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Oceanic feeding habits of chinook salmon, 

 Oncorhynchus tshawytscha, 

 off northern California 



Sharon L. Hunt 



Timothy J. Mulligan 



Department of Fisheries 



Humboldt State University, Areata, California 95521 



E-mail address (for T J Mulligan, contact author): t|m2@axe humboldt edu 



Kenichiro Komori 



School of Biological Sciences 



Macquarie University, New South Wales 2109, Australia 



The chinook salmon, Oncorhynchus 

 tshawytscha, is an important com- 

 mercial and recreational species 

 inhabiting rivers and nearshore 

 coastal waters from San Diego, 

 California, to the Bering Sea and 

 Japan (Miller and Lea, 1972). Many 

 West Coast populations are in a 

 serious decline (Pearcy, 1992). 

 Nehlsen et al. (1991) reported an 

 overall decrease in salmonid num- 

 bers in the coastal waters of the 

 Pacific Northwest and suggested 

 that northern California chinook 

 salmon runs may be at high risk of 

 extinction owing to 1 ) habitat dam- 

 age and mainstream passage prob- 

 lems; 2) overharvesting: and 3) hy- 

 bridization, predation, competition, 

 disease, and poor ocean survival 

 conditions. Surprisingly, little re- 

 search has been done off the north- 

 ern California coast regarding the 

 diet of salmonids during oceanic 

 migrations. 



Studies on the feeding habits of 

 adult chinook salmon have been 

 conducted from San Francisco, 

 California, to southeastern Alaska. 

 Northern anchovy (EngrauUs mor- 

 (iax), juvenile rockfishes iSebastes 

 spp. ), euphausiids. Pacific herring 

 (Clupea pallasii), osmerids, and 

 crab megalopae (Cancer magister) 

 have been reported as main prey 

 items of chinook salmon ranging 



from San Francisco to the Washing- 

 ton coast (Heg and Van Hyning, 

 1951; Merkel, 1957; Petrovich, 

 1970;Brodeuretal., 1987). Various 

 studies conducted in more northern 

 regions of the Eastern Pacific 

 Ocean have shown Pacific herring 

 and Pacific sandlance (Ammodytes 

 hexapteriis) as dominant food items 

 (Pritchard and Tester, 1944; Reid, 

 1961; Prakash, 1962). All of the 

 above studies noted seasonal and or 

 annual differences in the dominant 

 prey items. Overall, it appears that 

 northern anchovy and rockfishes are 

 the most important prey items for 

 chinook salmon in southern coastal 

 regions (i.e. San Francisco Bay 

 area) whereas the importance of 

 Pacific herring and Pacific sand- 

 lance increases in more northern 

 regions (Healey, 1991). To ad- 

 equately describe the trophic re- 

 sources utilized by a fish popula- 

 tion, it is necessary to sample at 

 consistent times throughout the 

 year (Bowen, 1996). Furthermore, 

 in an upwelling zone, such as north- 

 ern California, the food habits of 

 many fish species may fluctuate 

 considerably between years owing to 

 environmental variability (Brodeur 

 and Pearcy, 1992). For example, di- 

 ets of pelagic nekton may vary ow- 

 ing to changes in oceanographic con- 

 ditions, such as onshore and along- 



shore transport, primary productiv- 

 ity, and prey abundances (Brodeur 

 et al., 1987). In our study, we exam- 

 ined the diet of chinook salmon off 

 northern California. The main objec- 

 tives were 1 ) to compare the diet be- 

 tween two consecutive years and 2) 

 to examine seasonal variation in the 

 prey items consumed. 



Materials and methods 



Stomach samples were collected 

 from fish caught in coastal waters 

 off Humboldt Bay (40°46'N, 124° 

 14'W), Trinidad Bay (41°03'N, 

 124°09'W), and Crescent City 

 (41°46'N, 124°13'W), California 

 (Fig. 1). From May through Sep- 

 tember 1994, 196 stomachs were 

 collected from Chinook salmon 

 from the three areas. During 1995, 

 112 stomachs were collected from 

 the same ports but only in June and 

 September. Approximately 60*^, 

 10%, and 30% of the total stomachs 

 collected were taken from Hum- 

 boldt Bay, Trinidad Bay, and Cres- 

 cent City, California, respectively, 

 in both years. Stomach collections 

 were obtained from California De- 

 partment of Fish and Game (CDF 

 &G) port samplers and directly from 

 sportfishermen. Owing to changing 

 and sporadic season opening dates, 

 as well as to varied placement of port 

 samplers and weather conditions, an 

 unbiased random sampling scheme 

 was not possible. All fish were ob- 

 tained fi'om the recreational fishery 

 and were greater than or equal to 22 

 inches (>56 cm), the CDF&G mini- 

 mum size limit. Total length mea- 

 surements were obtained for 54 fish 

 in 1994 and ranged from 59 to 96 cm 

 (.v=74 cm). Although no fish were 

 measured in 1995, they were simi- 

 lar in size to those collected in 1994. 

 This finding suggests that all fish 

 sampled were three to five years of 

 age (Healey, 1991). 



Manuscript accepted 23 October 1998. 

 Fish. Bull. 97:717-721 (1999). 



