270 



BULLETIN OF THE BUREAU OF FISHERIES 



Here, again, the destructive effects of storms (p. 268) are evidenced by the 

 small returns from trips 4 and 6. The increase through December and January, 

 reaching its peak in late February about the time of the seventh cruise (February 

 24-25), and then the decline until overcome by the invasion from the east, corre- 

 sponds very well in the two tables. In each the influx became evident on cruise 11. 



In comparing these two tables it will be noticed that, although the number of 

 eggs taken in Massachusetts Bay on the twelfth cruise (April 21) was more than 

 four times that taken on any of the other cruises, the actual number of eggs in cleavage 

 stages on the Plymouth grounds had declined to approximately one-half that of the 

 trip in the previous month. The influx of eggs from the outer waters was supple- 

 mented to some extent, no doubt, by locally spawned eggs even on April 21, but by 

 May 20-22 local spawning had virtually ceased. This point is very well brought 

 out also by a comparison of Figures 8 and 11. The former, based on the 

 results of trips 9 to 12, shows the quantities of newly spawned eggs in the outer 

 waters but at the same time indicates clearly that considerable production was 

 still taking place on the Plymouth grounds. Figure 11, giving the quantitative 

 distribution of eggs a month later, shows that by May 20, 1925, production on 

 these grounds had virtually ceased, and the large numbers recorded for this cruise 

 (Table 5) are seen to be concentrated in the outer parts of the bay and along the 

 northern side. 



The large number of eggs entering from the east was probably the result partly of 

 contributions from the Ipswich grounds and partly of haddock eggs drifting in from 

 the northern part of Stellwagen Banks (favorite spawning grounds for that species) 

 or from the waters east of Cape Ann; for, as already stated, haddock do not spawn 

 in abundance in the inner parts of the bay and few, if any, on the grounds off 

 Plymouth. (Bigelow and Welsh, 1925.) 



According to Bigelow and Welsh (1925), the height of the cod spawning season 

 at Plymouth is reached during the period of falling temperature, the bulk of the 

 eggs being liberated before the winter minimum is reached. In 1924-25 the height 

 of the season apparently was reached in February and extended through the coldest 

 part of the year, when the mean surface temperature ranged from 2.54° to 0.75° C. 

 In this it agreed with previous observations at Ipswich, where the greatest produc- 

 tion is known to take place during the period of minimum temperatures. 



DRIFT AS INDICATED BY THE EGGS 



The determination of spawning areas by locating the centers of distribution of 

 eggs in early cleavage and then following the general movement from these centers 

 by charting the distribution of eggs in progressively advanced stages of development 

 is not new. In 1914 Hjort reported that cod eggs in early development had been 

 found to be restricted to very limited areas and suggested the possibility that these 

 eggs might serve as a basis for determining the position of the spawning shoals. 

 As early as 1900, however, this same author stated that in European waters cod fry 

 often are carried for hundreds of miles, being distributed over the banks off the 

 northern coast of Norway and especially in the Barents Sea, where they may be 

 taken the following year in fine-meshed nets on the bottom. 



