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



707 



given by Ward and Slaney (1988). Thus survival rates 

 to ocean age 2 of these two species appear to be roughly 

 similar, as is survival of repeat spawners of cutthroat 

 trout and steelhead trout (Giger 1972, Withler 1966, 

 Ward and Slaney 1988). Lower ocean survival or lower 

 postreproductive survival of cutthroat trout than steel- 

 head may not be a cogent explanation for cutthroat 

 trout spawning at an early age and small size. 



The second hypothesis is that small size at maturity 

 in cutthroat trout relative to steelhead trout may have 

 evolved as a result of the distance and rigor of spawn- 

 ing migrations. Schaffer and Elson (1975) concluded 

 that the mean age of first spawning of Atlantic salmon 

 increased with the difficulty of upstream migration, as 

 estimated by the distance ascended into freshwater. 

 Coastal cutthroat rarely penetrate inland more than 

 160 km (Johnston 1982), and hence may not need the 

 swimming performance or energy reserves required for 

 the long and arduous upstream migrations that some 

 steelhead undertake at the time of maturity. 



Finally, the small size at maturity attained by cut- 

 throat trout may permit utilization of small, shallow 

 tributaries for spawning and rearing where interspe- 

 cific competition with other anadromous salmonids is 

 reduced. Small streams are known to be important 

 spawning and rearing areas of cutthroat trout (DeWitt 

 1954, Needham and Gard 1959, Lowry 1965, Johnston 

 1982, Trotter 1989). Sea-run cutthroat trout general- 

 ly spawn in tributaries with a lower velocity and shal- 

 lower depth than steelhead (Hunter 1973). Hartman 

 and Gill (1968) reported that where both anadromous 

 cutthroat trout and steelhead were sympatric, juvenile 

 cutthroat were predominant in headwater tributaries 

 and steelhead in larger river reaches. Cutthroat trout 

 are behaviorally subordinate to steelhead and coho 

 salmon in agonistic encounters (Nilsson and Northcote 

 1981, Glova 1986, Griffith 1988) and their populations 

 appear to be suppressed by competition from anad- 

 romous salmonids (R. House, Unpubl.). The lack of mor- 

 phological specialization of cutthroat trout to either fast 

 or slow water may be another reason why this species 

 is dominated by coho salmon and steelhead in areas of 

 sympatry (Bisson et al. 1988). Anadromous cutthroat 

 are known to penetrate, spawn, and rear farther into 

 a watershed than steelhead trout (Michael 1983), 

 sometimes above natural falls or log jams (Mitchell 

 1988; R. House, Unpubl.) that were considered to be 

 barriers to anadromous salmonids (see Michael 1983). 

 Therefore, small size at maturity may be adaptive by 

 alkjwing anadromous cutthroat trout to spawn and rear 

 in numerous small tributaries of coastal streams where 

 other salmonids are absent or less abundant. 



Acknowledgments 



We thank B.R. Ward (British Columbia Recreational 

 Fisheries Branch), J. Nicholas and K. Kenaston (Ore- 

 gon Department of Fish and Wildlife), J. Light (Fish- 

 eries Research Institute, University of Washington), 

 J. Dambacher and B. Hicks (Department of Fisheries 

 and Wildlife, Oregon State University), and two 

 anonymous reviewers for helpful comments on the 

 manuscript. 



This publication is a result of research sponsored by 

 the Northwest and Alaska Fisheries Center (NA-87- 

 ABH-00014 and NA-88-ABH-00043) and the Oregon 

 State University Sea Grant College Program (Grant 

 No. NA-81-AA-b-00086, R/OFP-17). 



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