250r 



RAINBOW TROUT 

 (n= 1,852) 



t 



_1 I I 1 1 L. 



STEELHEAD TROUT 

 (n-- 1.816) 



_1 I 1_ 



20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 

 FORK LENGTH (cm) 



Figure 4. -Length-frequency distribution of mature (>20 cm) 

 rainbow trout and steelhead trout from the lower Deschutes 

 River, Oreg., 1972. 



spawn, the data indicate low likelihood of sig- 

 nificant overlap in length of mature rainbow and 

 steelhead trout. 



The mean egg size of steelhead (0.0936 ml) was 

 significantly greater (P< 0.001) than that of rain- 

 bow (0.0727 ml). Also, mean egg size was strongly 

 correlated with length of female (r = 0.829 and 

 0.791 for rainbow and steelhead trout, respec- 

 tively) (Figure 5), although there was much 

 variability of mean egg sizes between fish of a 

 similar length and of egg sizes from any one 

 female. For some fish, the largest egg was twice 

 the size of the smallest. 



The above r values between body size of dam 

 and egg size are higher than those reported in 

 many other investigations. Scott (1962) measured 

 FL and egg weight of rainbow trout and found no 

 significant correlation. Considering the narrow 

 range of FL (231-264 mm) and the great 

 variability of egg size within length classes, his 



18 

 16 



i 14 

 b 



3 12 

 5 10 



O = RAINBOW TROUT 



y = 0.341 + O.OISx 

 (j; = 0.829) 



m-- STEELHEAD TROUT 

 y = -2.974 + 0.196? 



(r = 0.791) 



1 F 



!i 



34 38 42 46 50 54 58 62 66 70 74 78 82 86 

 FORK LENGTH (cm) 



Figure 5.-Means and ranges of egg size plotted against length 

 of dam for rainbow trout and steelhead trout from the lower 

 Deschutes River, Oreg., 1972. 



results are not surprising. Galkina (1970) found 

 that length of rainbow trout, S. irideus, was not 

 highly correlated with mean egg weight (r = 0.48). 

 Although eggs of average size were found in all his 

 females, the smallest eggs were obtained only 

 from smaller females and vice versa. McFadden et 

 al. (1965) found a higher correlation (r = 0.73) 

 between egg size and length of brown trout, S. 

 trutta. Blaxter (1969), Galkina (1970), and Lindsey 

 and AH (1971) cited numerous authors who 

 examined this relationship in many species of fish; 

 although most authors reported a wide range of 

 egg sizes in females of similar length, there is 

 general agreement that a direct, and often high, 

 correlation exists between egg size and dam size. 



The presence or absence of any correlation 

 between egg size and ON size of the hatched fish 

 could not be determined directly for the rainbow 

 trout groups reared in Corvallis (egg size had been 

 measured for only four of the eight females whose 

 offspring were available, and we considered this 

 sample size too small). However, since there was a 

 high correlation between egg size and length of 

 dam (r = 0.829), this latter measurement was 

 regressed against ON size of offspring from the 

 eight matings (Figure 6). The r value of 0.489 and 

 the overlap of ranges indicate the relationship is 

 not strong; however, it is a positive correlation. 

 Also, the small sample size (8), the narrow range of 

 dam lengths (295-415 mm), the variation of egg 

 size within any dam, and the substitution of FL 

 for egg size are factors which may have obscured 

 the real extent of the relationship of fish size to ON 

 size. 



In summary, we found that because steelhead 

 ON are larger than rainbow ON, size of ON does 

 not change with growth of either race, and 

 correlations of ON size between dams and sires 

 and between wild and hatchery-reared fish of 

 either race are insignificant, ON size is an effec- 

 tive means of differentiating juvenile steelhead 

 trout and juvenile rainbow trout regardless of sex 

 or origin. We also concluded that because 

 steelhead trout are larger at maturity than rain- 

 bow trout and because egg size is a direct function 

 of body size, eggs of steelhead trout are larger 

 than those of rainbow trout; and although we did 

 not conclusively demonstrate that ON size is 

 directly related to egg size, other evidence was 

 offered to support the hypothesis that larger egg 

 size was the mechanism responsible for larger ON 

 size in steelhead trout as compared to rainbow 

 trout. 



658 



