Horizontal and Vertical Distribution of Juvenile 

 Salmonids in Upper Mayfield Reservoir, Washington 



By 



JIM ROSS SMITH, JOHN R. PUGH, and GERALD E. MONAN 



Fishery Biologists 



Bureau of Commercial Fisheries Biological Laboratory 



Seattle, Washington 98102 



ABSTRACT 



The proposed installation of a fish collecting device in the upper end of Mossy- 

 rock Reservoir, soon to be created on the Cowlitz River in southwestern Washington, 

 prompted this study to determine the horizontal and vertical distribution of juvenile 

 salmonids in such an environment. A section of the upper end of Mayfield Reservoir, 

 an existing body of water on the Cowlitz River, was systematically sampled with 

 gill nets and a trawl from April 1964 to June 1965. Eighty-seven percent of the 

 11,467 salmonids captured were taken in the upper 7.3 m. of water, which consti- 

 tuted 52.8 percent of the total sampling area. 



If the distribution of fish in upper Mossyrock Reservoir is similar to the dis- 

 tribution in Mayfield Reservoir, a collecting device running from shore to shore, 

 extending to a depth of 7.3 m., and designed to collect fish approaching from both 

 the upstream and the downstream sides should collect over 80 percent of all mi- 

 grating salmonids. 



INTRODUCTION 



Development of hydroelectric power is grad- 

 ually changing the fast-flowing rivers and 

 streams of the northwestern United States to a 

 network of reservoirs. This new environment 

 can alter the migration rates of young sal- 

 monids, prolong their stay in fresh water, and 

 subject them to increased predation. 



Until recently, most of the effort to pass 

 juvenile fish around hazardous turbines and 

 spillways at high dams has been with collec- 

 tion systems at the lower ends of reservoirs, 

 but even the most efficient of such systems 

 would not be satisfactory if downstream mi- 

 grants failed to reach the lower reservoir. 

 Fishery agencies, therefore, are considering 

 other methods, one of which is to collect 

 migrants in the upper reservoir. The 

 feasibility of such a system depends to 

 a large extent on the distribution of the 

 fish. 



Rees (1957) investigated the distribution of 

 seaward migrant salmonids in Baker Reser- 

 voir in northwestern Washington; Erho studied 

 the vertical distribution of smolts of coho 

 salmon ( Oncorhynchus kisutch ) in Merwin 

 Reservoir, southwestern Washington. Both of 

 these studies, however, were conducted in the 

 forebays, just above the dams. 



A proposal by the Washington State Depart- 

 ment of Fisheries to establish a collection 

 site in the upper end of Mossyrock Reservoir, 

 soon to be created on the Cowlitz River in 

 southwestern Washington, led us to investigate 

 the horizontal and vertical distribution of 

 juvenile salmonids in upper Mayfield Reser- 

 voir, an established reservoir on the same 

 river. 



Unpublished data. The vertical distribution of coho 

 smolts in the forebay of Merwin Dam in 1964 by Mike 

 Erho. Summary Report for the Fish-Passage Research 

 Program, U.S. Bureau of Commercial Fisheries. 



