Miller and Brodeur: Diets of and trophic relations among dominant marine nekton within the northern California Current ecosystem 557 



5 



Nekton prey 

 ^H Osteichthyes (a) 



^M Osleichlhyesll-J) 



Ostcichihycs (tissue) 

 t j Crustacea 



Euphausiidae 

 Bractiyura 



Hyperiidea 



Other 



Coho salmon 

 (yearling) 



Coho salmon 

 (adult) 



Chinook salmon 

 (yearling) 



Chinook salmon 

 (adult) 



Stcelhead trout 1 p^^^^^^^ 

 (juvenile) -I 



Figure 4 



Comparison of 2000 and 2002 nekton diets (percent wet weight) analyzed from northern California Current GLOBEC 

 (GLOBal ocean ECosystems dynamics) cruises. Scientific names of the species in the order they appear in the figure are 

 the following: coho salmon (Oncorhynchus kisutch), chinook salmon (O. tshawytscha), and steelhead trout (O. mykiss). 



Feeding by many nekton directly on phytophagous eu- 

 phausiids results in more direct food webs and fewer 

 trophic levels, and concomitant increases in food-web ef- 

 ficiency. Observations by Brodeur and Pearcy (1992) and 

 by the authors of the present study thus indicate that 

 trophic characteristics of the NCC system may have 

 periods of high production when euphausiids are con- 

 sumed across many trophic levels. Additional research 

 of trophic relationships between dominant nekton and 

 zooplankton within the NCC at varying levels of pro- 

 duction would further clarify whether this pattern is an 

 important characteristic of this ecosystem. 



Interannual variation 



Limited interannual variation in diets was observed; 

 only juvenile steelhead trout and adult coho salmon 

 expressed major differences between 2000 and 2002. 

 Other nekton, particularly yearling coho and chinook 

 salmon, showed relatively little variation in diet; only 

 euphausiids and brachyuran larvae (which represent 

 a small portion of nekton diet in general) varied most 

 between the two years. Whether this variability was an 

 artifact of sampling is uncertain; however, it is interest- 

 ing to note that the higher prevalence of euphausiids in 

 the 2000 diets and the higher prevalence of brachyuran 



larvae in 2002 diets indicated that variation between the 

 two years may have been a result of the relative abun- 

 dance of these prey. Other studies from the NCC have 

 also indicated high temporal variation in zooplankton 

 (Mackas et al., 2001, 2004) and in the diets of Pacific 

 hake (Emmett and Krutzikowsky, in press) and juvenile 

 salmonids (Brodeur and Pearcy, 1990). Although both 

 2000 and 2002 were years of high upwelling and high 

 production, substantial interannual variability in the 

 hydrographic environment was observed. During the 

 summer of 2002, an intrusion of cold, nutrient-rich, sub- 

 arctic bottom water resided on the Oregon shelf, result- 

 ing in high phytoplankton production in surface waters 

 and in hypoxic conditions near the bottom (Thomas et 

 al., 2003; Wheeler et al., 2003). Although it is possible 

 that this nutrient-rich water may have influenced the 

 prey field available to the different predators, no discern- 

 ible effect on their diet was observed in our study. 



The temporal variability in nekton diets reveals an 

 important characteristic of the NCC system in that 

 this variability reflects the duration of trophic interac- 

 tions between predator and prey in relation to popula- 

 tion-scale parameters. If variability in the time scale 

 of trophic links is short compared to nekton popula- 

 tion dynamics, then more short-lived species (such as 

 market squid) or those exhibiting a critical ontogenetic 



