374 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 3 



attack. Man has become a selective preda- 

 tor of large fishes. He has waged contin- 

 ual warfare on other fish predators, has 

 passed laws directed at controlling any 

 inclination on his part to take small fish, 

 and has developed an artificial propaga- 

 tion and stocking program to augment the 

 supply of small fish. Because the high 

 reproductive potential of the fishes has 

 remained unchanged, there has been a 

 great increase in the survival of young 

 fish, resulting in added competition for 

 food and space, in overpopulation of waters 

 and in stunting of fish. Intensive inter- 

 specific competition among the fishes them- 

 selves may be the key factor responsible 

 for eliminating or reducing populations of 

 many of the more desirable game fishes 

 in some waters, because the young of these 

 species are unable to compete with the 

 young of others under crowded conditions. 



A common belief among anglers is that, 

 in lakes in which the annual yield of game 

 fish has declined over a period of years, 

 intensive fishing is responsible for the pro- 

 gressively poorer catches. In most instances 

 in which an actual decline may be proved, 

 no data are at hand as to the kinds and 

 quantities of fish that remain in the lake 

 at the time of low production. However, 

 in 22 artificial lakes in Illinois, most of 

 which were censused at a time of low pro- 

 duction, only one showed a decline in yield 

 associated with intensive fishing. Whether 

 intensive angling is o.r has been directly 

 responsible for a reduction in game fish 

 yields of other Illinois lakes, particularly 

 those larger than 25 acres, is subject to 

 question. The efficiency of angling devices 

 is so low, and the patience of the average 

 angler so short, that, long before the pop- 

 ulation of the selected species has been 

 reduced to the danger point below" which 

 it cannot reproduce itself, the fishing for 

 that species has become so uninteresting 

 that it is greatly reduced, or it may even 

 be stopped. 



It is not uncommon for populations of 

 desirable fish to become depleted in, or 

 disappear from, waters that are fished very 

 little. This occurrence is frequent in arti- 

 ficial lakes and reservoirs where preda- 

 tors of small fish are at a minimum. Here 

 competition among those desirable species 

 artificially introduced, along with compe- 

 tition from undesirable kinds that gain 

 entrance through feeder streams, eventu- 



ally crowds out the more desirable species. 



The term "overfishing" as it relates to 

 lakes is loosely used by anglers. By some 

 it is applied to a process characterized by 

 the depletion of a single select species, a 

 depletion attributed to more or less inten- 

 sive fishing (usually an unspecified num- 

 ber of man-hours) for that species. Actu- 

 ally this decrease in yield may result from 

 competition with other fish wherein the 

 select species is gradually replaced by an- 

 other or several other species, less desir- 

 able to anglers. In such a case the water 

 cannot be said to be overfished, for it sup- 

 ports a fish population at least as great in 

 total weight as that when the select species 

 was present in numbers. 



In the strict sense, the term overfishing 

 involves the entire fish population of a 

 body of water. The yield taken by anglers 

 is not restricted to one or a few species 

 but includes all or almost all in near pro- 

 portion to their relative abundance. This 

 yield, considered in pounds of fish flesh, is 

 removed at a rate faster than it can be 

 replaced by conversion of available natural 

 food into flesh, and the poundage of fish 

 supported by the water remains below the 

 carrying capacity of that water for all 

 fish. Overfishing as defined here is a con- 

 dition that is extremely rare because it 

 requires a very great intensity of angling. 



The only case of overfishing ever 

 observed in Illinois by Natural History 

 Survey aquatic biologists occurred under 

 conditions favorable for study, 1938-1941. 

 Evidence of overfishing was available from 

 well-kept records of hook-and-line catches 

 for two complete fishing seasons and parts 

 of two others, and from sample catches 

 made with test hoopnets in 1938 and 1940. 

 In 1941 the lake was treated with a plant 

 alkaloid to kill the fish, and a careful cen- 

 sus of the fish population was made. 



This investigation would have been 

 impossible without the continual interest 

 and cooperation of members of the Fish-i 

 ing Club of the Owens-Illinois Glass Com- 

 pany of Alton, Illinois. 



Overfishing in Onized Lake was the 

 result of an intensity of angling far greater 

 than that observed in most waters. Only I 

 under conditions of heavy use of a small 

 body of water by a club, community or 

 industry, where fishing is associated with 

 other forms of outdoor recreation, would 

 this intensity of fishing be duplicated. 



f 



