Pearcy et al.. Oncorhynchus clarki clarki and O mykiss off Oregon and Washington 



703 



15 



10 



5 





 15 

 10 



5 





 15 



10 



5 





 15 

 10 \ 



CUTTHROAT 



LiJ 



03 



:s 



FL--^'» August 



STEELHEAD 



^^ May 



Xn =223 



June 



287 



July 



g'''^'^ FFT3 



120 



FORK LENGTH (mm) 



Figure 2 



Size-frequency distributions of cutthroat and steelhead trout, by 

 month, 1981-85 combined. 



Mean back-calculated FL at time of ocean entry for 

 steelhead was 199 mm (197 mm and 200 mm for fish 

 caught in May and June, respectively) and was quite 

 variable (SD 29 mm, n = 84). These estimated lengths 

 at time of ocean entry were similar to the mean lengths 

 of steelhead smolts caught during downstream migra- 

 tion in the Columbia River (~200 mm, Dawley et al. 

 1985, their figs. 4-10), and similar to those estimated 

 by Narver (1969, 1974) for steelhead smolts in two 

 British Columbia river systems (182 and 190 mm FL), 

 but were larger than mean lengths of all ages of wild 

 smolts from two Washington streams (156 and 165 m 

 FL; Loch et al. 1988). 



The mean lengths of both species increased during 

 the summer. This suggests either growth in length dur- 

 ing this time period or higher availability of larger fish 

 later in the summer (Loch 1982, Dawley et al. 1982). 



Mean lengths of cutthroat trout caught in the ocean 

 in July and August (299 and 318 mm FL, respectively) 

 were similar to mean lengths of age 2. and 3. cutthroat 

 trout (305 and 323 mm FL, respectively) caught in 

 coastal Oregon estuaries from July through September 

 on their initial spawning migrations (Giger 1972). 



More male than female cutthroat were captured, but 

 fewer male then female steelhead were caught. How- 

 ever, the respective sex ratios of 1.3:1 and 0.7:1 were 

 not statistically different from 1:1 (p>0.1, chi-square 

 test). None of the cutthroat trout examined had en- 

 larged testes or ovaries (>1% of body weight), which 

 is expected since cutthroat spawn in the winter-spring 

 period. 



Ocean growth 



Of cutthroat trout caught in the ocean in May, June, 

 July, and August, 65, 83, 90, and 100%, respectively, 

 showed an ocean growth pattern on their scales. Mean 

 estimated ocean growth rates of cutthroat trout (in- 

 cluding fish with and without ocean growth) were 0.47, 

 0.78, 1.03, and 2.60 mm/day, assuming an ocean entry 

 date of 1 April, 1 May, 10 May, and 31 May, respec- 

 tively. Since the median date of ocean entry of cut- 

 throat trout smolts is sometime in early May (Giger 

 1972, Loch and Miller 1988, Dawley et al. 1985), 

 growth rate estimates using 1 May or 10 May as an 

 ocean entry date (0.78 and 1.03 mm/day) are probably 

 closest to the true average growth rate of cutthroat 

 trout in the ocean. Ocean growth rates based on the 

 slope of the geometric mean regression (Ricker 1973) 

 of back-calculated ocean growth and Julian date was 

 1.22 mm/day {n = 101, r = 0.67), fairly close to the mean 

 growth rates of individual fish assuming early May 

 ocean entry dates. 



Almost all juvenile steelhead with readable scales 

 were caught in May and June. Only 30% and 50%, 

 respectively, of the steelhead caught in these two 

 months showed signs of ocean growth on their scales. 

 This suggests that the steelhead caught in May and 

 June were, on average, in the coastal ocean for less 

 time than the cutthroat trout caught in the same 

 months, either because of later ocean entry or rapid 

 migration of steelhead out of coastal waters. Back- 

 calculated ocean growth rates of juvenile steelhead (in- 

 cluding fish with and without ocean growth) were 0.21, 

 0.32, and 1.06 mm/day for assumed ocean entry dates 

 of 15 April, 1 May, and 17 May, respectively. Down- 

 stream migration of steelhead smolts in the Columbia 

 River estuary was half completed around the third 

 week in May (Dawley et al. 1985). Consequently, the 

 ocean entry date of 17 May probably gives the best 

 estimate of average growth rate in the ocean (1.06 

 mm/day). This growth rate was similar to the average 



