over 150 mm. long. The cumulative time the 

 electric current passed through the water 

 during sampling was used to calculate CPUE 

 (catch per unit of effort). We calculated the 

 CPUE as the number of fish collected at a 

 site divided by the elapsed sannpling time in 

 hundredths of an hour. 



Smaller fish, usually those less than 150 

 mm. long, were preserved in the field and 

 examined later in the laboratory; the larger 

 specimens were processed in the field. The 

 data taken from each fish were species, fork 

 length, weight, sex, gonad development, and 

 stomach contents. The data on each fish were 

 recorded on Hollerith cards. Two species are 

 included in each of the categories of salmon, 

 lamprey, and pumpkinseed. The identifications 

 were not reliable between the juveniles of 

 coho salmon and chinook salmon; between west- 

 ern brook lamprey, Lampetra richardsoni , 

 and Pacific lamprey, L. tridentata; and be- 

 tween pumpkinseed, Lepomis gibbosus , and 

 bluegill, Iv. macrochirus . 



Temperature, resistivity, and turbidity were 

 recorded at each sampling, partly to judge 

 effects on the efficiency of electrofishing. 

 Water temperatures were highest in the late 

 spring, summer, and early fall and usually 

 increased with distance downstream (fig. 2). 

 Temperature affects electrofishing to some 

 degree, but the effect appeared to be insig- 

 nificant in this study. Water resistivity read- 

 ings progressively decreased downstream and 

 did not vary greatly at a specific location 



throughout the year (fig. 3). The lower readings 

 indicate higher concentrations of ions, which 

 increase efficiency in electrofishing. Low 

 resistivity readings also indicate richer waters 

 (McFadden and Cooper, 1962). Turbidity, which 

 was measured by a platinum wire method 

 (Welch, 1948), fluctuated considerably between 

 sampling trips and sites (fig. 4). We had 

 difficulty in seeing shocked fish when visi- 

 bility was less than 35 cm. 



On some trips, conditions such as rain, 

 snow, wind, low intensity of light, and ice also 

 hampered our observations of fish. Although 

 these conditions may have biased our results 

 toward smaller catches of fish, CPUE could 

 not be accurately corrected. 



The development of fish fauna has probably 

 been influenced by the geological history of 

 the region. The Yakima River, in west central 

 Washington, drains the east slope of the 

 Cascade Mountains and empties into the Co- 

 lumbia River about 480 km. from the sea. 

 The lower Yakima River was not covered by 

 ice during the last major glaciation during 

 the Pleistocene era (Flint, 1957). The Cas- 

 cade Mountains had valley glaciers, but the 

 Piedmont ice sheet barely reached the Co- 

 lumbia Plateau, well above the lower Yakima 

 River. 



The geography of the watershed varies con- 

 siderably. The Yakima River flows through 

 three valleys, separated by mountainous 

 ridges. The river flows through deep canyons 

 between km. 250 to 233 and 217 to 177 and 



» 5 



SEPTEMBER 18 



TO OCTOBER 18,1957 



JULY 15 TO 

 AUGUST 7. 1957 



MAY 12 TO 20. 1958 

 NOVEMBER 12 TO 

 22, 1957 



APRIL 8 TO JUNE 



20, 1957 

 MARCH 3 TO 21, 1957 



JANUARY I TO 21, 

 1958 



50 100 150 200 250 



DISTANCE FROM MOUTH OF RIVER (KM ) 



Figure 2. — Water temperatures in 1957 and 1958 at sampling sites along 

 the Yakima River, by time of sampling. 



