PLANKTON OF THE GULF OF MAINE 217 



Massachusetts Bay, and makes the hypothesis that the latter is actually a center 

 of local production worthy of consideration. 



The two eastern centers are indicative of immigration, being continuous with the 

 more abundant occurrence of the species to the eastward along the outer coast of 

 Nova Scotia. More direct evidence that the comparatively rich gathering made 

 off the slope of Georges Bank on March 12, 1920 (station 20069), was such a wave 

 from the northeast is the fact that it was no more plentiful there a month later 

 (station 20109), though at most localities its numbers had about doubled in the interim 



(p. 214). 



It is, furthermore, entirely consistent with the probable flow of the currents in 

 this region in spring that there should be a drift of C. hyperboreus from northeast to 

 southwest along the continental edge and perhaps over the southern edge of Georges 

 Bank during March and April, continuing into June in some years, but the evidence at 

 hand suggests that few pass west of longitude 70° at any season. The large catches 

 in the northern channel and the eastern basin in March, April, and May, contrasted 

 with the scarcity of the species at all our Browns Bank stations irrespective of season, 

 point to the former as the chief route by which C. hyperboreus enters the Gulf of 

 Maine. If the data for the two years, 1915 and 1920, can fairly be combined, it 

 would seem that there is comparatively little movement in this direction before the 

 end of April; but with C. hyperboreus relatively much more plentiful in the northern 

 channel on March 20 (station 20078), and again on April 15 (station 20105), than 

 in the neighboring parts of the gulf, invasion only awaited the first considerable 

 movement of water westward past Cape Sable, which occurs by the first half of 

 May, for the richest catch of the species yet recorded in the gulf was made in the 

 eastern side of the basin on May 6 (station 10270) . I8 A comparatively large catch 

 (about twice the average for the month) in this general region six weeks later (station 

 10288, June 19) may have been evidence of continued immigration throughout May 

 and into June. 



There is nothing in the records of the distribution of the species for summer or 

 autumn to suggest that C. hyperboreus rounds Cape Sable in appreciable numbers 

 later in the summer; but to find it doing so would not be surprising, for the stock 

 existing along the outer Nova Scotian coast during the warm months fluctuates so 

 widely from year to year that Esterly did not detect it at all in the Grampus tows 

 between the cape and Halifax during the last week of July and first week of August 

 in 1914, whereas Willey (1919) records a moderate representation of the species 

 over the shelf generally nearly to the cape in August, 1915. Willey (1921) has 

 explained the presence of C. hyperboreus at St. Andrews in the winter of 1917 as an 

 invasion. 



Vertical distribution. — C. hyperboreus occurs to some extent on the surface in the 

 gulf in spring (table, p. 303), but more regularly deeper down, appearing in the lists 

 for about 80 per cent of all the vertical hauls during this period but in only about 50 

 per cent of the surface hauls, though the latter filtered much larger volumes of water. 

 Counting all the stations at which surface hauls were made in 1920 (table, p. 303), 



'• Doctor Wilson's analysis of the catch made in the vertical net at this station proves my earlier statement (Bigelow 1917, p- 

 292) that C. hyperborcui was rare or absent in this general region at the time, to have been incorrect. 



