BIOLOGY OF THE ATLANTIC MACKEREL 165 



as far as the intensity of fishing is concerned, there was no significant change between 

 1931 and 1932. The fleet numbered 112 seiners in 1931 (Fiedler, 1932, p. 211) and 

 114 in 1932 (Fiedler, Manning, and Johnson 1934, p. 97). 



Views may differ as to the relative part played by catch mortality and by natural 

 mortality in causing total mortality, but by taking divergent views, say three-quarters 

 catch mortality on the one hand and one-quarter catch mortality on the other hand, 

 one would arrive at 15 and 5 percent, respectively, as catch mortality; or, taking a 

 middle ground, it would be 10 percent. Similarly divergent views may be taken as to 

 the fraction of annual mortality suffered during the spawning season. Perhaps three- 

 quarters and one-quarter, respectively, may reasonably be taken as the extremes and 

 one-half (or 10 percent) as the middle ground. These would give as extremes 11 and 

 1.25 percent that the catch during the spawning season was of the total spawning 

 6tock. The middle view would be 5 percent. 



This results in an estimated total population between 45,000,000 and 400,000,000, 

 with a middle ground estimate at 100,000,000 individuals in the spawning population 

 on the spawning grounds as derived from catch statistics. 



It will now be recalled that the estimate derived from tow net hauls was 64,000 

 billion eggs spawned, and if 400,000 eggs are produced by the average female (p. 156) 

 the indicated spawning population would be 160,000,000 females, or 320,000,000 fish 

 of both sexes. This is within the extremes computed from the catch data and about 

 halfway between the middle and largest figures. Considering the approximate 

 nature of some of the elements in the estimates, this is a remarkable agreement be- 

 tween the two methods of computing the size of the spawning stock, and strengthens 

 the view that the total estimate of eggs is siifficieuth reliable to warrant the conclusion 

 that the egg production was in the order of 50,000 billion in 1932. 



This, of course, refers only to the spawning in the region south of Cape Cud, 



and it has been pointed out (p. 160) that important spawning occurs also in the Gulf of 



St. Lawrence. Since spawning in the latter region seemed to be of lesser magnitude 



than south of Cape Cod, it is probable that the entire spawning oil' the east coast of 



North America would not be more than double the estimated 64,000 billion, or, 



since the latter is an uncertain figure, let us say in the order of one hundred thousand 



billion eggs. 



SPAWNING HABITS 



According to Bigelow and Welsh (1925, p. 208), "Mackerel spawn chielly at 

 night." If this be true, the earliest egg stages should be relatively more abundant at 

 certain times of the day than at others. From material collected at a number of 

 stations in 1929, the eggs in "early cleavage" and "late cleavage" were counted, 

 representing respectively the first and second 10 hours of development at the tem- 

 peratures prevailing at the time. If spawning took place chiefly at night the early 

 cleavage eggs should predominate between midnight and 10 a. m. and be in the mi- 

 nority during the remainder of the day. At the 14 stations from each of which more 

 than 10 eggs of both stages were examined, the average percentage of early cleavage 

 in the midnight to 10 a. in. group was 45 and in the 10 a. m. to midnight group 33. 

 The difference between the two groups was not statistically significant(f=0.91 and 

 P=0.3 +, according to the method of fisher, 1932, p. 114) and it may be concluded 

 that the diurnal variation in percentage of early stage eggs does not indicate a tendency 

 toward more spawning by night than by day. Tabulation of percentages according 

 to the hours of the day did not indicate that any other particular part of the day was 

 favored. 



