206 



BULLETIN OF THE BUBEATJ OF FISHEBIES 



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 jinmarchicus 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 : 



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 '. jinmarchicus 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. jinmarchicus 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 half 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. jinmarchicus were identified 

 among a rich catch of young copepods off Gloucester (station 20124). e 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). 



