BIOLOGY OF THE ATLANTIC MACKEREL 167 



that thereafter, during the period of embryonic development, 6 the eggs are suspended 

 in the sea water mostly near the surface and all above the thermocline. 



As is true with most cold-blooded organisms the rate of development depends on 

 the temperature at which it takes place, being slower at low temperatures and faster 

 at high temperatures. According to Worley (1933), who examined this feature of the 

 development at the U. S. Fisheries Biological Station, Woods Hole, Mass., the time 

 elapsing between fertilization and hatching was 50 hours at 21°, 70 hours at 18°, 

 95 hours at 16°, 115 hours at 14°, 150 hours at 12°, and 208 hours at 10°. There is 

 no reason for believing that the rates differ at sea, though this is difficult to demon- 

 strate. 



According to Worley (1933, p. 857), "Experiment showed that typical develop- 

 ment (and survival) could be realized only between 11° and 21°." At sea in 1932, 

 however, eggs were most abundant at temperatures below 11°, as appears from the 

 following average numbers taken at each degree (centigrade) of surface temperature 

 encountered in the survey: 



7 - 



8 HI 



9 2, 117 



10... - 3,360 



U 2,432 



12 - 1,390 



13 1,380 



14 150 



15 555 



16 44 



17 5 



18 74 



19 



20.__- 



The embryos in eggs from water below 11° C. differed in no perceptible way from 

 those found in warmer water, and there is no reason for believing that development 

 was not proceeding as "normally" at the lower as at the high temperatures. 



Worley also found (loc. cit.) that "The total mortality during the incubation 

 period was least at 16° C. where it amounted to 43 percent." He had three experi- 

 ments at this temperature with mortalities of 37, 40, and 53 percent respectively 

 (loc. cit. p. 847). At sea, in 1932, the average mortality was 59 percent (from inter- 

 polation to the hatching point from the data of the 5th column in table 7), or only a 

 little greater than in the least favorable of the laboratory experiments. The weighted 

 mean temperature of the water from which these sea-caught eggs were taken was 

 10.9° C. Worley's laboratory eggs suffered 90 and 95 percent mortality in his two 

 experiments at 11°. 



Obviously, both the range for normal development and the point of maximum 

 survival were at lower temperatures at sea than in the laboratory experiments of 

 Worley. The explanation for this disparity between results in the laboratory and 

 observations at sea probably lies in the fact thai Worley's experiments took place at 

 a time when temperatures of the sea water from which he took his fish were in the 

 neighborhood of 10° C. The lesser mortality at and near this temperature was 

 connected no doubt with the lesser change involved in bringing the eggs from the 

 temperature of the parent to the temperature of the experiment. It is obviously de- 

 sirable that laboratory experiments be repeated on material taken from water of 

 lower temperature. 



Vertical distribution. — Although it has been known that mackerel eggs are sus- 

 pended in the sea, usually near the surface, there has been in American waters no 

 previous determination of vertical distribution, apart from the general observation 



« For the minutiae of the embryology of mackerel, the reader is referred to Moore (1S95, pp. 5-14), and to Wilson's (1891) desr-rip 

 tion of the sea bass, which the mackerel in its embryology closely resembles. 



