216 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



and in which they develop. On the Massa- 

 chusetts Bay grounds breeding commences when 

 the whole column of water has cooled to about 47° 

 to 49°, and is at its climax (late in December) 

 in temperatures of 40° to 43°, while the major 

 production of eggs takes place long before the 

 water has cooled to its winter minimum of 35° 

 to 36° F. at the level at which the fish lie. Thus 

 the pollock spawns on a falling temperature, with 

 most of the eggs produced within a comparatively 

 narrow range and in water several degrees warmer 

 than that in which haddock spawn most actively 

 (p. 208). This agrees closely with the European 

 pollock which spawns only in temperatures near 

 44.5°, so far as is known. 



As the successful propagation of any fish 

 depends as much upon the incubation of its eggs 

 as on its spawning, we should note that incubation 

 proceeds normally, and that the resultant larvae 

 are strong and active over the whole range of 

 temperature just outlined, that is, from about 

 38° to about 48° as proved by experience in the 

 Gloucester hatchery. 



The Massachusetts Bay spawning takes place 

 in salinities ranging from as low as 32 per mille 

 to as high as 32.8 per mille, according to precise 

 locality, depth, and season, salinities much lower 

 than those in which pollock breed on the other 

 side of the Atlantic (35.14 to 35.26 per mille). 



The number of eggs produced by a female pol- 

 lock averages about 225,000, but more than 

 4 million eggs were reported in one fish of 23 % 

 pounds. The egg is buoyant, has no oil globule, 

 and averages about 1.15 mm. in diameter. Thus 

 it is decidedly smaller than the egg of the cod 

 or of the haddock. Incubation occupies 9 days 

 at a temperature of 43°; 6 days at 49°. 



The larvae are about 3.4 to 3.8 mm. long at 

 hatching, slender, with large yolk sac, and with 

 the vent situated on one side of the body at the 

 base of the ventral fin fold as it is in other larval 

 gadoids; they are sprinkled with black pigment 

 cells. About 5 days' time is required for the 

 entire absorption of the yolk sac and for the 

 formation of the mouth; meantime the pigment 

 of the post-anal section of the trunk becomes 

 grouped in longitudinal bars, two dorsal and two 

 ventral, the former longer than the latter. At 

 this stage pollock closely resemble cod of the 

 same size, but the ventral bars are longer than 

 the dorsal bars opposite them in the cod, and 



usually three in number in the cod instead of 

 two as in the pollock. These bars persist until 

 the pollock grows to a length of about 15 mm., 

 when the pigment becomes more scattered. The 

 caudal fin rays appear at about 9 mm., all the 

 dorsal and anal rays and the ventral fin rays 

 at about 15 mm., the dorsal fins are separate from 

 one another and also the anal fins at 20 mm. (at 

 about 2 months), and fry of 25 to 30 mm. show 

 most of the characters of the adult. 



In European seas the young pollock lives 

 near the surface for its first 3 months. The 

 young fry have been taken similarly, in the tow 

 nets near the surface at Woods Hole from Jan- 

 uary to May, and they are to be expected in 

 Massachusetts Bay then, though we have no 

 actual record of them there. 



Rate of growth. — Thanks to the shortness of its 

 breeding season and to the readiness with which its 

 scales can be "read" European students 7 have 

 found it easy to trace the rate of growth of these 

 "saithe" or "coal fish"; and this has been done for 

 the American pollock by Mavor, 8 also by us. 

 Judging from scale studies and from the sizes of 

 the fry that are caught near Woods Hole in the 

 spring, pollock hatched in mid-winter are about 

 1 to 2 inches long by the following spring, growing 

 to 3-5 inches by late summer; to 5-7 inches in 

 their first winter, when a year old; to 12-13 inches 

 at two years of age; to 17-18 inches at three years; 

 to 21-22 inches at four years; to an average of 

 about 23 inches at 4% years; of 25 inches at 5% 

 years; and of about 27 inches at 6X years. Thus 

 the 11-12 inch and 15-16 inch pollock that appear 

 in such numbers along the New England coast 

 late in summer are about 1% and 2% years old, 

 respectively. The annual rate of growth thus is 

 about 5 or 6 inches for the first three years, 2-4 

 inches for the next three years, and 1-2 inches for 

 the next few years, after which they grow still 

 more slowly. 



These sizes are somewhat larger than the aver- 

 ages given by Damas for European fish of corre- 

 sponding ages, but the difference is so small that 

 it is safe to apply the European figures to older 

 Gulf of Maine fish, for which we have no data. 

 On this basis we may expect the American pollock 

 to average about 28 inches at 7% years; about 29 



» For resume 1 see Damas (Rapp. et Proc. Verb., Consell Perm. Internat. 

 Explor. Mer, vol. 10, No. 8, 1900, p. 167). 

 • Contr. Canad. Biol., (1917-1918) 1918, No. 6. 



