116 
BULLETIN OF THE BUREAU OF FISHERIES 
Boreal Arctic. 
Chsetoceros atlanticum. 
boreale. 
criophilum. 
decipiens. 
Oceanic 
Temperate. 
Nitzschia seriata. 
Rhizosolenia hebetata (semispina), 
Thalassiothrix longissima. 
Chaetoceros densum. 
peruvianum. 
willei. 
Rhizosolenia alata f. genuina. 
f. gracillima. 
Tropical 
Thalassiothrix frauenfeldii. 
Chsetoceros coarctatum. 
Antarctic 
peruvianum. 
Corethron valdivia;. 
In 1922 the summer swarm was composed almost entirely of members of the 
genus Rhizosolenia (figs. 15 and 16). Rhizosolenia semispina and R. shrubsolei 
appeared about June 15, followed in July by R. setigera. The latter two species 
were never as numerous as the former. R. semispina increased rapidly until July 5, 
when the swarms literally filled the waters of the bay and sound, clogging even the 
coarsest plankton nets with a slimy brown ooze. Shortly after this it began to 
decline, disappearing about September 9. The 1923 maximum was very similar, 
except that the two minor species terminated their season earlier than in the previous 
year, while R. semispina declined more slowly, remaining in small numbers through- 
out the fall and early winter. 
The occurrence of this species at Woods Hole during the summer months is 
rather interesting. It is a northern oceanic form, known from both the Arctic and 
Antarctic regions, and was found by Ostenfeld (1913) to thrive best in the areas 
of the North Atlantic where cold currents seek southward. It is particularly abun- 
dant in the spring in the region of the Labrador Current about Newfoundland and 
Nova Scotia. Bigelow (1917), in July, 1914, found a large maximum off Marthas 
Vineyard at the time when the great swarms appear in local waters. In 1923 the 
author found them extending from Cape Cod along the eastern side of Marthas 
Vineyard to Nantucket and in Vineyard Sound as far as Menemsha Bight. None 
were found at the western end of the Sound or in the waters about No Man 's Land. 
This indicates that they enter the region from the northeast, as would be expected. 
Miss Ogilvie (1923) found a maximum in July, 1920, off the south coast of Ireland. 
The summer maximxma at Woods Hole, then, is not wholly dependent upon 
local conditions. Either of the two neritic species, Rhizosolenia shrubsolei and R. 
setigera might dominate if hydrographical conditions prevented the appearance of 
R. semispina. The abundance of the latter species will depend partly on the num- 
bers blown into the bay and sound and partly upon the food material present there. 
Although it is a northern form it must have an extremely broad temperature range, 
because its distribution in Buzzards Bay in 1923 showed conclusively that great 
production was taking place there at a time when the temperature was ranging 
from 19 to 
21° C. 
