314 Relations within the Species 



When a large number of salmon have arrived at a suitable spawning 

 place, the fish react more definitely to each other, pair off, and begin 

 their breeding activity. After spawning the adults of the Atlantic 

 salmon eventually return to the sea, but in the Pacific species the 

 adults die in the stream and huge numbers of dead fish may choke 

 the narrow waterways. The great quantity of decomposing organic 

 matter thus added to the streams may deplete the oxygen supply 

 locally and produce further ecological consequences. Nutrient mate- 

 rials resulting from the decomposition may possibly stimulate the 

 growth of plankton on which the young fish depend. When the eggs 

 hatch, a large number of young fish appears simultaneously in the 

 headwaters of each salmon stream and represents another occurrence 

 of aggregation in this species. A further discussion of the ecological 

 problems involved in the life history of the salmon will be found in 

 a symposium sponsored by the American Association for the Advance- 

 ment of Science (1939). 



Mutual Attraction. Another manner in which aggregations of the 

 same species are formed is by an initial direct attraction of the in- 

 dividuals to each other. Mutual attraction of individuals is found 

 among lower animals, but frequently the reaction is largely non- 

 specific. If isopods are distributed over a surface of uniformly low 

 moisture, the animals stop against the first individual to come to a 

 halt and soon a dense cluster will be built up. Brittle starfishes placed 

 in a bare aquarium will move to form closely entwined aggregations, 

 but, if the aquarium contains eel grass, or even glass rods simulating 

 eel grass, the starfishes will remain spaced out in contact with these 

 objects. In these instances the other individuals present are merely 

 satisfying physical needs of moisture or of contact. In schools of 

 fish, flocks of birds, and herds of mammals the origin of the group 

 results from mutual attraction on a highly or a completely specific 

 basis. "Birds of a feather flock together" because of a definite attrac- 

 tion to others of the same species. The members of these groups 

 are not necessarily from the same parents and are usually quite un- 

 related, but during the breeding season units are formed within some 

 populations in which the members are breeding partners and family 

 relatives, as discussed above. 



Sometimes the reaction to keep in close contact, or to follow others 

 of the same species, is very strong indeed. The manifestation of this 

 in groups of ungulates, such as sheep, has given rise to our term 

 "sheep-minded." Anyone who has visited sheep country knows the 

 very great strength of the tendency of these animals to keep close 

 together. So strong is this reaction that if the leaders of a flock 



