FISHES OF THE GULF OF MAINE 



187 



a 75-pounder. But the average production of eggs 

 is perhaps not more than 1,000,000 for the general 

 run of Gulf of Maine fish. 



The eggs are buoyant, transparent, without oil 

 globule, and 1.10 to 1.82 mm. in diameter. Gulf of 

 Maine eggs, artificially fertilized and measured 

 by Welsh, averaged about 1.46 mm. in diameter, 

 but the size varies somewhat with the temperature 

 of the water, being larger in cold than in warm. 79 



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° F., in 14 or 15 days at 43° F., in 20 to 23 

 days at 38° to 39° F. and not for 40 days or more 

 if the water is as cold as 32° F. Fertilization can 

 take place and development commence in tem- 

 peratures even lower than this, as proved by ex- 

 periments by Krogh and Johansen. 77 But their 

 observation that the mortality is great among eggs 

 incubated at 32° F. (although full development can 

 take place) corroborates the experience of the 



11 Fish (Bull. U. S. Bur. Fish., vol. 43, 1929, p. 292) found cod eggs taken in 

 the tow net in Massachusetts Bay to average about 1.63 mm. in February, 

 smaller (1.46 to 1.49 mm.) in December and in May. 



" Dannevig, Canadian Fisheries Exped. (1914-1915)., 1919, p. 44. 



Figure 87. — Egg. After Heincke and Ehrenbaum. 



Figure 89. — Larva, 4.5 mm. After Schmidt. 



hatcheries, where it has proved impossible to hatch 

 more than 25 to 50 percent of the eggs in water as 

 cold as that. And the relative strength of the 

 larvae that are hatched at different temperatures 

 points to 41° to 47° F. as most favorable for in- 

 cubation. 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 to be found any- 

 where in the open sea, p. 195), but by its effect on 

 the developing eggs. And it is interesting that cod 

 in the tank at Woods Hole produced eggs in Febru- 

 ary, when the water may have cooled to 30° F. 

 (and quite normally to judge from the fact that the 

 eggs incubated successfully in the warmer water of 

 the hatchery), for these same fish would have 

 spawned naturally in temperatures at least as high 

 as 36°-38° F. if they had been left at liberty. 



Newly spawned cod eggs are indistinguishable 

 from those of the haddock, with which they inter- 

 grade in size. But shortly before hatching, the 

 pigment of the cod gathers in 4 or 5 distinct 

 patches: one over the region of the pectoral fins, 

 one above the vent, and the others equally spaced 

 behind the latter (fig. 87) ; whereas in the haddock 



Figure 88.- — Larva, just hatched, 4 mm. After Masterman. 



Figure 90. — Larva, 9 mm. After Schmidt. 



Figure 91. — Fry, 20 mm. After Schmidt. Figure 92.— Young, 40 mm. After Schmidt. 



Cod {Gadus callarias), developmental stages, European. 



