The upper Mississippi River supports an important sport and commer- 

 cial fishery. To obtain better management of this fishery as well as the 

 other ra.ldlife resources of the river, the Upper Mississippi River Con- 

 servation Committee was organized in 19U3- Represented on the committee 

 were the official conservation agencies of the states of Missouri, Illinois, 

 Iowa, Wisconsin and Minnesota, and the United States Fish and Wildlife 

 Service. 



Under guidance of the committee a project was set up for gathering 

 information about the fish population of the Mississippi River by a 

 series of test-netting surveys. These were carried out during the years 

 19iili-Ii8 and consisted (1) of netting by traveling crews at stations be- 

 tween Caruthersville, Missouri and Hastings, Mnnesota and (2) of netting 

 throughout the open-water season at permanent stations near La Crosse, 

 Wisconsin. The latter work was done in I9I18 and it is with the fluctua- 

 tions of trap net catches at these permanent netting stations that the 

 present paper is concerned. 



Quantitative interpretation of net catches presents many difficulties. 

 As hac' been pointed out by Hartly (19U7) and Moyle (19^0), the catch of 

 passive fishing gear such as hoop, fyke, gill and trap nets depends not 

 only on the abundance of fish large enough to be taken and held by the nets 

 but is also influenced by the rate of activity of the fish. These factors 

 as well as others related to them must be considered if net catches are to 

 be used as a quantitative measure of the s"'ze and structure of fish popu- 

 lations. This paper attempts to evaluate some of the factors found to 

 influence the catch of trap nets in the Mississippi River during the sum- 

 mer of 19lt8. 



AREA FISHED 



The catches considered here were all made in the upper third of 

 Pool 8, formed by the dam at Genoa, Wisconsin. This part of the Miss- 

 issippi consists of a large number of backwater lakes and interconnect- 

 ing channels all directly or indirectly connected with the main naviga- 

 tion channel. The area fished is shown in Figure 1. 



Most of the backwater lakes have no current in them except during 

 high water periods in the early spring. There is usually a slight cur- 

 rent in the connecting channels and there is always current in the main 

 navigation channel, hihere current is rapid the bottom is usually of 

 shifting sand. Where there is little or no current the bottom is usually 

 of mud or silt. 



At most places aquatic vegetation is not abundant, being limited by 

 shifting bottoms, turbidity, current, and fluctuating v.ater levels. Sub- 

 mersed aquatic plants, mainly coontail (Ceratophyllum demersum) , were 



