TROPHIC RELATIONS OF THE BLUE ROCKFISH, 

 SEBASTES MYSTINUS, IN A COASTAL UPWELLING SYSTEM 



OFF NORTHERN CALIFORNIA 



Edmund S. Hobson and James R. Chess^ 



ABSTRACT 



The planktivorous Sebastes mystinus in nearshore habitats off northern California feeds primarily on 

 relatively large, gelatinous zooplankters that originate offshore, including thaliaceans, ctenophores, and 

 pelagic hydrozoans. These prey organisms increase in number during spring and summer when surface 

 waters driven seaward by northerly winds carry upwelled nutrients to diatoms that nourish offshore 

 zooplankton populations. But the resulting increases in zooplankton during this upwelling season become 

 available to S. mystinus in the nearshore habitats only when the surface flow turns shoreward during 

 intermittent episodes of downwelling. Although some of this shoreward flow is driven by southerly winds, 

 much of it occurs during calms, or under northerlies lacking the velocities needed to drive surface waters 

 seaward. There is increasing shoreward transport during fall and winter, when downwelling episodes 

 are more frequent, but progressively fewer zooplankters are carried into the nearshore habitats. This 

 is because as less nutrients come into the system with the reduced upwelling, and as available sunlight 

 declines, the offshore zooplankton populations suffer from shortages of diatoms. Although S. mystinus 

 compensates for decreased numbers of zooplankters during most of the year vnth increased consump- 

 tion of specific plant materials, i.e., Nereocystis sori, or the monostromatic epiphytes Porphyra nereocystis 

 and Smithora naidum (depending on the season), these too are in short supply during winter. In winter, 

 therefore, S. mystinus experiences its poorest feeding conditions. Thus, S. mystinus is adapted to feeding 

 opportunities created by alternating episodes of strong upwelling and strong downwelling, and is most 

 abundant within its range along the west coast of North America where both conditions are well developed. 



Coastal marine fishes in temperate latitudes experi- 

 ence major seasonal changes in their environment. 

 In study off northern California's Mendocino coast 

 (lat. 39°13'N, long, 123°14'W), we studied the ef- 

 fects of seasonal change on the trophic relations of 

 the blue rockfish, Sebastes mystinus (Fig. 1). 

 Sebastes mystinus, a major species in the recrea- 

 tional fishery off northern and central California 

 (Frey 1971), is a planktivore that feeds on scypho- 

 zoans, ctenophores, copepods, amphipods, thalia- 

 ceans, fishes, and algae (Gotshall et al. 1965; Love 

 and Ebeling 1978; Hallacher and Roberts 1985). 

 Although its diet is known to vary with the season, 

 relationships involving specific environmental 

 features, and the availability of prey, remain 

 unclear. 



The marine environment off California is pro- 

 foundly affected by seasonal variations in wind- 

 driven movement of the surface water (Reid et al. 

 1958; Bolin and Abbott 1962; Bakun and Parrish 



^Southwest Fisheries Center Tiburon Laboratory, National 

 Marine Fisheries Service, NOAA, 3150 Paradise Drive, Tiburon, 

 CA 94920. 



1980). This force, known as Ekman transport 

 (Ekman 1905), is strongest off the Mendocino coast 

 (Bakun 1973), where it must be a major influence 

 on the trophic relations of S. mystinus. Because 

 Ekman transport is the basis of coastal upwelling 

 (e.g.. Smith 1968), it is of great importance to 

 biological productivity (e.g., Boje and Tomczak 1978) 

 and, therefore, to the availability of food. 



Upwelling develops along the coast of northern 

 California when surface waters driven seaward by 

 northerly winds, such as those characteristic of 

 spring and summer, are replaced by colder subsur- 

 face waters from offshore that flow up into the near- 

 shore habitat (Smith 1968; Bakun 1973). On the 

 other hand, an opposing condition develops when 

 surface waters driven shoreward by southerly 

 winds, such as those characteristic of winter storms, 

 flow over the colder nearshore waters to produce 

 the condition sometimes called downwelling (Bakun 

 1973). But despite the strong seasonality evident in 

 both upwelling and downwelling, short-term rever- 

 sals lasting just a few days occur throughout the 

 year (see Bakun [1973] and Mason and Bakun 

 [1986]). 



Manuscript accepted July 1988. 



FISHERY BULLETIN: VOL, 86, NO. 4, 1988. 



715 



