Table 1. — Summary of limnological data for the John Day pool of 

 the Columbia River, from Hjort et al. (1981). All data collected in 

 August 1979 except for surface temperatures, which were taken in 

 1981. 



Methods 



We collected walleye for this study in the first 23 

 km (tailrace) downstream of McNary Dam in the 

 John Day pool of the Columbia River at lat. 45°55'N 

 (Fig. 1). Walleye were collected from 2 April to 30 

 September 1980 and from 30 March to 30 Septem- 

 ber 1981. In 1980, we captured walleye with either 

 a 38.1 X 1.8 m sinking gill net with 3.81, 5.08, 6.35, 

 7.52, and 10.16 cm variable stretch mesh, or a 

 76.2 X 3.7 m monofilament floating gill net with 

 15.25 cm stretch mesh. All gill net sets were of a 

 maximum 2.5-h duration. In 1981 we used these 

 gill nets and a 6.15 m electroshock boat with a 

 3,500-W generator and front-mounted electrodes, 

 utilizing pulsed DC current of 1-4 A to capture 

 walleya Sampling was conducted in the day and 

 night. 



We recorded the fork length (FL, mm), weight (g), 

 sex and whether or not the fish were sexually mature 

 (Eschmeyer 1950), and removed a scale sample from 

 beneath the tip of the left pectoral fin of each wall- 

 eya Many authors report difficulty using scales to 

 determine the age of older walleye (Carlander 

 and Whitney 1961; Campbell and Babaluk 

 1979); therefore, we took a subsample of sagitta 

 {n = 86), which we preserved in 50:50 glycerine and 

 water. 



We mounted scales between two glass microscope 

 slides and viewed them using a microfiche projec- 

 tor at 43x. We identified annuli using the criteria 

 described by Carlander and Whitney (1961). We 

 found that the easiest way to detect annuli on sagit- 

 tae was to burn the whole otolith in a flame, immerse 

 it in oil or alcohol, and examine it under a dissect- 

 ing microscopa Reburning was often necessary un- 

 til very distinct, dark annuli appeared. Christensen 

 (1964) proposed a similar technique; however, he 



broke the burned otolith and examined the cross sec- 

 tion. There was 92% agreement between at least one 

 otolith reading and one scale reading so we ter- 

 minated the collection of otoliths. We examined 

 scales and otoliths twice and a person experienced 

 in reading walleye scales (W. R. Nelson, U.S. Fish 

 and Wildlife Service, Vancouver, WA) examined a 

 subsample of scales (n = 63). 



Age determinations for walleye collected in 1980 

 were based on either two scale readings, three scale 

 readings, two scale readings and two otolith 

 readings, or three scale readings and two otolith 

 readings. All age determinations of walleye collected 

 in 1981 were based on two scale readings. There was 

 90% agreement between at least two of the five 

 possible age determinations for walleye collected in 

 1980, and 75% agreement between the two age 

 determinations for walleye collected in 1981. After 

 the final age determination, we measured the scale 

 radius and scale length to each annulus (43 x) at 

 about 45° off of a straight line from the focus 

 through the anterior field. In this area of the scale 

 it was much easier to detect the annuli because of 

 crowding and anastomosis of circuli in the lateral 

 fields. 



We back-calculated length at each annulus 

 (i.e, year of life) assuming a straight line body- 

 scale relationship (r^ = 0.69) and using the 

 Fraser-Lee method as recommended by Carlander 

 (1982): 



L, = a -t- i,- 



where L^ = fish fork length at capture 



Lj = calculated fork length at age i 

 S(. = scale radius at capture 

 Si = scale measurement at annulus i 

 a = intercept of body-scale regression = 55 

 mm. 



We converted these back-calculated fork lengths to 

 total lengths (TL) using a conversion factor of 1.06 

 FL, which is the unweighted mean of the TL/FL 

 relationships reported by Colby et al. (1979). This 

 conversion allowed us to more easily compare our 

 data with data from other areas. 



During the spring 1981 spawning season, we 

 removed the ovaries from 27 mature, but unspawn- 

 ed walleye We preserved the ovaries in Bouin's solu- 

 tion and subsequently estimated the number of eggs 

 by means of the gravimetric method recommended 

 by Wolfert (1969). We performed regressions of life 



702 



