BIOLOGY OF THE ATLANTIC MACKEREL 203 



Mortality rates. — When the logarithms of the fully adjusted survival numbers are 

 plotted, as in figure 17, the series describes Dearly straight lines over certain portions 

 of its extent, indicating that in each of these straight-line segments, mortality must 

 have proceeded at a uniform percental rate. The major feature to be noted is the 

 break at about 35 days when the larvae are 10 mm. long. At this point there is a 

 change of level and of slope which may be considered as dividing the curve into three 

 portions: (1) egg, yolk-sac, and larval stages, (2) transition between larval and post- 

 larval stages, and (3) post-larval stage. Each will be discussed separately. 



The first portion representing stages up to 10 mm. in length is subject to alterna- 

 tive interpretations due to the nearly, but not wholly, linear arrangement of points. 

 The simplest interpretation is that the mortality rate was uniform and that the devia- 

 tions from linearity were due to defective sampling. If so, the single heavy straight 

 line drawn from A to 9 mm. in figure 17 expresses the mortality. Accordingly, this 

 mortality was at a constant rate, and amounted to 14 percent per day. On the other 

 hand, it has been shown in the previous section that there is little ground for sus- 

 pecting serious defects in sampling, and also that the slope of the line A to C differs 

 significantly from that of the line 4 to 8 mm. This being true, the mortality rate 

 would be better described by the three fine lines of figure 17, the one extending from 

 A- to C-stage eggs; another from 4- to 8-mm. larvae; and still another joining their 

 ends across the 3-mm. (yolk-sac) stage. According to this interpretation, the initial 

 rate, i. e., the rate during the egg stage, was 5 percent per day. The next rate, i. e., 

 during the yolk-sac stage, was 23 percent per day, and the third rate, i. e., during the 

 larval stage, was 12 percent per day. 23 However, according to both interpretations, 

 mortality has reduced the population to about one-tenth of its original numbers by 

 the time the larvae reach 4 mm. long, and when they attain 9 mm. in length at 35 

 days of age, to one-thirtieth of the original number. 



If any one period is to be singled out as the most critical, it must be the ensuing 

 period during the transition from larval to post-larval stages, when in passing from 

 9 to 11 mm., the numbers are reduced by 90 percent in the short space of about 3 

 days. The rate of mortality may be variously computed, depending on the choice 

 of straight lines in figure 17. The lowest is 30 percent, and the highest, 44 percent 

 per day. Either of these rates is distinctly higher than the highest alternative esti- 

 mate (23 percent per day) in the yolk-sac stage. The high mortality during this short 

 period, coupled with the losses previous to this stage, reduced the survivors to only 

 one three-hundredth of their original numbers; thus the population was already 

 severely decimated on entering the post-larval stage. 



During the post-larval stage, the rate of mortality apparently was more mod- 

 erate than in earlier stages. The data on which the rates are based appear fairly 

 reliable up to the 22-mm. stage, or 62 days of age, and the fitted line for the segment 

 11 to 22 mm. in figure 17 represents a mortality of slightly over 10 percent per day. 

 Beyond 22 mm. the catches of larvae were few and were confined to only one cruise, 

 so that the reliability of their relative numbers is in doubt; but the evidence, such 

 as it is, points towards the continuation of the same rate of mortality to the size of 

 50 mm., or age of 85 days. 



Restating the history of mortality, it appears that there was a general basic 

 rate of 10 to 14 percent mortality per day throughout the period studied. The most 

 important deviation from this general rate was during the 9- to 11-mm. stage, when 

 the population suffered about 30 to 45 percent mortality per day. Other deviations 



" Also, according to this interpretation, the data In the last column of table 7 should be taken as representing the number of 

 survivors per 840,000 newly spawned eggs Instead of per million, as given In the column beading. 



