In the spring and summer of 1964, surface 

 temperatures were generally higher than those 

 at depths, but the experimental area had no 

 definite thermocline. Water temperatures be- 

 came stratified in the fall; between the surface 

 and 21.3 m., the temperature decreased from 

 15.5° to 12.2° C. in September and from 12.2° 

 to 9.9° C. in October. From November through 



February this section of the reservoir was 

 nearly homothermous, and the percentage of 

 fingerling chinook salmon captured in the upper 

 layers decreased. 



The mean monthly flow of the Cowlitz River 

 during 1964-65, as recorded 25 km. upstream 

 at Kosmos, Wash., was compared with monthly 

 catches (fig. 6). Juvenile coho salmon and 



i i r 



APRIL JUNE AUG. 



1964 



JUNE 



Figure 6.' 



-Monthly changes in the catch of juvenile salmonids per net day in Mayfield Reservoir, and mean monthly 



flows in the Cowlitz River. 



rainbow trout reached their greatest abundance 

 during maximum water discharge in June; the 

 largest catch of age-group I chinook salmon 

 per unit of fishing effort was during the period 

 of minimum water flow inSeptember. Stockley 3 

 found a similar timing pattern for coho sal- 

 mon; however, he found age -group I chinook 

 salmon most abundant from October to Janu- 

 ary, the period of winter floods. 



3 Unpublished data. The migration of juvenile salmon 

 past the Mayfield Dam site, Cowlitz River, 1955 and 1956. 

 Washington State Department of Fisheries. 



SUMMARY AND CONCLUSIONS 



Information on the horizontal and vertical 

 distribution of juvenile salmonids was needed 

 to assess the feasibility of installing a collec- 

 tion system at the upper end of Mossyrock 

 Reservoir, soon to be formed on the Cowlitz 

 River near Mossyrock, Wash. Monofilament 

 nylon gill nets were systematically fished 

 across a section of the upper end of Mayfield 

 Reservoir, an established reservoir on the 

 same river, from April 6, 1964, to June 17, 

 1965. Trawling was done during the spring of 



10 



