PLANKTON OF THE GULF OF MAINE 93 



Although it was often the deeper haul that yielded the larger amount of plankton, 

 all the very rich tow-net catches (2,000 cubic centimeters or more) made in the ^ulf 

 during that summer (six in number; see Bigelow, 1917, p. 312) were from depths of 

 100 meters or less, with the average volume (about 900 cubic centimeters) of all the 

 subsurface catches made shoaler than 100 meters, almost three times that of the 

 deeper hauls (about 350 cubic centimeters), although the latter fished through a 

 longer column of water on their journey down and up. Thus, it seems that the gulf 

 is usually richer in zooplankton above than below 100 meters depth during the 

 summer season, and very rich catches were made in vertical hauls shoaler than that 

 at the few stations which the Grampus occupied in the gulf during July, 1916 

 (p. 92; Bigelow, 1922, p. 136). 



With the plankton often concentrated at some one level, it becomes more or 

 less a matter of chance whether a net fishing horizontally hits or misses the richest 

 zone. Consequently, the yields of the two sorts of hauls, horizontal and vertical, are 

 often far from parallel. When there is a wide discrepancy between the two it has 

 usually been in favor of the horizontal net (especially in deep water), for we have 

 usually made at least one horizontal tow in the productive stratum between 40 and 

 100 meters at each station, whereas the vertical catch mirrors the plankton content 

 of the barren strata combined with that of the rich. Occasionally, however, the 

 tables are turned, as was the case on July 23, 1914, on the eastern part of Georges 

 Bank (station 10223), where the volume per cubic meter taken by the vertical haul 

 was more than seven times as great (2.2 cubic centimeters) as that taken by the 

 horizontal haul (about 0.3 cubic centimeter) although the depth of water — that is, 

 the length of the column fished through — in the case of the former was only 82 meters, 

 whereas the latter worked for about three-quarters of a rrule. Thus, the vertical 

 net must have passed through water much more productive than the level at which 

 the horizontal net was fishing. In 1913 and 1914, too, the richest catches with 

 horizontal nets were not at the stations where the volumes per square meter or per 

 cubic meter were largest, as calculated from the vertical hauls. 



It follows from these facts that while the ostensible volumes per cubic meter 

 may be a satisfactory index to the density of the planktonic population of the Gulf 

 of Maine in winter or early spring, and in summer at stations where no stratification 

 is apparent from the yields of the horizontal hauls, and while this calculation may 

 approximate the truth in very shallow waters generally at most times of year, as a 

 rule it greatly understates the actual maximum density of aggregation of the 

 plankton in deep water, making such regions appear much less prolific as feeding 

 groimds for pelagic fishes than their richer layers actually are, while crediting far too 

 high a plankton content to their more barren strata, as I have pointed out else- 

 where (Bigelow, 1917). 



Owing to the tendency of the zooplanktonic community as a whole to con- 

 gregate in the upper 100 meters of water during the warm months, but at the same 

 time to keep some few meters down (p. 24), the seasonal difference between the 

 volumes of plankton per cubic meter present in March, on the one hand, and in 

 July and August, on the other, is actually much greater than the ratio arrived at 

 by any calculation which fails to take account of its vertical stratification. A more 



