PLANKTON OF THE GULF OF MAINE 139 



end of Georges Bank during these months certainly was not accidental, for we made 

 two traverses of the bank four weeks apart, and it was equally wanting at our several 

 stations on the western end of the bank on May 17, a month when we have previously 

 found it widespread in the inner parts of the gulf. 



It will require more than the one year's data to prove whether this vernal con- 

 traction of the range of Th. inermis on the offshore side, which must be followed by a 

 corresponding expansion in June to repopulate these waters to the extent that 

 obtains in midsummer, is an annual occurrence. 



We have yet to learn how far the maintenance of the local stock of Th. inermis 

 in the Gulf of Maine depends on the reproduction which takes place there and how 

 far on immigration around Cape Sable from the colder waters of the Nova Scotian 

 current, no attempt having yet been made to trace the life history of this shrimp in 

 the gulf. It is probable that Th. inermis breeds successfully at least as far west as 

 Cape Cod, and that it is represented among the considerable numbers of larval 

 euphausiids which we have taken there side by side with medium-sized specimens 

 and large adults of this species. 



Thysanoessa inermis has never been found in abundance at the surface in any 

 part of the gulf except at Eastport, though it has often occurred in small numbers 

 in the catches of the surface nets. On the other hand, our deepest hauls in the gulf 

 have never yielded many, and the largest catches have all been in nets working at 40 

 to 80 meters depth. Thus it tends to congregate at about the same level as Calanus 

 and is not associated with the Euchseta community of the deep basins, as its relative 

 Aleganyctiphanes norvegica so often is. 



I can offer no data bearing on the actual numerical strength of Th. inermis in 

 the gulf, nor could much dependence be placed on the results of vertical hauls in the 

 case of so active an animal unless with larger nets than we have used. Our largest 

 catches of it have been made near Cape Ann (August 22, 1914, station 10253), on the 

 eastern end of Georges Bank (July 23, 1914, station 10223), near Cape Sable (August 

 11, 1914, station 10243), and off Marthas Vineyard (August 25, 1914, station 10259). 



Tliysanoessa longleaudata (Kr0yer) 74 



This species, as Kramp (1913) and Holt and Tattersall (1905) have pointed out, 

 is generally distributed in Arctic Seas and in the northern part of the Atlantic, 

 ranging south to the west coast of Ireland and northern North Sea in European 

 waters. On the whole, it is more northern and more oceanic in its affinities than 

 Th. inermis, but, like the latter, the records for it in the Gulf of Maine are so widely 

 distributed that it is to be expected anywhere in the offshore parts of the latter in 

 summer (fig. 50), late winter, and early spring. Only three times in all our 

 experience, however, have we detected it in the coastal zone inside the 100-meter 

 contour at any season, and never in inclosed bays or estuaries. 



Thysanoessa longicaudata is far less numerous in the gulf than its relative Th. 

 inermis, and occurs there far less regularly, having been detected at fewer than 25 

 per cent of our summer stations (fig. 50), and then usually in small numbers; nor 



" For the occurrence of this species in 1912 to 1916 see Bigelow, 1914a, 1917, and 1922. In the spring of 1920 it was taken at 

 Stations 20045, 20046, 20054, 20057, 20060, 20064, 20065, 20066, 20069, 20070, 20073, 20075, 20076, 20077, 20079, 20080, 20086, 20087, 20100, 

 20101, 20107, 20112, 20116, and 20129. It was also taken in December, 1920, and January, 1921, at stations 10490, 10494, and 10502. 



