166 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



THE EGG 



Description.- — According to published descriptions, (Ehrenbaum, 1921, p. 4 for 

 the European mackerel ;Dannevig, 1919, p. 11, and Bigelow and Welsh, 1925, p. 208, for 

 the American mackerel) the mackerel egg is 0.97 to 1.38 mm. in diameter and contains 

 an oil globule 0.28 to 0.35 mm. in diameter. Measurements of eggs taken at sea 

 during this investigation had a similar range in dimensions. By far the commonest 

 dimension (modal) was 1.2 to 1.3 mm. for the egg and 0.31 to 0.32 mm. for the oil 

 globule. 



There is a tendency toward a decrease in size of mackerel eggs as the season ad- 

 vances. Data given by Ehrenbaum (1921, p. 4) show the same tendency in the egg 

 of the European mackerel. This could be due to the seasonal trends of either tem- 

 perature or salinity, but the experiments of Fish (1928, pp. 291-292), who found cod 

 eggs fertilized in cold water to be larger than those fertilized in warm water, suggest 

 that temperature alone could be responsible. Whatever its mechanism, the phenom- 

 enon of decrease in size as the season advances probably holds true for all species 

 occuring in the tows of the present investigation. It was my practice to make scatter 

 diagrams in which oil globule diameter was plotted against egg diameters for all eggs 

 in hauls containing troublesome mixtures. Invariably, when mackerel eggs were 

 near the limits of the over-all range of their dimensions and thus might be expected 

 to overlap the range of the eggs of other species, the latter were also near the cor- 

 responding limits of theh respective over-all range and the groups remained discrete, 

 showing that tendencies for smaller or larger than average size were shared simul- 

 taneously by all species. Thus, in individual collections the range in dimensions was 

 much less than the relatively large range of all collections, and a feature that might 

 have been a hindrance in identification was in reality not very troublesome. 



In the collections made during the course of this investigation there were eggs of 

 four species whose dimensions approached those of the mackerel. The egg of the 

 common bonito (Sarda sarda) is 1.15 to 1.33 mm. in diameter, but in its early stages 

 it has a cluster of small oil droplets instead of a single large one. In its late stages, 

 these droplets often become united into a single oil globule. In this condition there 

 might be some difficulty in distinguishing the two, were it not that bonito eggs occur 

 later in the season (hi areas we have prospected) when the mackerel eggs are consid- 

 erably smaller. For instance: Mackerel eggs taken in Cape Cod Bay, July 19, 1929, 

 were 1.00 to 1.12 mm. in diameter while bonito eggs taken July 25, 1929, in the offing 

 of No Man's Land were 1 .12 to 1 .27 mm. in diameter. The eggs of the cusk (Brosmius 

 brosme) and the tilefish (Lopholatilus chamaehonticeps) are similar in size but have oil 

 globules distinctly smaller (0.19 to 0.23 mm.) than those in the mackerel's eggs. 

 Closer to the mackerel egg hi its dimensions was that of a species not yet identified. 

 Although overlapping the mackerel egg in dimensions, its modal size was distinctly 

 smaller and the oil globule somewhat larger, and in its late stages the embryonic 

 pigment was arranged in bars unlike the diffuse arrangement in the embryo of the 

 mackerel. Inasmuch as eggs of this type were found only at the edge of the con- 

 tinental shelf, their distribution was discontinuous with that of the mackerel; and 

 since no mackerel larvae were later found in the same or neighboring localities this 

 egg caused no confusion. 



Bate oj embryonic development. — Although mackerel have never been observed in 

 the act of spawning, it is generally supposed that both eggs and sperm are discharged 

 into the surrounding water, where fertilization takes place. Observations have shown 



