404 



BULLETIN OF THE BUREAU OF FISHERIES 



fish, or fishermen's reports, that any pollock eggs are produced in the deep basin 

 of the Gulf. In European waters, however, this fish is described as breeding 

 only in depths greater than 75 fathoms, a difference difficult to account for- 

 Although the pollock is not a ground fish at other seasons, the gill-netters describe 

 it as spawning on hard bottom. 



The brief duration of the breeding season and the fact that the vertical tem- 

 perature gradient then covers a range no greater than 3° to 5° down to 50 fathoms, 

 makes it easy to establish the physical conditions under which the eggs are spawned 

 and in which they develop. On the Massachusetts Bay ground breeding com- 

 mences 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°, with the major 

 production of eggs taking place long before the water cools to its winter minimum 

 of 35° to 36° 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. 442). This agrees closely with the European pollock which, so far as 

 known, spawns only in temperatures closely approximating 44.5°. 



The Massachusetts Bay spawning takes place in water as fresh as 32 permille 

 and as saline 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 At- 

 lantic (35.14 to 35.26 permille), a difference obtaining for almost allspeciesof fish 

 that spawn both in the Gulf of Maine and in north European seas. 



As the successful propagation of any fish depends as much upon the incubation 

 of its eggs as on its spawning, we should note that hatchery experience proves that 

 incubation proceeds normally and with the resultant larvae apparently strong and 

 active over the whole range of temperature just outlined — that is, from about 38° 

 to about 48°. This fact is evidence that regional variations of temperature are 

 not the factor that localizes the breeding pollock in the southwestern part of the 

 Gulf and prevents it from spawning on the north shore of the Bay of Fundy, for the 

 temperature of Massachusetts Bay differs by only a couple of degrees from that of 

 Passamaquoddy Bay at the commencement of the breeding season. While the 

 coastal water as a whole is cooler east than west of Cape Elizabeth at the height of 

 the spawning period, the differences from station to station have been small, and 

 all the readings we have taken during late December and early January have fallen 

 well within the extremes between which pollock spawn freely in Massachusetts Bay, 

 as appears in the following table. This applies equally to salinity. 



Water temperatures, Massachusetts Bay to Lurcher Shoal, 1920-1921 



