PLANKTON OF THE GULF OF MAINE 201 



summer, consequent on fluctuations in its actual abundance, and in the abundance 

 of the other species of copepods. Twenty-one vertical hauls at as many stations 

 for September and October, 1915, give an average of only 38 per cent and 42 per 

 cent of Calanus, respectively— that is, little more than half of the May percentage. 



The percentage of Calanus averaged somewhat higher in the horizontal hauls 

 of December, 1920 (about 58 per cent; table, p. 304). However, this does not reflect 

 an increase in the actual abundance of the species (which, on the contrary, decreases 

 markedly in numbers during the late autumn and early winter), but a still more 

 pronounced decrease in the local stock of other species of copepods. Thus, while 

 curves for the actual and for the relative abundance (percentage) of C. finmarchicus 

 would be similar for the spring, they would be contradictory for the September- 

 December quarter, and to this extent the percentages taken by themselves would 

 give a totally false picture of the seasonal fluctuations of the species in the Gulf of 

 Maine. 



From the economic standpoint this means that any copepod-eating fish in the 

 Gulf of Maine is likely to make Calanus its chief diet from May until August and 

 in October, but to depend less on it and more on other copepo ds during the early autumn 

 and again in late winter and early spring. 



The average percentages need further qualification to bring out the great 

 irregularity in the relative abundance of the species which we have encountered 

 from station to station on most of the cruises and from month to month at individual 

 stations, irregularities connected with the streaky way in which C. finmarchicus 

 often occurs, and with the formation and dissipation of its shoals. In Massachusetts 

 Bay, for example, the percentage fell from 80 to 45 at one locality off Gloucester 

 between March 1 and April 9, 1920, but increased from 6 per cent to 50 per cent 

 off Boston Harbor, near by, during approximately the same interval. In the western 

 basin, at three successive stations, the percentage of C. finmarchicus was 90 on 

 February 23, 25 on March 24, 75 on April IS, and at three stations along a line run- 

 ning out from Ipswich Bay toward Platts Bank, on April 9 and 10, the percentages 

 were alternately 75, 25, and SO. Seventy-five per cent of Calanus in the south- 

 western part of the basin on February 23, but only 2 per cent on the neighboring 

 part of Georges Bank the same day, was evidence of a corresponding difference in 

 the actual number of C. finmarchicus per square meter — respectively, 6,562 and 

 25 — but on the southeastern slope of the bank the percentage fell only from an 

 average of about 75 per cent on March 12 to 60 per cent on April 15, although this 

 interval saw the dissipation of a very dense swarm of Calanus, occasioning a shrinking 

 in the number per square meter from 103,000 to about 600. 



Apart from the question of vertical stratification (p. 24), the percentages of 

 Calanus have proved more nearly uniform over considerable areas in the later 

 spring and summer. In early May, 1920, for example, it constituted 60 to 80 per 

 cent of the copepods at most of the stations in the southwestern part of the gulf 

 and on the western end of Georges Bank (table, p. 302). In July and August, 1914, 

 its percentages in the horizontal hauls at most of the stations inside the continental 

 edge approximated the average (71 per cent) for all the stations, irrespective of 

 regional variations in the actual abundance of the species. In September and the 



