5o Recruitment of the I9U6 and 19h7 year classes of black crappies, 

 bluef;ills and white crappies influenced catch rates by (1) causing a rise 

 during July when these year classes first appeared, and (2) magnifying 

 the effects of increased activity rate among the bluegills and black 

 crappies during August and September. 



6, There was some evidence of high losses among older crappies and 

 bluegills during the season. Fewer members of the 19Ui| and 19U3 year 

 classes were caught as the season progressed. Loss of older fish from the 

 population is thought to have b een largely the result of natural mortality. 



7. The stabilization of water levels and t he warming of waters to 

 above 70 degrees F., in June may have been related to a decreased rate of 

 activity noted among the centrarchids. 



8, No relationship was noted between changes in water turbidity and 

 catches of trap nets. 



9. Spawning activities were apparently related to the high catches 

 of channel and flathead catfish during June and the first part of July and 

 to the high catch of carpsuckers during the last half of June. 



10. Species composition of the total catch ty weight changed markedly 

 throughout the season (Figure 10). During Period III (June 12-26) 68 per- 

 cent of the catch was of commercial species 5 13 percent of panfish and I8 

 percent of game fish. A month later in Period V (July 18-27) 60 percent 

 of the catch was of commercial speciesj 27 percent of pan fish and l5 per- 

 cent of game fish. In Period VII a month later (August 18-27) 25 percent 

 of the catch was of commercial species; 59 percent of panfish and 10 per- 

 cent of game fish. Such changes in composition of the catch probably do 

 not represent similar changes in the structure of the fish population of 

 the area fished. Ihey do demonstrate that several factors influence the 

 catch of stationery nets and these must be taken into account if catches 

 from such nets are to be used to estimate fish population, size and 

 structure. 



ACKNOWLEDGEMENTS 



The writer wishes to acknowledge aid and encouragement received from 

 conservation agencies of the states of Minnesota, Wisconsin and Iowa and 

 the U.S. Fish and Wildlife Service. Special thanks are due Mr. Joe 

 Martel for aid in the netting, Mrs. Wentzel Satek for assistance in the 

 laboratory, Dr. John T. Greenbank who supervised the field work and gave 

 generously of his time and advice and Dr. Raymond E. Johnson and Dr. 

 John B, Moyle who helped prepare and revise the manuscript. Work inthe 

 field was carried out from the U.S. Fish Cultural Station at La Crosse, 

 Wisconsin where space and facilities were made available by Mr. C. F. 

 Culler, Mr. Harry Canfield and Mr. Harry Carson. The photograph of the 

 trap net was supplied by Mr. John Dobie. 



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