of spawning has been little studied so far; however, 

 it is of great importance for a rational fishery, particularly for planning the 

 fishery regulation necessary during the spawning season. 



