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Fishery Bulletin 88(4) 1990 



diets of the trout species (juvenile roctcfishes, hex- 

 agrammids, anchovy, Cancer spp. megalopae, and in- 

 sects) are commonly found in the neustonic layer 

 (Brodeur et al. 1987b, Shenker 1988) which suggests 

 that these trouts feed in surface waters. The occur- 

 rence of a juvenile salmon in the diet of juvenile cut- 

 throat is noteworthy since larger individuals (>300 

 mm) of this species were identified as one of the few 

 fish predators on juvenile salmonids among the 20 

 species of nekton examined from these same purse 

 seine catches (Brodeur et al. 1987a). 



Dietary overlap of 39% between cutthroat and steel- 

 head trout based on percent similarity was higher than 

 the overlap values between these species of trout and 

 the juveniles of four species of salmon caught in purse 

 seines, with two exceptions. The highest overlap values 

 were between cutthroat trout and juvenile chinook 

 salmon (49%) because of the common utilization of 

 fishes in the diet, and between juvenile steelhead trout 

 and juvenile sockeye salmon (43%) because of the com- 

 mon occurrence of euphausiids. 



The average growth rate of cutthroat trout in the 

 ocean based on our scale analysis was 0.8 and 1.0 

 mm/day (based on 1 May and 10 May dates of ocean 

 entry, respectively) and 1.2 mm/day (based on change 

 in back-calculated ocean growth with time). These 

 estimates are similar to the growth rates of about 1.0 

 mm/day for age 2. wild cutthroat and 0.9 mm/day for 

 hatchery cutthroat, assuming a 3-month ocean resi- 

 dence (Giger 1972) and 0.7-0.8 mm/day for fish after 

 their first 5-6 months in the ocean (Sumner 1962). 

 Johnston (1982) reported an average growth rate of 

 1.0 mm/day during the time spent at sea for cutthroat 

 trout stocks from the Columbia River, coastal rivers, 

 and Puget Sound rivers. Our average growth rate for 

 steelhead of 1.1 mm/day (based on a 17 May date of 

 ocean entrance) is similar to that for cutthroat trout. 

 It is about the same as the average ocean growth of 

 1.3 mm/day calculated by Everest (1973), the 0.8 mm/ 

 day estimated by K. Kenaston (Oreg. Dep. Fish Wildl., 

 Corvallis, pers. commun. 16 June 1989) for summer- 

 run steelhead "half-pounders" of the Rogue River, and 

 the 1.5 mm/day estimated for first-year ocean growth 

 of steelhead from Vancouver Island (Hooten et al. 

 1987). Data on ocean growth of steelhead during their 

 first year in the ocean is also provided by recovery of 

 fish with coded-wire tags. These include growth rates 

 of about 1.0 mm/day for a fish caught in the Gulf of 

 Alaska, based on the mean size of the same tag code 

 of steelhead smolts when caught about 60 days earlier 

 in the Columbia River (Pearcy and Masuda 1982), and 

 growth rates of 0.7, 1.0, and 1.2 mm/day for coded-wire 

 tagged steelhead recovered 198, 186, and 173 days, 

 respectively, after release (Pacific States Marine Fish- 

 eries Comm. unpubl.). Lengths at release were esti- 



mated from release weight using the length-weight 

 relationships for steelhead smolts given by Everest 

 (1973). The average growth rate of steelhead during 

 their first full year in the ocean was about 1.0 mm/day 

 for fish returning to California streams (calculated from 

 data in Shapovalov and Taft 1954), 0.6 mm/day for fish 

 caught on the high seas (Sutherland 1973), and 0.85 

 mm/day for fish from the Keogh River, British Colum- 

 bia (Ward and Slaney 1988). These estimates suggest 

 fairly similar growth rates for both cutthroat and 

 steelhead trout during early ocean life. Apparently cut- 

 throat trout are small when they return to spawn 

 because they spend less time in the ocean, not because 

 of an inherently lower growth rate. 



If coastal cutthroat trout have the potential for a 

 large increase in size during ocean life, why do they 

 curtail marine growth by returning to freshwater each 

 winter rather than remaining in the ocean for several 

 years like most steelhead trout? We present three 

 hypotheses for the adaptive value of this behavior. 



Early maturation of cutthroat trout, after only a few 

 months in the ocean, may have evolved as a response 

 to low survival (Cole 1954), either in the ocean or fresh- 

 water, especially if postreproductive survival is low 

 after the age at first breeding (Schaffer 1974). Cut- 

 throat trout do not appear to have distinctly lower 

 ocean sui-vival than steelhead, however, based upon the 

 reports in literature. Giger (1972) states that coastal 

 cutthroat exhibited comparatively high rates of survival 

 during summer periods of ocean residence between 

 spawnings. He estimated ocean survival rates of 

 20-40%! for hatchery cutthroat smolts between release 

 in the spring and return in the fall, and survival rates 

 of 14-39%, 17-35%. and 12-25% between first and 

 second, second and third, and third and fourth spawn- 

 ings, respectively, based on trap or net catches of fish 

 returning to four Oregon coastal streams. Sumner 

 (1962) estimated that 17-50% of coastal cutthroat trout 

 survived between successive spawnings, and Jones 

 (1978) reported marine survival of 17%. Tomasson 

 (1978), on the other hand, noted that 92% of the fish 

 returning to the Rogue River were first migi-ants, sug- 

 gesting low survival of repeat spawners, and Michael 

 (1983, 1989) reported marine survival rates of 2-20% 

 between smolt outmigration and first return to fresh- 

 water for coastal cutthroat trout. 



Cutthroat survival to an ocean age of 2, the age when 

 many steelhead trout return for their initial spawning 

 (Wit'hler 1966, Shapovalov and Taft 1954, Chapman 

 1958), is about 5% (assuming 20% survival from smolt 

 to first spawning and 25% survival from first to second 

 spawning, based on Giger's estimates), a value which 

 is close to the average for smolt to adult return foi- 

 steelhead (Bley and Moring 1988), but less than the 

 mean ocean survival of 16% based on maiden-run fish 



