206 
BULLETIN OP THE BUREAU OF FISHERIES 
In the San Diego region Esterly (1911) took it in abundance as deep as 400 to 500 
meters, to which depth diurnal migration was effective. 
Physical factors offer no apparent explanation for the comparative scarcity of 
Calanus in the deepest water of the gulf as compared with the intermediate levels, 
both temperature and salinity being well within the optimum for it; and it is more 
likely that the cause lies in the distribution of the food supply, Calanus tending to 
congregate at the levels where the microscopic plants on which it feeds are most 
abundant. 
Reproduction . — It is now well known that Calanus finmarchicus deposits its eggs 
singly in the water, where they float until the young copepod hatches in the “nau- 
plius” stage. Being of characteristic appearance (Damas, 1905), Calanus eggs are 
easily recognized in the plankton. The larval stages are distinguishable by the 
number of thoracic and abdominal segments and developed legs, as well as by their 
size. The stages are described by Lebour (1916). Damas’s (1905) notation of 
them, now generally adopted, is as follows: 
Stage 
Thoracic 
segments 
Abdom- 
inal 
segments 
Fully 
devel- 
oped legs 
I - 
2 
2 
2 
II- 
3 
2 
3 
Ill 
4 
2 
4 
IV 
5 
3 
5 
V 
5 
4 
5 
VI, adult female 
5 
4 
5 
VI, adult male 
5 
5 
5 
The proportionate numbers in which the different stages in development have 
occurred in the many samples, American and European, which have now been 
studied by various authors, indicate that C. finmarchicus passes most of its existence 
in the late postlarval stages, living only for a short time as an adult, to perish shortly 
after breeding; but much is yet to be learned of its breeding habits in detail. 
Only a few scattered observations have been made on the occurrence of eggs 
or juveniles of C. finmarchicus in the Gulf of Maine; enough, however, to show that 
it is regularly endemic there and that the local stock is chiefly the product of repro- 
duction in the Gulf, though more or less recruited by immigration from colder waters 
to the east and north. 
As previously remarked (p. 194), swarms of copepod nauplii and young copepods 
appeared off Gloucester during the first week of May, 1915, a decided increase in 
juvenile Calanus took place in the neighborhood of the Isles of Shoals during the 
first fyalf of the month, and there were great numbers of young Calanus in Mas- 
sachusetts Bay off Magnolia on the 17th. In 1920, again, copepod nauplii newly 
hatched swarmed in the surface waters of the bay on May 4 (stations 20120 and 
20121, fig. 27 and 28), and on the 16th juveniles of C. finmarchicus were identified 
among a rich catch of young copepods off Gloucester (station 20124). 6 The fact 
that the Calanus that swarmed off Cape Ann on May 4, 1915 (p. 297), were mostly 
in the younger, intermediate stages of growth is sufficient evidence that a production 
6 These juvenile stages were taken chiefly on the surface and in some abundance in the vertical hauls as well (see table, p. 297) . 
