82 BULLETIN OF THE BUREAU OF FISHEBIES 



about 25 cubic centimeters, graduated as required, serves well for copepods and all 

 smaller animals. The chief difficulty is that it is not always easy to make sure that 

 the diluted plankton is evenly distributed in the fluid while the sample is being 

 taken, because the various animals settle at different rates. Therefore, it is usually 

 advisable to take two or sometimes three samples from each haul and average the 

 results. 



Animals as large as amphipods, Sagittae, and euphausiids are seldom so numer- 

 ous but that it is easy to count the entire number caught in a vertical haul, and as a 

 rule it is necessary to remove them before taking the sample of copepods, etc., lest 

 they clog the mouth of the pipette. Fish eggs, also, can usually be counted directly 

 from the entire catch, though they sometimes occur in such numbers that it is neces- 

 sary to take a sample for this purpose. The copepods have been counted for most of 

 the vertical hauls, the results being discussed in the chapter on that group (p. 167). 

 Notes on -numerical strength of other animals will be found under the particular 

 species. 



The unit of measurement best available for the volume depends upon whether 

 horizontal or vertical nets are used. If the former, calculation of the amount per 

 hour's hauling, as employed by Jespersen (1924), can hardly be bettered; but vertical 

 hauls lend themselves to a somewhat more exact measure, namely, the amount present 

 under some chosen area of the surface of the sea, which is usually expressed in cubic 

 centimeters of plankton per square meter. This would be a sufficient index to the 

 total productivity of any locality at any given time, and hence is often extremely 

 instructive from the biologic viewpoint; but, as I shall have occasion to emphasize 

 later (p. 90), it does not necessarily throw any light on the density with which the 

 plankton is aggregated, since it neglects the possible stratification of the latter at 

 different levels. 



On this basis the animal plankton of the gulf as a whole, like the phytoplankton 

 (p. 399), is apparently at its lowest annual ebb late in February and during the first 

 half of March, when it was only in the western basin and over a tongue extending 

 from the Eastern Channel and eastern edge of Georges Bank northward along the 

 axis of the eastern basin to the 100-meter contour off Grand Manan (fig. 37) that we 

 found as much as 75 cubic centimeters per square meter in 1920. Nor did we make 

 any rich hauls then even in these comparatively productive zones, judged by mid- 

 summer standards (p. 83). In all other parts of the gulf at the time, both inshore and 

 over the basin, except as just qualified, and on Georges Bank as a whole, the water 

 supported less than 25 cubic centimeters of plankton per square meter of sea surface, 

 with several of the catches too small to measure, while on one occasion (off Cape 

 Elizabeth, March 4, station 20059) the vertical net yielded nothing whatever. 



If the minimal catches of February and March, 1920 (less than 25 cubic centi- 

 meters), be credited with 15 cubic centimeters of zooplankton per square meter 

 (probably an excessive estimate), the average for the whole gulf at this season was 

 only about 40 cubic centimeters, contrasted with about 100 cubic centimeters in 

 midsummer, and the distinction between rich and barren was decidedly more sharply 

 marked than we have found it during the more productive seasons of the year. 



