1 70 The Life Story of the Fish 



full-grown trout. Fish, or most of them, anyway, continue 

 to grow as long as they live and as long as they obtain food 

 in a quantity greater than the amount necessary to maintain 

 the body in a normal state of upkeep and repair. The rate of 

 growth varies in different species, and varies within the 

 species at different ages. In general, it is faster in young fish 

 than in old fish, the most marked slowing-down coming at 

 the age of maturity, by which is meant the age at which the 

 fish is ready to spawn. From that time on the growth is apt 

 to be much slower, probably because so much food is needed 

 for the manufacture of sex products, and also because many 

 fish take no food during the spawning period and therefore 

 have to rebuild their depleted tissues, when they start feeding 

 again, before they can add anything to their growth. Such 

 fish may show only very slight annual increases in size after 

 the age of maturity, but a definite and measurable increase 

 can nevertheless occur every year. 



The arrival at maturity — at that condition where the eggs 

 or spermatozoa are ready, where the creation of new indi- 

 viduals is possible — is the great moment in the fish's life. It 

 may come early, or it may come late, measured not only in 

 absolute units of time but also in comparison to the life-span 

 of the fish. Cichlids like the jewel are ready to spawn at the 

 age of four months, when they are about two inches long; 

 and jewels have been known to live five years and to reach 

 a length of five inches. In other words, the jewel matures 

 when it has lived only one-fifteenth of its possible life, and 

 reached only one-fifteenth of its possible weight.^ The golden 

 trout in the upper Cottonwood Lakes of California matures 



^ Since in almost all species of fish, weight is proportional to the cube 

 of length, a two-inch jewel weighs only about one-fifteenth of a five-inch 

 jewel : 



23 _ 8 _ I 



5^ 125 15 



