and steelhead trout (r = 0.916). Neither seemed 

 easier to read. Primarily due to problems of 

 developing methodology, 189 ON (29% of 641 

 examined) were not sufficiently distinct to permit 

 measurement. Usually, the hyaline center of the 

 nucleus was visible, but the metamorphic check 

 could not be distinguished. Because measurement 

 of the larger dimension would likely be more 

 precise than widths, ON length was used in the 

 following analyses. 



The mean ON lengths of steelhead trout (0.354 

 mm; n = 114) and rainbow trout (0.245 mm; n = 

 145) differed significantly (P< 0.001). The length- 

 frequency plot of these data (Figure 3A) demon- 

 strates an overlap of lengths. Most unexpected in 

 this plot are the nucleus lengths for steelhead less 

 than 0.26 mm. These values occur in direct 

 proportion to the values for rainbow; also, these 

 steelhead are from Wizard Falls Hatchery, where 

 both rainbow and steelhead are reared. Perhaps 

 these fish are rainbow offspring which were inad- 

 vertently mixed with steelhead during hatchery 

 operations or hybrids of the two species. The 

 length-frequency plot of steelhead ON excluding 

 those from Wizard Falls Hatchery increased the 

 normality of the histogram (Figure 3B); the nadir 



ta = RAINBOW TROUT 



(n=l45) 

 O = STEELHEAD TROUT 



(n=ll4) 



16 20 24 28 32 36 40 44 48 52 



^ = RAINBOW TROUT 



(n= 145) 

 EZ3 = STEELHEAD TROUT 



(n=72) 



16 20 24 28 32 36 40 44 48 52 



LENGTH OF OTOLITH NUCLEUS (10"^ mm) 



Figure 3.-Length-frequency distribution of otolith nuclei of (A) 

 rainbow trout and all steelhead trout and (B) rainbow trout and 

 steelhead trout excluding those from Wizard Falls Hatchery. All 

 fish. were captured from the lower Deschutes River, Oreg., 

 1971-73. 



at 0.46-0.48 mm is probably due to the small sample 

 size of each interval. Therefore, although overlap 

 of ON size of the two races occurs between 0.28 mm 

 and 0.34 mm, the race of most juvenile S. gairdneri 

 from the Deschutes River can be determined 

 reliably on the basis of this measurement. 



The histogram for rainbow more closely 

 approximates a normal distribution, probably the 

 result of a larger sample size and of the many 

 sources of variation operating within a more 

 narrow size range of spawning fish. The ON length 

 of one adult rainbow was 0.48 mm. Although no 

 hatchery marks were noticed, scale characteristics 

 suggested a hatchery origin; because we have ob- 

 served that almost all hatchery-reared rainbow 

 succumb to Ceratomyxa before reaching maturity 

 in the lower Deschutes River, this may have been a 

 nonmigratory steelhead. In general, though, data 

 from our samples do not support the suggestion of 

 Wagner and Haxton (1968) that there may be a 

 great number of such nonmigrants in the 

 Deschutes River. 



To determine whether size of ON changes dur- 

 ing growth of fish, we regressed length of ON 

 against FL of fish: For rainbow, r = 0.060. For all 

 steelhead, r = 0.694; however, if Wizard Falls fish 

 are excluded, r = 0.018. Even with this exclusion, a 

 wide range of steelhead FL (504-762 mm) was 

 tested; and if the relationship was strong it should 

 have been noticeable in these data. 



Mean length of ON was 0.339 mm for all females 

 and 0.317 mm for all males; they are not sig- 

 nificantly different (P >0.20). Also, the data sug- 

 gest no significant male-female difference within 

 either race. 



Mean length of ON of wild steelhead (0.395 mm; 

 n = 52) was not significantly different (P >0.20) 

 from that of hatchery-reared steelhead excluding 

 those from Wizard Falls Hatchery (0.405 mm; n = 

 20). A similar comparison between adult 

 hatchery-reared and wild rainbow cannot be made 

 since there are few, if any, adult hatchery-reared 

 rainbow in the lower Deschutes River (as men- 

 tioned earlier, hatchery fish released in spring 

 succumb to Ceratomyxa by summer). 



Fish, Egg, and ON Size Relationships 



The lengths of rainbow trout and steelhead 

 trout from the lower Deschutes River are dis- 

 tributed into discrete size ranges (Figure 4). 

 Although these fish are not necessarily ready to 



657 



