The Relation of Mollusks to Fish in Oneida Lake 115 
and streams. The vigorous growth and reproduction of plants 
furnishes a large food supply for the smaller animals, which 
in their turn can reproduce more abundantly and provide a 
greater amount of food for the fish. 
“ Barring enemies and artificial hindrances to increase, 
such as overfishing, fish will multiply up to the limit of the 
food supply, but can never overstep that limit. If the food 
supply can be increased, an increase in the number of fish 
will naturally follow.” 
Other American authors, as well as some foreign writers, 
have touched more or less extensively on this very important 
subject. It remained for two Danish ecologists, Dr. C. G. 
Joh. Petersen and Dr. P. B. Jensen, the former the Director 
of the Danish Biological Station, to elaborate some of the 
most important methods of investigating this subject. Peter- 
sen has been studying the marine waters of Denmark for the 
purpose of determining the amount of life on the sea bottom, 
the relation of this life to the plant life and to the main- 
tenance of the food fishes of these waters, especially the 
Plaice. These studies were inaugurated in 1883 and have 
progressed each year, the notably increasing success being 
attested by the brilliant reports of the Danish Biological Sta- 
tion (see Bibliography). The work of these Danish ecologists 
does not appear to have received adequate attention from 
American ecologists and fish culturists. 
Petersen realized at an early period that to understand 
fully the conditions governing the habits of fish, especially as 
regards their food, a knowledge must not only be obtained of 
the kind of food eaten, a knowledge secured by means of an 
examination of the stomach contents of recently feeding fish, 
but that we must also know the variety and amount of the 
possible food supply. In other words, a biological survey of 
the fish habitat is necessary before we are in a position to 
understand the conditions governing the physiology of the 
piscene fauna. In short, such studies lead to a broad con- 
sideration of the metabolism of the whole lake or body of 
water under discussion. Petersen observed that the Plaice 
from the western part of Limfjord practically ceased growing 
