THE BEHAVIOR OF BROOK TROUT EMBRYOS 57 



If someone throws a stone into the water the jar startles the 

 little fish into swimming about rapidly for a few seconds, after 

 which it sinks again into some shady nook. Here it rests, until a 

 sudden eddy caused by an animal swimming through the water, 

 makes it dart a few inches into the current. Other tiny fish 

 touch and jostle it continually. If the water becomes filled 

 with carbon dioxide, it becomes more active, and overcoming 

 its impulse to avoid the light, swims about restlessly from place 

 to place- until it comes into purer water, where it again sinks 

 down beneath the stones. Thus far its responses have been 

 largely avoiding reactions, serving to keep it from unfavorable 

 conditions. 



When the trout is about six weeks old, it becomes more sen- 

 sitive to objects outside itself. The sight of other animals pas- 

 sing by sends it scurrying under the cover of moss and stones. 

 Shortly after this it begins to be curious, nosing nearly every 

 object which it sees. It swims to the top of the water in pursuit 

 of a bubble. It explores the bottom of the stream, often swim- 

 ming head downward, passing in and out among the rocks, 

 stones, and algae. Many particles on the bottom or floating 

 above are taken into the mouth. If found to be good to eat, 

 they are swallowed; if not, they are expelled. As the fish eats, 

 it takes food more and more eagerly until it is satisfied, when it 

 ceases to react, and hides in the algae. 



Throughout its larval life the Brook Trout is reacting to 

 external and internal stimuli, responding to nearly every cur- 

 rent, object, or ray of light that strikes it. 



In general, the behavior suits the needs of the fish. As long 

 as the trout are very young, and are encumbered with the large 

 yolk sac, which renders them unable to swim any distance, 

 their reactions are such as would naturally tend to keep them 

 lying quietly out of the sight of their enemies. They exhibit 

 no curiosity, but avoid the light, hiding beneath the rocks and 

 stones, reacting to current just enough to keep them from being 

 carried down stream. The trout appear not to notice external 

 objects, except as their approach jars the fish, making them 

 struggle to regain their equilibrium. 



As was previously stated, an excess of carbon dioxide renders 

 them temporarily indifferent to light, so that they swim about 

 restlessly until they reach a place where the water is purer. 



