5 



Reproduction, Competition, 



and Predation 



If the habitat is suitable for fish, the success of various components of a 

 fish population may depend upon the interrelationships of three forces, 

 namely, reproduction, competition, and predation. Reproduction produces 

 new individuals, not only to replace losses of mature animals but also to 

 enter the trophic cycle— the young fishes feeding on lesser animals and 

 themselves becoming food for other larger ones. Thus, the products of 

 reproduction push the population toward expansion. In opposition, the 

 forces of competition (inter-specific and intra-specific ) and predation 

 tend to counteract population expansion. This interrelationship of re- 

 production, competition, and predation is normal and necessary to die 

 well being of the population, although considerable variations exist. Thus, 

 some populations, that appear engaged in a constant struggle for exist- 

 ence, may show comparatively little fluctuation from year to year in 

 actual number of surviving individuals. However, other populations may 

 require a cycle of two or more years to achieve maximum abundance, 

 while still others may fluctuate irregularly between maximum and mini- 

 mum limits. 



Geological evidence has shown that fishes have existed on the earth's 

 surface for many milHons of years. Yet in spite of the fact that they have 

 furnished food for many forms of vertebrates— fishes, amphibians, reptiles, 

 birds, and mammals including man— and some kinds of invertebrates, they 

 must be considered as one of the more successful groups of vertebrates. 

 A number of species of fishes alive today are scarcely changed in form 

 from those found as fossils in deposits representing the Devonian Period, 

 360 million years ago. 



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