106 BULLETIN OF THE BUREAU OF FISHERIES 



relationship between the fluctuations in the amount of zooplankton present in the 

 sea and in the seasonal and yearly catch of mackerel, corroborated by experience 

 for herring, also, in the Irish Sea (A. Scott, 1924) ; and E. J. Allen (1908) aroused an 

 interesting discussion by his tentative hypothesis that the abundance of mackerel 

 at any given locality depends on the amount of sunshine during the previous months, 

 sunny weather favoring the multiplication of diatoms and thus affording a rich 

 pasture for copepods, an abundant stock of which attracts mackerel. Dr. C. B. 

 Wilson, in a letter, suggests that the diurnal migrations of copepods upward toward 

 the surface at night and downward by day may be the reason why mackerel and 

 herring most often school at the surface at night, following the daily migrations of 

 their prey. 



To attempt to connect the fluctuations in the stock or the movements of the 

 fish population of the gulf, even of such typical plankton feeders as the herring, with 

 variations in the supply of plankton is as yet out of the question, neither digested 

 statistics of the catch of the former nor sufficiently definite information as to the 

 latter having been gathered. However, it is evident that a correlation between the 

 two must exist, and, as Dr. C. B. Wilson writes, "anything that contributes to a 

 detailed knowledge of the presence and movements of the copepods throughout 

 the year will give us information as to the movements and distribution of the fish," 

 and is therefore of as direct interest to the fisherman as to the scientist. 



FOOD OF THE PLANKTON 



The study of the stomach contents of the smaller pelagic animals, which to- 

 gether make up the zooplankton, is, as Steuer (1910, p. 622) points out, beset by 

 many obstacles, principal among which is the rapidity with which the various organic 

 substances are digested after being eaten, leaving as recognizable in the masticated or 

 half-digested state only such objects as are provided with spines, bristles, etc., or with 

 calcareous or silicious shells of characteristic outline. Then, too, it is a common 

 experience to find whole series of animals, even of the larger species, perfectly empty. 



In spite of these difficulties, however, so considerable a body of observations has 

 been accumulated that the general diet of most of the important planktonic groups 

 can now be stated with some confidence, and although little attention has yet been 

 paid to the diets of the plankton of the Gulf of Maine, there is no reason to suppose 

 that the feeding habits of its various members differ essentially from those of their 

 north European representatives. 



Among the zooplankton, as among the pelagic fishes, some species or groups are 

 carnivorous while others depend for subsistence on the unicellular vegetable life of the 

 high seas, but within the various groups the smaller planktonic animals are decidedly 

 uniform in their feeding habits. Perhaps as striking an illustration of the carnivorous 

 habit as any is afforded by naked pteropods such as Clione limacina, which, so far as 

 known, live exclusively on other pelagic animals and most often on their own shell- 

 bearing relatives (for instance, on Limacina), which they devour by thrusting the 

 protrusible proboscis into the shell and tearing the inmate to pieces in spite of its 

 futile efforts to escape by contracting into the smallest possible compass, as Schie- 

 menz (1906, p. 29) has so graphically described. 



