Growth 85 



in central Illinois ^ was followed during August, September, and October 

 of 1941 by a rapid growth of largemouth bass. These plants were pro- 

 tecting a large population of small fish which suddenly became easily 

 available. Bass that were between 10.5 and 11.0 inches before the plant 

 die-off, averaged 13.0 inches in October; those about 7.0 to 8.0 inches 

 before the die-off averaged 10.5 inches in October; and the surviving 

 members of the current year class averaged nearly 6.5 inches by October. 

 No comparable growth rate increase was shown among bluegills, even 

 though the pond developed a bloom of plankton algae following the death 

 of the higher aquatic plants. In this instance, a large supply of food 

 suddenlv became available to the bass with no comparable increase in 

 foods for bluegills. The cause of death of the rooted aquatic plants was 

 unknown, but its effect was highly favorable to a species of fish having 

 little direct ecological relationship to aquatic plants. 



Interpretation of Growth from Fish Scales 



Variations in growth rates and the occurrence of growth stoppage are 

 recorded on the scales of the fishes. When a fish is growing rapidly, the 

 circuli (fine lines of new material) laid down on the edges of the scales 

 are relatively coarse and spaced far apart; on the other hand, when growth 

 is slow, the circuli are fine and close together. A fish subjected to a period 

 of starvation not only loses body flesh, but erosion with resorption of 

 material on the edges of the scales may also take place. However, on a 

 maintenance diet where the condition of a fish remains constant, there 

 appears to be neither increment nor erosion of the scales. Thus, the 

 correct interpretation of the marks on the scale surface will give an 

 accurate growth history of a fish. 



The years of a fish's life are recorded as a series of annuli or distinct 

 rings laid down around the focus or center of the scale, each one repre- 

 senting a year. The circuli are between the focus of the scale and the first 

 annulus and are also between the other annuli. A newly-hatched fish may 

 be scaleless, but if it is of a scaled species, the scales soon form. Once 

 the fish becomes covered with scales, the number remains constant 

 throughout life, and to form a covering for the fish, the scales must grow 

 as the body grows. Any natural or artificial phenomenon that will stop 

 feeding and growth of a fish for about 14 days or longer will be followed, 

 once growth is resumed, by the appearance of an "annulus," on the margin 

 of the scales. The so-called "true annulus" was first called a winter ring 

 because it was not visible until new circuli were laid down in the spring 

 when the fish had resumed growth. Further studies of annulus formation 

 showed that some fishes in some locations did not begin to grow until the 

 middle of summer ^^ so that the winter ring became a summer ring. Hubbs 

 and Cooper -" recorded a double annulus in green and long-eared sun- 



