lected from three locations in the Deschutes 

 River, OR. Resident rainbow trout, which were 

 collected from the main stem near the mouth of 

 Nena Creek in March 1985, were mature and 

 smaller (280-450 mm FL) than the steelhead, 

 and, based on analyses of scales and otoliths 

 (McKern et al. 1974), had never entered salt- 

 water. Juvenile progeny of steelhead were col- 

 lected from Round Butte Hatchery on the 

 Deschutes River in 1984. Wild juvenile rainbow 

 trout (<200 mm FL) of unknown parental origin 

 were collected in 1984 and 1985 from Bakeoven 

 Creek, an important spawning tributary for steel- 

 head in the Deschutes River. 



Sagittae removed from rainbow trout were 

 stored in 90"^^ ethanol for up to two months. Be- 

 fore they were viewed, one otolith from each pair 

 was mounted (concave face up) with epoxy on a 

 glass slide. The back of the slide was blackened 

 with indelible ink. The otolith was ground by 

 hand with 600 grit wet sandpaper and periodi- 

 cally inspected under a light microscope at 100 x 

 until the microstructure of the nucleus, as de- 

 scribed by Neilson et al. (1985), was visible. The 

 otolith was rinsed with 59f HCl for several sec- 

 onds to remove scratches and improve resolution. 



To reduce bias, we coded each slide with a ran- 

 dom number and ordered the slides sequentially 

 for viewing. Otoliths were examined with a Zeiss^ 

 dissecting microscope at 125 x. A camera lucida 

 attachment enabled us to use a computer digitizer 

 to measure three dimensions of the otolith. In 

 measuring length and width of the central nu- 

 cleus, we used the first growth increment encir- 

 cling all the central primordia, which was the 

 nuclear boundary defined by Neilson et al. (1985). 

 In addition, we measured the maximum length 

 along the longest axis through an area defined by 

 the first metamorphic check, a narrow hyaline 

 ring surrounding an opaque ring with a hyaline 

 center, to replicate the measurements of Rybock 

 et al. (1975). 



We used analysis of variance (ANOVA) to test 

 for significant differences in each dimension of 

 the otolith nuclei among groups in our study. 

 Where adequate data were available, we tested 

 for significant differences between groups in our 

 study and similar groups described by Rybock et 

 al. (1975) and Neilson et al. (1985) for mean di- 

 mensions of otolith nuclei. Neilson et al. (1985) 

 showed that the mean length of otolith nuclei for 



rainbow trout incubated at 6.5°C was signifi- 

 cantly less than those for trout incubated at 9.5° 

 or 15.0°C. Because of this discrepancy, we evalu- 

 ated the potentially confounding effects of incuba- 

 tion temperature on the comparisons of otolith 

 dimensions between our samples and those of Ry- 

 bock et al. (1975), by testing the hypothesis that 

 water temperatures during 1967-69 were higher 

 than those during 1982-83. We used a paired t- 

 test of average daily water temperatures recorded 

 by the U.S. Geological Survey on the 1st and 15th 

 day of each month from 1 January to 1 August 

 during 1967-69 and 1982-83 (U.S. Department 

 of the Interior Geological Survey 1967, 1968, 

 1969, 1982, 1983 1. These dates represent the incu- 

 bation periods for most of the resident rainbow 

 and steelhead trout sampled in our study and by 

 Rybock et al. (1975). Incubation temperature for 

 steelhead at Round Butte Hatchery is from hatch- 

 ery records. We estimated spawning and incuba- 

 tion periods for resident rainbow and steelhead 

 trout on the basis of reports of the Oregon Depart- 

 ment of Fish and Wildlife (Fessler 1972) and per- 

 sonal observations. 



Results 



For each dimension, we failed to reject the hy- 

 pothesis (a = 0.05) that rainbow trout collected 

 from different populations for our study had 

 otolith nuclei of the same size (Table 1). There- 

 fore, we concluded that these dimensions could 

 not be used to discriminate between the resident 

 and steelhead forms of rainbow trout sampled in 

 our study. 



Water temperatures during 1967-69 were 

 slightly greater than those during 1982-83 

 (t = 2.03, df = 14, P = 0.03). Mean difference be- 

 tween the two periods was 0.8°C. Spawning dates 

 for resident rainbow trout and steelhead differ; 

 steelhead spawn from January to April and resi- 



Table 1. — Means, standard errors (in parentheses), and sample 

 size for three otolith dimensions in resident rainbow trout and steel- 

 head from three Deschutes River populations. 



2Reference to trade name does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



161 



