FISHES OF THE GULF OF MAINE 429 



The period of incubation for cod eggs depends on temperature. According to 

 experience at the hatcheries hatching may be expected in 10 or 11 days at 47°, 

 in 14 or 15 days at 43°, in 20 to 23 days at 38° to 39°, and not for 40 days or more 

 if the water is as cold as 32°. Fertilization can take place and development com- 

 mence in temperatures even lower than this, as proved by experiments by Krogh 

 and Johansen,'" but their observation that the mortality is great among eggs incu- 

 bated at 32° (although full development can take place) corroborates the experience 

 of the hatcheries, where it has proved impossible to hatch more than 25 to 50 per cent 

 of the eggs in water as cold as this, while the relative strength of the larva? hatched 

 at different tempei'atures points to 41°to47° as most favorable for incubation. All 

 this suggests that extreme cold prevents the successful reproduction of the cod, not 

 by interfering with spawning (for this can take place in the lowest temperatures 

 found anywhere in the open sea) but by its effect on the developing eggs. 



Newly spawned cod eggs are indistinguishable from those of the haddock, 

 with which they intergrade in size, but by the time the embryo is as long as the 

 circimiference of the egg (that is, shortly before hatching) the pigment of the cod 

 gathers in 4 or 5 distinct patches — one over the region of the pectoral fin, one above 

 the vent, and the others equall}- spaced behind it (fig. 205) — whereas in the haddock 

 the pigment cells are arranged in a row along the ventral side of the trunk (p. 444). 

 There is also danger of confusing newly spawned cod eggs with those of the witch 

 floimder (p. 515), which they overlap in size; but the black pigment of the cod eggs 

 identifies them as gadoid as soon as it appears, the embryonic pigment of the 

 witch being yellow. (See also under the haddock on p. 443.) 



At hatching the larvae are about 4 mm. long with the vent (which is close 

 ' behind the yolk sac) located at the base of the ventral fin fold on one side instead 

 of at its margin, so that the intestine apparently ends bUndly as is the case with 

 haddock and pollock larvae, also. At this stage young cod much resemble the latter 

 but are easily separable from them by the fact that the pigment is in two dorsal 

 and three (rarely two) ventral bars, with the dorsal bars shorter than the ventral 

 bars opposite them, whereas in pollock larvae up to 10 mm. long the dorsal bars 

 are longer than the opposite ventral bars (p. 405). Neither is there any danger 

 of confusing cod larvae with haddock even at this early stage, for the latter are 

 not barred but have a continuous row of pigment cells along the ventral margin 

 of the trunk behind the vent besides other patches on the nape and in the lining 

 of the abdomen (p. 444). 



When first hatched the young cod float helplessly, yolk uppermost, but they 

 assume the normal position in about 2 days, the yolk being absorbed and the mouth 

 formed in 6 to 12 days, according to temperature, when the larvae are about 4.5 mm. 

 long. As the little cod grows the pigment bars gradually fuse, and at 8 to 10 mm. 

 a median band forms, but cod 10 to 20 mm. long may easily be distinguished from 

 pollock by the fact that the pigment extends to the tail, whereas in the latter it 



» Dannevig. Canadian Fisheries Expedition, 1914-15 (1919), p. 44. 



