PLANKTON OF THE GULF OF MAINE 
249 
on the second occasion with 100 specimens in a total of only 400 copepods of all 
kinds. Willey (1919) also records it much more often from vertical than from 
surface hauls in the Gulf of St. Lawrence and off Nova Scotia. 
I can offer no data on its presence or absence at the surface in the Gulf of 
Maine during the summer months; hut Willey’s (1919) tables, which show that a 
larger proportion of the records of it obtained by the Canadian Arctic expedition 
were from the surface in May and June than in July and August, suggest that 
it tends to sink down into cooler strata as the seasonal warming of the top of 
the water progresses. 
The vertical distribution of this species in other seas makes it probable that it 
ranges right down to the deepest levels in the Gulf of Maine, but the data are not 
sufficient to show whether it tends to gather at any particular level or is more evenly 
and indifferently distributed vertically. 
When the locality records for M. Tonga are plotted (fig. 75) it is evident that 
in the years when it is most plentiful in the gulf it becomes generally distributed 
over the entire area of the latter, indifferently in the peripheral zone, in the central 
basin, and over the offshore banks as far west as Marthas Vineyard. It should be 
noted that the absence of summer and autumn records on Georges and Brown’s 
Banks, and in the southeastern part of the gulf generally, is actually not a contra- 
diction, because there were no, or at least very few, M. Tonga in the gulf during 
1914, the year when we made our chief midsummer cruise in this region. The 
apparent predominance of records in the western side of the gulf is equally deceptive, 
due simply to the fact that we have worked more there than elsewhere. 
Immigration and breeding. — The periodic appearances and disappearances of 
M. Tonga in the Gulf of Maine, coupled with its Arctic nature in general, identify 
it as primarily an immigrant to the gulf from the north, depending on frequent 
accessions from more prolific centers to maintain the local stock. But the fact that, 
unlike most of the immigrant species, it is not localized in the eastern side and around 
the peripheral belt of the gulf is evidence either that the visiting specimens come 
in such abundance and live so long that they spread universally over the entire 
extent of the latter before they perish, or that they succeed in breeding within the 
gulf to an extent sufficient for the dispersal of the resulting generations to hide 
the routes of entrance of their parents. In this connection it is instructive to find 
the distribution of M. Tonga paralleling the spring status of Catanus Tiyperboreus, 
a species similarly of northern affinities but for which a certain amount of local 
reproduction within the gulf seems sufficiently demonstrated. 
The locations of the stations (fig. 76) where more M. Tonga have been taken 
than the average numbers per square meter for their respective months (in which 
respect M. Tonga closely parallels CaTanus Tiyperboreus ) are further evidence of this. 
In spring and early summer (the season when the influx of northern water is at its 
height, and when consequently the greatest invasions of M. Tonga are to be expected) 
two distinct lines of immigration are suggested by the rich catches — the one inward 
into the eastern side of the gulf via the northern and eastern channels, and the other 
westward along the continental edge of Georges Bank. The rich spring catches made 
in the western side of the gulf in 1920 might have been the result either of local 
