Fish Population Adjustment 151 



studying the growth of warmouth in Park Pond,^^ he found that the 

 warmouth had made unusually rapid growth (114 and 128 per cent of 

 expected annual length increment) during 1945 and 1946. This he could 

 not explain until it was discovered that the years of good warmouth 

 growth corresponded to years of population thinning through partial 

 poisoning. 



Experience in partial poisoning operations has shown that gizzard shad 

 are killed with lighter dosages of rotenone than almost any other warm- 

 water fish.^* In general most of the centrarchids (sunfishes) are moder- 

 ately sensitive to rotenone, but smaller individuals of a species are gener- 

 ally more susceptible than larger ones. For this reason and because young 

 fishes of many species inhabit the warm, quiet, shallow waters near the 

 shore on bright summer days, a shoreline rotenoning operation can be 

 used to kill numerous fish too small to interest anglers. ^^^ 



Timing in Partial Poisoning. The timing of a partial treatment is im- 

 portant because the final effect may vary, depending upon whether 

 the operation is done in the spring, mid-summer, or early fall. Sup- 

 pose, for example, that a lake contained an excessive number of small 

 bass and one wished to thin this population to allow for an expansion of 

 a relatively small population of bluegills. A partial poisoning operation 

 in May or June, but after the bass had spawned, would reduce the severity 

 of predation by young bass on newly-hatched bluegills (the spawning 

 season for bluegills lasts from late May to mid-September in the latitude 

 of central Indiana and Illinois) and would allow a greater survival of 

 these young in June, July, and August. Many of these bluegills might 

 grow fast enough to exceed the size of easy predation before the next 

 year class of bass was produced the following spring. 



In another instance, a lake might contain a large population of stunted 

 bluegills and a few large bass unable to reproduce successfully because 

 of predation on bass eggs and fry by hoards of hungry bluegills. In such 

 a situation partial poisoning should be performed either ( 1 ) immediately 

 before the bass spawning season in the spring or (2) at the end of the 

 bluegill spawning season, in September or early October. If the operation 

 were done between these specific times, the food and space gained by the 

 removal of a portion of the excess of bluegills would be taken up almost 

 immediately by new hatches of young bluegills. However, population 

 reduction by partial poisoning, just prior to the bass spawning season 

 ( and the bluegill spawning season as well ) , would curtail bluegill preda- 

 tion on the bass eggs and fry, resulting in a proportionate increase in the 

 survival of young bass. Similarly, if partial poisoning were performed in 

 early fall after the bluegills had stopped spawning, the space gained at 

 the expense of a part of the population would not be filled, either through 



