96 



BULLETIN OF THE BUREAU OP FISHERIES 



in July, 1916 (station 10342, at least 4.5 cubic centimeters per cubic meter); but 

 occasionally it is much more dense than this at one level or other, the volumes just 

 listed being the minima possible. For example, a horizontal haul of 15 minutes' 

 duration at 40 meters depth, with a net 1 meter in diameter, off Cape Cod on July 

 22, 1916 (station 10344), yielded over 6 liters, mostly copepods, which is equivalent 

 to about 12 cubic centimeters per cubic meter for the water fished through (the tow 

 covered about one-third of a mile). In fact, it was the richest tow-net catch we have 

 ever made in the gulf, although the vertical haul indicated only about 2.8 cubic 

 centimeters of plankton per cubic meter. 



ANNUAL VARIATIONS IN ABUNDANCE 



Annual variations in the amount of zooplankton living in the waters of the 

 gulf will mirror the long-time fluctuations in its physical state — may, indeed, be 

 the best clue to such — and exert an important influence on the growth, local repro- 

 duction, and distribution of the adults of such important plankton-feeding fishes 

 as herring, mackerel, and pollock. 



It is certain that considerable fluctuations of this sort in the plankton do take 

 place from year to year, as illustrated by the following table of the volumes per 

 square meter of sea surface for corresponding localities in the summers of 1913-14 

 and the first week of September, 1915. 45 



' July hauls. 



' A few miles west of the corresponding stations, 1912 to 1914. 



* From horizontal hauls. 



According to these measurements the volume of the plankton was greater in 

 1913 than in 1914 at all but two stations. As between 1913 and 1915, however, 

 one year was the richer at some, the other at other localities. However, since the 

 average is practically the same (or at least did not differ as widely as the probable 

 error) for the three years, there was apparently no important general change in the 

 amount of plankton existent in the gulf from 1913 to 1915, though both these years 

 were apparently decidedly more productive, on the whole, than was 1912 during 

 the corresponding months (Bigelow, 1915, p. 337). During the summer of 1916 

 (a year of low temperatures) the waters off Massachusetts Bay proved more produc- 



« Although different types of nets were used during these years, the results, reduced to the common standard, will allow 

 a rough and ready comparison. 



