Winterkill and Sumyncrkill 53 



plankton cells would be present to improve borderline oxygen conditions 

 for fishes. 



Results of Partial Winterkill 



It is seldom that all of the fishes in a lake or pond are killed when sub- 

 jected to adverse conditions under ice. This is because some are more 

 resistant to low oxygen tensions than others and because adverse condi- 

 tions throughout a lake or pond may not be uniform, so that certain ones 

 in more favorable locations may survive while the rest may die. However, 

 most game and pan fishes require larger amounts of oxygen than do the 

 coarse ones— carp, buffalo, and bullheads. For this reason, a few of the 

 more undesirable fishes may survive to repopulate the water. 



Studies of fish populations subjected to partial winterkill have been 

 made by a number of biologists. In Michigan a partial winterkill was 

 followed by changes in growth rate of the surviving fishes.- A dominant 

 year class of bullheads developed in Lost Island Lake ( Iowa ) following 

 a partial winterkill --' ^^; this same phenomenon was reported for Spring 

 Lake near Savanna (Illinois). 



The fish population of 10-acre Gale Lake near Galesburg, Illinois, was 

 examined about 20 months after a winterkill had occurred during January 

 or February of 1945.^ Prior to the winter of 1944-45, the lake contained 

 largemouth bass, bluegills, white crappies, carp, bigmouth buffalo, golden 

 shiners, and catfish. When the ice went out in early March, 1945, about 

 4000 pounds of fish carcasses were collected. 



When the lake was censused by treatment with rotenone on September 

 16, 1946, it contained white crappies, green sunfish, black bullheads, 

 carp, buffalo, and golden shiners. Table 3.1. Approximately 106,500 fish 

 weighing 5275 pounds were collected. The larger fish shown in Table 3.1 

 are those which had survived the winter of 1944-45; the smaller indi- 

 viduals of the same species represented the young produced in 1945 and 

 1946. It is inconceivable that this population could have ever produced 

 satisfactory hook-and-line fishing. 



Studies of fish populations that have undergone partial winterkill il- 

 lustrate well the danger of this phenomenon. Usually, it would have been 

 far better if the adverse winter conditions had killed all of the fish. Tlien 

 one could restock the lake with useful kinds and numbers of fingerlings 

 and obtain satisfactory fishing within the second season after the winterkill 

 occurred. Nonetheless, where the winterkill is partial and some desirable 

 pan fish species survive, a stocking of a small number of sexually mature 

 largemouth bass might result in the production of a dominant brood 

 of these fish to prevent an overpopulation of the pan fish. Such a stocking 

 probably could have been accomplished prior to the bass spawning 

 season in a lake as small as Gale. Larger waters that have lost a part of 



