FECUNDITY OF XORTH AMERICAN SALMONIDAE 



459 



fry better survival chance.s. A selection pressure ii' 

 favour of large eggs therefore certainly exists and this 

 selection must work until the eggs are so few that no note- 

 worthy competition for food exists among the fry. 



While we must agree that larger fry generally 

 have better survival rates, the reason given by 

 Svardson — intraspecifie competition — may some- 

 times liave little bearing on the matter; un- 

 doubtedly, there are other important factors. For 

 instance, Robertson (1922) has pointed out that 

 the race of small-sized sockeye salmon that spawns 

 in Harrison Rapids, a tributary of the Fraser 

 River, produces larger eggs than the other races of 

 sockeye in the Fraser. This may be related to 

 the fact that this is one of the few races of sockej^e 



in which the young go to sea as fry, since large, 

 vigorous fry would be required to survive in 

 sufficient numbers to maintain the population. 



It should be noted, moreover, that among the 

 Pacific salmons (table 2) the smallest eggs are 

 found in tlie sockeye which normally spend the 

 longest time in fresh water. Size can be attained 

 only by the sacrifice of number. In each ecolog- 

 ical situation there is some point at which, on 

 the average, the forces favoring size are exactly 

 balanced by those favoring number. This point 

 must vary between river systems, tending to 

 produce genetic variation between populations for 

 egg size and number. 



Table 2. — Size and weight oj eggs and fry of certain North American Salmonidae 

 [Asterisk (•) indicates diameter calculated from volumetric measure by Von Bayer conversion table] 



