Natural Mortality 201 



gain entrance through skin abrasions and produce a toxin that causes the 

 death of the fish. Achhja sp., one of these fungi, is reported to attack 

 healthy fish without breaks in the skin. 



Biologists active in state fisheries departments learn to anticipate a 

 spring period each year when many phone calls and letters report deaths 

 of fish in various state and private waters. Part of these originate directly 

 from winterkill: the fish that have died over the winter decompose and 

 float to the surface in spring soon after the ice goes out. Other spring 

 reports of dead or dying fish can be assigned to disease or fungus in- 

 festations. Usually, nothing can be done to stop the fish from dying and 

 the situation must be left to run its course. In no case that I know of has 

 a partial loss of fish had serious consequences, unless severe winterkill 

 was the cause of death. When fish suffocate under the ice, a partial kill 

 may have serious consequences to later fishing ( see Chapter 3 ) . 



Many fish die during the summer and fall. Usually these never appear 

 at the surface or else float up in numbers too small to receive attention. 

 The complete disappearance of a year class of crappies during summer is 

 not unusual. In one instance where 1-inch mesh wing nets with leads were 

 used at 30- to 40-day intervals over a period of years for catching crappies, 

 a year class was followed from the time its members were first large 

 enough to be caught until they suddenly and permanently disappeared, 

 indicating a complete mortality for that year class. -^ Angling returns of 

 marked crappies in Lake Chautauqua (Illinois) showed that fishermen 

 were taking less than 5 per cent of the available large fish; thus, about 

 95 per cent of the large crappies were dying from old age. Most of these 

 deaths apparently occurred during the warm months. ^^ 



Scavengers 



As a result of pollution, many fish die at about the same time, making 

 available a large amount of carrion for such scavengers as survive in the 

 lake. However, these remaining scavengers are unable to assimilate such 

 an abundance of protein. As a result, the fish decay and float to the surface 

 where they may be consumed by terrestrial scavengers and blowfly larvae. 



Underwater scavengers are probably not so eflicient as terrestrial carrion 

 feeders. However, they do well enough to consume a seasonal quantity 

 of carrion of as much as 100 pounds per acre in some waters, without 

 allowing any of these fish to appear on the surface. Crayfish are important 

 as underwater scavengers and will attack injured or disabled fish before 

 they are dead."' Certain kinds of turtles also act as scavengers. 



Length of Life of Fishes 



Growth studies based on scale analyses furnish valuable information on 

 the length of life of fishes. Most species do not live as long as is popularly 



