90 



BULLETIN OF THE BUREAU OF FISHERIES 

 Volumes of plankton per square meter, western basin 



Feb. 23,1920. 

 Mar. 24, 1920. 

 Apr. 18,1920. 

 May 5, 1915 



Cubic centi' 



meters of 



zooplank- 



ton per 



square 



meter 



175 

 95 



150± 

 250 



Date 



Cubic centi- 

 meters of 

 zooplank- 

 ton per 

 square 

 meter 



June 26, 1915. 



July 15, 1912. 



Aug. 22,1914 



Aug. 31,1915 



250 

 65 

 200 

 165 



There is, likewise, less fluctuation with the seasons on the western part of Georges 

 Bank than on the eastern. The largest volume of plankton per square meter yet 

 recorded for the Gulf of Maine was 425 cubic centimeters in the eastern side of the 

 basin on September 1, 1915 (station 10309), while the smallest was a bare trace. 

 In fact, the animal population may be so sparse locally that a vertical haul may catch 

 nothing at all, as has been our experience at several stations along the coast of Maine 

 and in the Grand Manan Channel (p. 84) ; but even then, a half hour's tow with the 

 horizontal net has invariably yielded a few copepods or other animals, proving that 

 although the planktonic community may fall to a very low ebb, indeed, at its season 

 of scarcity, it never vanishes wholly from any part of the gulf at any time of the year. 



DENSITY OF ASSOCIATION OF THE ZOOPLANKTON 



A statement of the volume of zooplankton existing in the total column of water 

 below any chosen unit of sea area — e. g., each square meter — serves to illustrate the 

 total regional and seasonal production of the gulf; but unless the water in question 

 be very shallow, it throws little light on the density in which the animals concerned 

 are congregated, because the catch of the vertical haul may be distributed generally 

 over a column so long that even a considerable volume of plankton might mean only a 

 sparse population. To meet this need, another unit of measurement is required, the 

 one usually employed in other seas, and of which I have made use in previous re- 

 ports (Bigelow, 1915 and 1917), being the volume of plankton present in each cubic 

 meter of water. This , of course, is simply the product of the volume per square meter 

 of sea surface divided by the depth (in meters) covered by the haul in question. 



Were the zooplankton of the gulf uniformly distributed from the surface down 

 to bottom, this simple calculation would not only "establish the relative richness of 

 different regions in plankton, and hence in food for the pelagic fishes " (Bigelow, 1915, 

 p. 327), a question naturally of much importance in the economy of the gulf, but go 

 far to explain many biologic problems even more far reaching. Unfortunately for 

 the statistician, however, such is not the case, all our experience tending to show that 

 the zooplankton is often more or less stratified and that the degree of stratification 

 varies widely from place to place with the time of day and with the change of the 

 seasons. Consequently, the results always require analysis in the light of any 

 information bearing on the vertical distribution of the planktonic communities 

 represented in the catches in question. Otherwise one is apt to be led to conclusions 

 so widely astray as to be worse than none. 



