Natural Mortality 199 



as abundant as in 1937. It seems probable tliat the total yield of fish in 

 1953 was considerably less than in 1937 but more selective for larger 

 fish. 



From the standpoint of the angler the relationship in 1937 between 

 anglers, commercial fishermen, and fish-eating birds was near optimum, 

 and the combined action of these cropping agencies was taking a reason- 

 able annual fish crop. Thus the benefits of an expanding population were 

 evident: fish had food and space to grow. Because the total annual 

 mortality of fish in this lake was nearly optimum (72 plus pounds per 

 acre), fishing remained good. Without the help of the commercial fisher- 

 men, the fish-eating birds and other natural predators available in 1937 

 could not have kept up with the reproductive potential of the fishes. What 

 happened to the population is fairly evident, for the number of fish caught 

 per fisherman in 1953 remained about the same as in 1937,^^ yet the weight 

 of fish taken was less than one-third that of the earlier year. 



NATURAL MORTALITY 



Natural mortality includes all causes of death of fish exclusive of pollu- 

 tion, angling, and commercial fishing. Deaths may result from predation, 

 injuries received through unsuccessful attempts at predation, competition 

 for food and space resulting in fatal injury or starvation, disease or ex- 

 cessively heavy infestations of parasites, catastrophes such as adverse 

 weather conditions, floods, etc., as well as from senile degeneration or a 

 combination of several of these factors. 



Causes of Natural Death 



Important causes for the deaths of fish change with a fish's age and size 

 as related to its normal life span. In the embryo and early free-swimming 

 stages when fish are very small, high mortality rates are probably caused 

 by predation from aquatic insects and larger fish.^^ 



The approximate numbers of largemouth bass fry in enumerated schools 

 at Ridge Lake (Illinois) were compared with the bass taken by lake 

 draining at the end of the second succeeding growing season. It was 

 estimated that the survival rate of schooling bass fry ranged from 1 in 29 

 to 1 in 195.^ These bass fry were exposed to very favorable conditions 

 because many potential predators in the form of small bluegills, large 

 predaceous aquatic insects, and crayfish had been removed from the lake 

 prior to the bass spawning season. A survival ratio of schooling bass fry 

 to yearlings of at least 15 to 1 was attained by the first year class of bass 

 spawned in Ridge Lake after water was first impounded. Predation rates 

 on embryos and fry of other nest-building centrarchids may be higher than 



