236 Mr Saunders, A Note on the Food of Freshwater Fish. 
A Note on the Food of Freshwater Fish. By J.T. SAUNDERS, 
B.A., Christ’s College. 
[Read 5 May 1913.] 
Very little is known about the food of freshwater fish, records 
in the past being very scanty and, owing to the difficulty of 
identifying remains in the stomach, sometimes inaccurate. Again, 
if an animal happens to be found two or three times in the stomach 
this was recorded as the chief food of the fish, without having 
any regard to the season of the year and the natural conditions 
under which the fish was living. This fault is very noticeable in 
semi-popular works like those of Tate Regan (British Freshwater 
Fishes, London, 1911) and Yarrell (A History of the British 
Fishes, London, 1886). Here we find it recorded that certain fish 
will eat such things as insect larvae, shrimps, small bivalves and 
the fry of other fishes. The natural conclusion that one draws 
from a statement such as this is that the fish swim about in search 
of food and snap up anything palatable that they meet. But this 
is far from being the case. 
On opening up the alimentary canals of some sticklebacks 
(Gasterosteus aculeatus, L.) caught at Madingley, I found that their 
stomachs were full of Diatoms, but it was only one species of © 
Diatom, viz. Nitzschia sigmoidea, Ehbr. It is true that there 
also occurred a few trichomes of Oscillatoria nigra, but they were 
so few that it is reasonable to suppose that they were ingested 
accidentally when the fish was seizing a Diatom. I have examined 
the stomachs of a great many sticklebacks at all times of the year 
caught in this pond and I found in them nothing but this Diatom, 
Nitzschia sigmoidea. All the specimens whose stomachs were full 
of Diatoms were caught with a worm and were large individuals, 
probably full grown. But finding it rather arduous to catch them 
singly with a worm I adopted the device of luring a number of 
sticklebacks over a net in the water by means of a worm. A great 
number of sticklebacks came to bite at the worm, and on quickly 
withdrawing the net fifteen to twenty could be caught at once. In 
this way fish of all sizes were taken. 
An examination of the stomachs of fish that were not fully 
grown gave very different results from the examination of those 
that were full grown. I found at once that the younger fish do 
not exhibit such an exclusive preference for NV. sigmoidea as do 
the older fish. N. sigmoidea is found only sometimes in their 
stomachs and it seems that the larger the fish the more restricted 
is its preference. The smaller ones are not so particular as the 
