PLANKTON OF THE GULF OF MAINE 



197 



about 1,000 per cubic meter, with 2,300 and 3,700 at two stations at the mouth of 

 Massachusetts Bay, these being among the densest aggregations of the species yet 

 demonstrable from our vertical hauls. 



In marked contrast to this rich region and to a second center of abundance in 

 the eastern basin (1,900 per cubic meter), there was a sparse stock of Calanus along 

 the coast of Maine east of Penobscot Bay (40 to 100 per cubic meter) in June, and 

 it was only moderately abundant on Browns Bank (120 per cubic meter, station 

 10296). 



In the cool year of 1916, when it is probable that the vernal cycle in the lives 

 of planktonic animals lagged behind its normal schedule, Calanus was extremely 

 plentiful in the Massachusetts Bay region and off Cape Cod in July, as already 

 described (p. 195); and while the numbers per square meter fell somewhat short of 

 the maximum for May, the numbers per cubic meter — both maximum and average — 

 were slightly greater because of the shoalness of the localities where the vertical 

 hauls were made. 



Numbers of copepods and Calanus finmarchicus per cubic meter, assuming the latter to average 70 per 



cent of the former, July 1.9 to S2, 1916 ' 



1 The exact proportions of the several species of copepods have not been determined as yet for these hauls, but preliminary 

 examination suggests at least 70 per cent Calanus and probably more. 



The copepod population being confined largely to the deeper layers, as evidenced 

 by the comparative poverty of the surface catches, Calanus finmarchicus was evidently 

 more densely aggregated locally than even these amounts per cubic meter would 

 suggest. For example, the haul at 40 meters (station 10344), with the 1-meter net, 

 yielded about 6 liters in 15 minutes, chiefly copepods, and contained upward of 

 2,500,000 large Calanus (Bigelow, 1922, p. 136). This compares favorably with 

 200,000 in a five-minute haul near Iceland, listed by Paulsen (1906) as one of his 

 richest. 



In the daytime the stock of Calanus at, say, the 10 to 30 meter level, becomes 

 to some extent enriched by the tendency of this little crustacean to sink when the 

 sun is high; at night it is correspondingly impoverished. 



The July hauls for 1916 represent the richest Calanus pasture for mackerel, 

 herring, etc., that has come to our notice, and hence may be regarded as containing 

 about the maximum number per cubic meter to be expected in any part of the gulf 

 at any season, except in years for some reason unusually productive. When and 

 where this crustacean food supply is at its best, therefore, a plankton-feeding fish 

 finds at least 2,000 Calanus per cubic meter at some level, and probably many more 

 at others, for this copepod has often been reported in shoals. On such occasions 

 every few mouthfuls of water taken by an adult mackerel, herring, alewife, or shad 



