256 BULLETIN OF THE BUREAU OF FISHERIES 



it, than anywhere else in the gulf. Its occurrence has been nearly as universal over 

 the whole eastern half of the basin and in the southern part right across to Cape 

 Cod (recorded at 75 to 80 per cent of all the stations) , but it has been decidedly less 

 regular in the northwestern part of the basin generally (about 63 per cent of the 

 stations) , and the percentage of occurrences has been much lower in the deep trough 

 off Cape Ann than anywhere else. The trough between Jeffreys Ledge and the 

 Isles of Shoals, however, seems a definite center of abundance for it. On the whole, 

 M. lucens occurs rather less regularly over the coastwise belt out to the 100-meter 

 contour (about 59 per cent of the stations) than in deeper water (about 72 per cent 

 of all the stations in the basin) . 



In the richer region outlined on this chart no seasonal variation is apparent in 

 the regularity of occurrence of the species for the periods June to October and Decem- 

 ber to May, the number of occurrences being the same (28) and the number of 

 stations at which M. lucens was not detected as nearly equal (6 and 3) as could be 

 expected with the constant possibility that one net will pick up and another miss 

 any particular animal unless it is present in abundance and uniformly distributed. 



In the coastwise belt and the northwestern part of the basin it occurs somewhat 

 more regularly during the winter and spring, when it has been detected at about 

 66 per cent of the stations for which the copepods have been listed by Doctor Esterly 

 and Doctor Wilson, than in summer and autumn, when it figured in only about 45 

 per cent. M. lucens has proved similarly but more definitely seasonal on Georges 

 Bank and over the continental shelf off Marthas Vineyard, having been taken at 

 all the late winter and spring stations of 1920 but at only 30 per cent of the summer 

 and autumn stations; as pointed out in the foregoing regional analysis, this also 

 applies to the waters outside the continental shelf as far offshore as our lines have 

 extended. 



When the stations where M. lucens was more plentiful than the average for the 

 month are plotted (fig. 78), a definite regional separation can be drawn between the 

 northeastern part of the gulf, where it has been found in relatively large numbers 

 on several occasions in August, September, and October but never in the spring, 

 and the southeastern and southern parts of the area generally, including Georges 

 Bank and its offshore slope and the eastern and northern channels, where rich catches 

 of Metridia have been made in February, March, and April but never from May to 

 October. In the coastwise belt in the western side of the gulf there are "rich" 

 stations both for spring and for summer-autumn. 



Seasonal variations in the actual numerical strength of the stock of M. lucens 

 in the gulf can only be stated in a tentative way until more extensive data have 

 been gathered, because the annual fluctuations in its abundance introduce a source 

 of error of unknown magnitude into calculations based on a combination of the 

 data for different years; and unfortunately the only year when vertical hauls were 

 taken at frequent intervals from spring until autumn (1915) was one in which this 

 copepod occurred less regularly than it sometimes does. Furthermore, M. lucens, 

 like most other copepods, has proved decidedly "streaky" in its distribution. This 

 phenomenon was illustrated off Gloucester on May 4, 1920, when, with the Albatross 

 lying at anchor, a vertical haul at 3 p. m. (station 20120) yielded this species at the 



