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Fishery Bulletin 88(4), 1990 



growth rate for cutthroat trout using an early May date 

 of ocean entry. The lack of steelhead caught late in the 

 summer precluded estimating mean growth rate from 

 change in back-calculated ocean growth with time. 



Food habits 



Cutthroat trout Fishes were by far the dominant 

 prey of juvenile cutthroat trout in terms of frequency, 

 number, and weight. Hexagrammids, scorpaenids, 

 northern anchovy Engraulis mordax, and the brown 

 Irish lord Hemilepidotus spinosus were the dominant 

 fish taxa identified (Appendix Table 1). One uniden- 

 tified Pacific salmon, Oncorhynchus sp. [81 mm total 

 length (TL)], was found in the stomach of a 221 -mm 

 cutthroat trout collected 11.4 km off the mouth of the 

 Columbia River in July 1981 . Fishes made up more than 

 75% of the biomass consumed during all years, but they 

 decreased in importance in the diet as the summer pro- 

 gressed, so that by late summer several other prey taxa 

 such as euphausiids, hyperiid amphipods, and decapod 

 larvae were important. Prey fishes found in cutthroat 

 stomachs averaged 52.5 mm TL (SD 21.8) and ranged 

 from 21 to 101 mm. No significant relation (n = 46, 

 r = 0.30, p = 0.16) was found between the lengths of 

 cutthroat trout and their fish prey. 



Steelhead The diet of juvenile steelhead trout was 

 more diverse than that of cutthroat trout. Both arthro- 

 pods and fishes were important prey items. Prey of 

 steelhead trout ranged from small barnacle larvae and 

 copepods to larger juvenile fishes and squids. Euphau- 

 siids, mainly Thysanoessa spinifera or Euphausia 

 pacifica, accounted for over 75% of the total IRI for 

 all years combined (Appendix Table 1). Fishes were 

 more important, however, on a weight basis making 

 up about 60% of the total biomass consumed. Juvenile 

 rockfishes (Sebastes spp.), sandlance Animodytes hex- 

 apterus, brown Irish lord Hemilepidotus spinosus, and 

 greenlings (Hexagrammos spp.) were the dominant fish 

 taxa identified. With the exception of barnacle cypris 

 larvae and hyperiid amphipods which were quite 

 numerous in stomachs collected at several stations, all 

 other prey taxa were relatively unimportant. No rela- 

 tionship between predator and fish prey size was found 

 (n = 38, r = 0.19, p = 0.25). However, steelhead, which 

 had a smaller mean length than cutthroat trout, con- 

 .sumed a smaller mean size (x 35.9 mm TL, SD 16.6 

 mm) and size range (7-72 mm) of prey fishes than cut- 

 throat trout. 



The relative proportions by weight of the major prey 

 categories varied substantially among the years sam- 

 pled. Usually, fishes contributed the majority of the 

 weight to the diet, but during the relatively strong 

 upwelling years of 1982 and 1985 (Fisher and Pearcy 



1988) euphausiids were more important. The largest 

 number of prey categories occurred in steelhead and 

 cutthroat stomachs during the relatively weak upwell- 

 ing summers of 1981 and 1984. 



The Percent Similarity Index between cutthroat 

 trout and steelhead was 39% for all cruises combined. 



Discussion 



Cutthroat and steelhead trout have evolved contrasting 

 migratory behavior in the ocean. Both species were ab- 

 sent in purse seine catches during September of their 

 first summer in the ocean (Miller et al. 1983, Loch and 

 Miller 1988; this paper), apparently because they 

 migrated out of the coastal ocean. Cutthroat return to 

 estuaries and freshwater streams from mid- to late 

 summer (Giger 1972, Loch 1982), whereas most steel- 

 head migrate far offshore during their first summer 

 in the ocean (Hartt and Dell 1986). 



Although some aspects of the life histories of cut- 

 throat and steelhead trout are similar, their ecologies 

 in marine waters differ. Steelhead commonly spend 

 2-3 years in the ocean and attain a large size before 

 initiating their spawning migrations to freshwater. 

 Some, however, return after only one full year in the 

 ocean, and some return, though usually do not spawn, 

 after several months in the ocean (Everest 1973). Ap- 

 parently anadromous cutthroat trout return to fresh- 

 water after only a few months in coastal waters and 

 rarely overwinter at sea. Giger (1972) did not identify 

 any coastal cutthroat that had overwintered in the 

 ocean, based on analyses of circuli patterns from scales 

 of fish returning to coastal rivers of Oregon. Fish that 

 migrated to sea in the spring invariably returned to 

 freshwater in the summer or fall of the same year. 

 Similarly, Armstrong (1971), Jones (1982) and Loch, 

 and Miller (1988) did not report any cutthroat that 

 spent an entire year in the ocean. J. Johnston (Wash. 

 Dep. Wildl., Olympia, pers commun. 8 March 1990) 

 found a few cutthroat trout in Hood Canal, WA, dur- 

 ing February and March, fish that evidently over- 

 wintered in marine waters. Most cutthroat spawn after 

 their return to freshwater, but according to Johnston 

 (1982) a large percentage of the Columbia River, Puget 

 Sound, British Columbia, and Alaska stocks of cut- 

 throat overwinter but do not spawn in freshwater after 

 their first summer in the ocean. Jones (1977) reported 

 that less than 50% of the cutthroat migrating into 

 Petersburg Creek in southeast Alaska were approach- 

 ing sexual maturity. In some instances, coastal cut- 

 throat may migrate downstream but not into the ocean. 

 Smolts from the Cowlitz River in Washington may re- 

 main in the Columbia River estuary for a year (Tipping 

 1981 as cited by Johnston 1982), and some adults that 



