106 
BULLETIN OF THE BUBEAU 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. 
