BIOLOGY OF THE ATLANTIC MACKEREL 



193 



taken on the first 4 cruises, the treatment includes the population resulting from the 

 major portion, perhaps 70 percent, of the season's spawning. It of course ignores 

 the fate of the fewer eggs spawned prior to or later than the first four cruises, but the 

 neglected portion is probably so small that it is unlikely that the survival of the whole 

 season's brood of young differs from that of the treated portion. It could do so only 

 if the mortality of the neglected portion differed widely from the included portion. 

 There appears to be no reason for believing that there was any such wide difference. 

 On the contrary, examination of the relative numbers of the various stages and sizes 

 caught on those cruises which included a part of the history of the neglected portions 

 suggests that these had a survival rate similar to that of the included portion. 



Having the average relative numbers of each category of egg and larva from this 

 selected series (table 7, column headed "Average per cruise") there remained the 

 necessity of adjusting the numbers to compensate for the differences in the duration of 



EGG 



ABC 



T 



LARVAL LENGTHS 

 5 6 7 8 9 10 



MILLIMETERS. 



15 20 



25 



30 



T 



40 



nr 



50 



T 



- 100,000 



10 ZO 30 40 50 



ASE. DAYS. 



Figure 17.— Survival of young stages of mackerel In 1932. Solid dots represent the means of three or four cruises each. Open 

 circles represent the less reliable values based on only one cruise. The heavy lines represent a simple interpretation of survival 

 rates, and the fine lines, a more complex alternative interpretation. Solid lines are fitted to the solid dots by the method of 

 least squares. Lines of dashes connect their ends, and the line of dots and dashes is an extrapolation. 



time represented by each egg stage and each larval-length class. The stages or classes 

 representing a long period of development would be passed slowly and the catches of 

 such a category would represent a larger accumulation of individuals than a category 

 representing a shorter period of development. Since the accumulation would be 

 directly proportional to the duration of the category, the true relative values were 

 obtained by dividing the numbers of individuals in each category by the number of 

 days required to pass through that category, according to the schedule, given in the 

 column headed "Duration of category" of table 7. This, in effect, reduces the data 

 to represent what the relative numbers would have been had it been possible to sub- 

 divide the material into categories that occupied uniform time intervals — in this 



