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



699 



a = the y intercept of the regiTssion of FL on 

 scale radius. 



The y intercepts (a) for each species were determined 

 from the geometric mean regressions of FL and scale 

 radius (Ricker 1973) from juvenile fish caught in the 

 ocean (cutthroat repeat spawners excluded): 



Steelhead 



FL{mm) = 51.1 + 1.69 -Sc (mm at 88xX n =84, 

 r = 0.73, FL range = 143-288 mm 



Cutthroat 



FL(mm) = 62.1 + 2.26  Sc (mm at 88 x), n = 101, 

 r = 0.70, FL range = 175-369 mm. 



Sc-FL relationships for both species appeared linear 

 over the length ranges of fish examined. Back-calcu- 

 lation of size at ocean entrance using natural logarithm 

 transformations of FL and Sc (Bartlett et al. 1984, 

 Hooten et al. 1987) produced only very small differ- 

 ences {x 1 mm) in back-calculated lengths compared 

 with untransformed data. 



We estimated ocean growth for each steelhead and 

 cutthroat trout by subtracting the back-calculated FL 

 at time of ocean entry from the FL at time of capture 

 in the ocean. Growth rates in the ocean of individual 

 fish were estimated from ocean growth back-calculated 

 from scales divided by estimated days in the ocean. To 

 arrive at estimates of time spent in the ocean we used 

 dates of ocean entry corresponding to the beginning, 

 middle, and end of the period of smolt migration into 

 the ocean, yielding several estimates of growth rate 

 for each fish. Seaward migration of cutthroat trout 

 smolts through the Columbia River and coastal Oregon 

 estuaries generally takes place from late March or early 

 April through May (Giger 1972. Loch and Miller 1988, 

 Dawley et al. 1985), with maximum numbers of smolts 

 occurring in the Alsea estuary in the second week of 

 May (Giger 1972). By the end of May very few cut- 

 throat tr(.)ut smolts were found in the Alsea estuary 

 (Giger 1972). Almost all steelhead smolts passed river 

 kilometer 75 in the Columbia River estuary between 

 15 April and 15 June, with half the migration completed 

 by the third week in May (Dawley et al. 1985). We used 

 ocean entry dates of 1 April, 1 May, 10 May, and 31 

 May to estimate growth rates of cutthroat trout, and 

 15 April, 1 May, and 17 May to estimate growth rates 

 of steelhead. Although ocean entry of steelhead smolts 

 probably continued until about the middle of June, 

 almost all of our catch of steelhead occurred before 15 

 June. 



We also estimated average growth rate of cutthroat 

 trout from the slope of the regi'ession of back-calculated 

 ocean growth on Julian date, a method in which no 



assumptions are made about a precise time of ocean 

 entry. This method was possible because captures of 

 cutthroat in the ocean extended well past the period 

 of smolt entry into the ocean. 



Stomach contents were preserved in 10% buffered 

 formalin, transferred to 70% ethanol prior to examina- 

 tion, and weighed to the nearest mg after removal of 

 excess moisture. Contents were examined with a 

 dissecting microscope and were identified to the lowest 

 possible taxon. Total lengths of intact fish prey were 

 measured to the nearest mm. Percent frequency of 

 occurrence in non-empty stomachs (F), percent of the 

 total number of prey organisms (N), and percent of the 

 total wet weight of prey organisms (W) were deter- 

 mined and combined in the Index of Relative Impor- 

 tance (IRI = F(N -I- W)). Dietary overlaps were calcu- 

 lated from a Percent Similarity Index (= 2 minimum 

 percent weights of taxa in common between two 

 groups of fish). A total of 67 cutthroat (<300 mm in 

 length) and 98 steelhead stomachs were examined from 

 the collections made in 1980-85. The food habits of 48 

 larger (>300 mm) cutthroat collected during this study 

 were presented by Brodeur et al. (1987a). 



Results 



Abundance 



A total of 163 cutthroat and 134 juvenile steelhead 

 trout were collected during our cruises in 1981-85. 

 These species combined comprised about 3% of our 

 catches of juvenile salmonids (Pearcy and Fisher In 

 press). Cutthroat and steelhead trout were captured 

 from May through August, and occurred in 0-30%i and 

 2-24%i, respectively, of the seine sets during these 

 months (Table 1). 



Frequencies of occurrence and catches per set of 

 steelhead were generally highest in May and June and 

 were much lower in July and August (Table 1). Steel- 

 head were absent in September of all years. Abun- 

 dances of cutthroat trout in 1981, the year of most 

 regular sampling, were highest in July and about the 

 same in May, June, and August. Like steelhead, cut- 

 throat trout were absent in September of all years 

 (Table 1). 



Distributional trends 



In most years, average catches of cutthroat trout were 

 higher in the region off southern Washington and 

 northern Oregon, near the mouth of the Columbia 

 River (Zone B), than off northern Washington (Zone 

 A) or Oregon south of the Columbia River (Zone C) 

 (Table 2). No obvious latitudinal trends were noted 



