COPPER SULPHATE IN ALGAL CONTROL AND IMPLICATIONS 81 



fish populations, mostly buffalo and carp. Rough fish have been removed 

 regularly since 1923. 



Five untreated lakes in this chain have been selected for a comparison 

 of rough fish yields from treated and untreated waters. The untreated 

 lakes, North Silver, South Silver, Wilmert, Martin, and Iowa, range in 

 size from 186 to 443 acres. They are shallow, fertile, and very similar to 

 the treated waters. As far as can be ascertained these lakes, with the excep- 

 tion of South Silver in 1921, have never been treated with copper sulphate. 

 Because most of the rough fish removal has been carried out in the winter 

 by seining under the ice, the fishing records are considered by seasons 

 rather than by calendar years. In all there have been a total of 137 fish- 

 ing seasons on the 9 lakes, 64 on treated and 73 on untreated waters. The 

 number of rough fish removal operations on any one lake ranges from 

 12 to 22, with an average of 15 for all. 



For the last 24 years (1923-1947), the average rough fish yield per 

 fishing season was 132.1 pounds per acre from the 4 treated lakes and 117.7 

 pounds per acre from the 5 not treated. On the average, the yield of 

 the untreated lakes has been 9% less than of those treated with copper 

 sulphate. If fishing records are broken down into two long periods, 1923- 

 1933 and 1934-1943, to show trends, and a shorter period, 1944-1947, to 

 evaluate present conditions, a similar decline in rough fish production will 

 be observed for both treated and untreated waters (Table I). It appears, 



TABLE I 



Rough Fish Yields in Pounds per Acre per Fishing Season in Two 

 Groups of Southern Minnesota Lakes '^ 



1 Parenthetical figures are the number of fishing seasons on which the average is based. 



therefore, that most of the decline in the catch of rough fish in the treated 

 lakes must be attributed to causes other than copper sulphate — to such fac- 

 tors as repeated rough fish removal, fishing skill, weather, and the price of 

 fish. The somewhat greater decline in catch from the treated lakes may be 

 due to copper sulphate but there are so many variables involved that the 

 statistical validity of difference in yield cannot be proven. 



From such information as we have on the Fairmont lakes, it appears 

 that the continued treatment with copper sulphate has not seriously affected 

 angling. Creel census carried out during the months of July, 1941, and 

 July, 1942, on 2 of the treated lakes. Amber and Budd, showed an average 

 catch rate of 2.28 fish per hour of fishing. The average catch rate dur- 

 ing these months for all Minnesota lakes was 1.61 fish per hour of fishing 

 (Hiner 1943). 



