PLANKTON OF THE GULF OF MAINE 



263 



spring (e. g., stations 20056, 20058, 20060, 20061, 20077, 20081, and 20083, March, 

 1920) makes it unlikely that any temperature that may be experienced in the open 

 gulf is fatally cold for this species, though it may not be able to survive the subzero 

 temperatures of ice-laden seas. On the other hand, one of the records of it on the 

 surface of the western basin (station 10256, August 23, 1914) was from nearly as high 

 a temperature (19.56°) as it has ever been found in, although it could have reached 

 decidedly cooler water by sinking a few meters. Most of the records of this copepod 

 in the gulf have been from temperatures between 4 and 15°, but, like Esterly (1912), 

 I have found it impossible to correlate its regional and seasonal variations in 

 abundance with changing temperature. Nor is it likely that its distribution within 

 the gulf is governed by local differences in salinity, the whole of that body of water 

 being well within the limits within which M. lucens occurs commonly elsewhere. 



Economic importance. — While no definite observations seem to have been made 

 on the extent to which H. lucens is eaten by plankton-feeding fishes, it is generally 

 assumed to be an important article in the diet of the mackerel in Irish waters. No 

 doubt mackerel, all the herring tribe, and the other copepod eaters consume it to 

 some extent in the Gulf of Maine, but it averages such a small numerical percentage 

 of the catch of copepods compared with the dominating swarms of Calanus fin- 

 marchicus, which its adults about equal in size, that it can vie with the latter in 

 economic importance only when local shoals gather. 



Average percentage of Metridia lucens, by months, in the total catches of copepods 



Hauls 



March, 1920, verticals 



April, 1920, verticals 



May, 1915 and 1920, verticals 



June. 1915, verticals 



August, 1913 and 1915 



Per- 

 centage 



Hauls 



September, 1915 



October, 1915 



December, 1920, horizontals 

 January, 1921, horizontals.. 



Per- 

 centage 



4 

 12 



On three occasions in October, 1915 (stations 10327, 10328, and 10329), M. 

 lucens, forming 25 to 30 per cent of a moderately abundant copepod community 

 (table, p. 298) and about equaling Calanus, would have offered an attractive pasture 

 for the schooling fishes. This was also the case off Gloucester on May 4, 1920 

 (M. lucens constituted 30 per cent at station 20120). In every other instance, 

 however, when we have found it forming 25 per cent or more of the copepods the 

 total catch of all kinds has been extremely scanty. 



Monstrilla serricornis Sars 



G. O. Sars described this species in 1921 from two male specimens taken off 

 the west coast of Norway. Occasional specimens from four surface hauls in the 

 Gulf of Maine in March and April, 1920 (table, p. 303), are the second record of its 

 occurrence; but these four, including Browns Bank, the northeastern part of Georges 

 Bank, the neighborhood of Lurcher Shoal, and Mount Desert Island, indicate that 

 it is to be expected anywhere in the gulf. It is the only representative of its family 

 yet reported there. 



