62 BULLETIN OF THE BUREAU OF FISHERIES 



The copepod Oalanus liyperboreus affords a second example of an Arctic immi- 

 grant that finds an environment in the gulf favorable for the growth of the indi- 

 vidual and to some extent for reproduction. Its recorded occurrence in the Gulf of 

 Maine illustrates the care with which such data must be analyzed before general 

 conclusions can be drawn from them, for if its Arctic nature were not well estab- 

 lished, the fact that there is a center of abundance for it in the western side of the 

 gulf and a second in the eastern might easily lead one to assume a totally erroneous 

 faunal status for it. In reality it is probable that its comparative abundance off 

 Massachusetts Bay is the result of a certain amount of local reproduction, though 

 replenishment of the stock depends directly on immigration via the Nova Scotian 

 current, as emphasized hereafter (p. 215). 



The routes by which C. liyperboreus enters the gulf are discussed in the general 

 account of the species. Once past Cape Sable they spread so generally over the 

 gulf that it is impossible to trace their further drift from the actual locality records, 

 probably because the large ody adults, on which most of our records have been based, 

 live long enough to become dispersed far and wide, as well as because of the local 

 production just mentioned. 



OTHER IMMIGRANTS 



The indraft of water through the eastern channel and over the neighboring 

 parts of the banks is not only fairly constant in its physical characters but carries 

 with it various planktonic animals as characteristic of this source as those previously 

 discussed are of an Arctic or Tropic origin. They include in their ranks, however, 

 perfectly successful colonists, which, consequently, are also regularly endemic in 

 the gulf (for example, the mammoth copepod Euchasta and the amphipod genus 

 Euthemisto), as well as species that evidently find the gulf a less favorable environ- 

 ment than the Salter and heavier mixed water, as evidenced by their comparative 

 scarcity near shore and the smaller size attained there at sexual maturity. Others, 

 too, are included, which are unable to breed at all in the gulf, though they may live 

 there for some time, in which respect they correspond to S. serratodentata, of the Tropic 

 group, and to L. helicina, of the Arctic category. 



The influx of this mixed water into the gulf being more or less continuous through- 

 out the year, either via the two channels, Northern and Eastern, or across Georges 

 Bank, the mechanical agency for replenishing the stock of visitors from this source 

 is always available, their life histories and chiefly their seasons of reproduction 

 determining whether they are in evidence in the gulf at any given season of the year. 



As I have pointed out, Tropic and Arctic visitors are brought into the gulf 

 chiefly in the superficial water stratum, but the whole column of water down to the 

 bottom of the deepest trough of the eastern channel serves as a medium for the dis- 

 persal of the immigrants entering with the mixed water, the precise "sailing routes" 

 (to borrow a nautical term) followed by its inhabitants depending upon the courses 

 of the inflowing water at the different levels at which they live. For the most in- 

 structive animal index to the movements of the surface layers of the mixed water, 

 because the most abundant and conspicuous, we need only refer back to Sagitta 

 serratodentata (p. 58) ; for, although this cha^tognath primarily originates in the Gulf 



