200 Fishing and Natural Mortality 



those of largemouth bass because the former produce larger numbers of 

 eggs and probably o£Fer less protection to their young. 



If the young fishes of the larger species escape predation and can find 

 sufficient food, they may survive the first growing season and be well on 

 their way to adulthood. Many of the smaller species reach sexual maturity 

 and spawn during the early part of the second summer of life. Most of 

 these yearlings are small enough to be preyed upon by some kinds of 

 fishes and nearly all of the predaceous amphibians, reptiles, birds, and 

 mammals; however, they are beyond the size for predation by most 

 aquatic insects. By the end of the second growing season, direct preda- 

 tion may become a minor cause of death and other mortality causes may 

 take over. 



Ricker ^^ investigating the natural mortality rate among the fishes of 

 several Indiana lakes concluded that once the fish reached sizes larger 

 than 5 inches, senility must account for most of the natural mortality. 

 Ricker believed that senility in fish was active over a wide range of ages, 

 relatively much wider than that of domestic animals and man. He based 

 this assumption on his discovery that natural mortality in the bluegill 

 was rather constant in fish from three to six or seven years, the approxi- 

 mate maximum age of this species. 



Once a fish reaches a size beyond that of "easy" predation its death 

 may result from a combination of several factors. A 5-inch green sunfish 

 may fall prey to a 16-inch bass because a bacterial infection of the sun- 

 fish's fins has caused it to swim in such an abnormal manner as to attract 

 attention. The internal parasites of a minnow may reduce its swimming 

 activity so greatly that it cannot avoid being captured by a crappie or a 

 heron. Even senility may be followed by predation before the individual 

 fish has time to die from organic degeneration.^^ 



When Do Fish Die? 



Fish may die at any time of year but it is probable that most of them 

 expire during spring, summer, and fall ( one must discount deaths caused 

 by suflFocation under ice resulting from unusual circumstances ) . However, 

 Snieszko ^^ believes that winter conditions are responsible for reducing 

 the resistance of fish to bacterial diseases in the spring and that reported 

 deaths associated with rising water temperatures are due to a deficiency 

 of antimicrobial components in the fish's blood; the studies of carp blood 

 by Schaeperclaus ^^ and Plancic ^- substantiate this hypothesis. There is 

 little question that many fish die in the spring, a large number of which 

 appear to be diseased or infested with aquatic fungi ( Saproligniales ) . In 

 the case of fungus infestations, it is believed that injuries acquired by 

 fish might heal during other times of the year. In the spring, however, 

 conditions are optimum for the growth of aquatic fungi, and they readily 



