HOBSON and CHESS: TROPHIC RELATIONS OF THE BLUE ROCKFISH 



cedure, and how the samples were processed and 

 analyzed.) We visually assessed the larger zooplank- 

 ters in the water column during each collection, as 

 well as at other times during each sampling session, 

 and these observations greatly enhanced our abil- 

 ity to interpret the samples. 



Benthos 



Although S. mystinus is a planktivore, it is impor- 

 tant to include the benthos when documenting 

 environmental changes that affect its feeding. Many 

 of the actual or potential foods of 5. mystinus 

 originate on the underlying substrata, and their oc- 

 currences in the water column relate to changes in 

 the benthos. Thus, assessments of the benthos done 

 during other studies concurrently under way in the 

 study area, including visual counts and airlift sam- 

 ples, produced data that are incorporated into this 

 report where pertinent. 



Ranking Prey Taxa 



To estimate the relative importance of the various 

 prey organisms in the diet of S. mystinus, we 

 grouped related forms in multispecies categories 

 that were then ranked. The ranking was based on 

 an index calculated as: relative frequency of occur- 

 rence in diet x mean number of individuals con- 

 sumed X percent of total diet volume that was 

 represented by that category. This index is similar 

 to the widely used Index of Relative Importance 

 (IRI) of Pinkas et al. (1971), but puts more weight 

 on numbers consumed. It is important to emphasize 

 numbers consumed in quantifying the feeding ac- 

 tivity of S. mystinus (and most other planktivores) 

 because the many small prey are ingested indi\adual- 

 ly, making the capture of each a discrete act. 



RESULTS 



During the four years of this study, spring and 

 (to a lesser extent) summer constituted an upwell- 

 ing season, while fall and (to a greater extent) winter 

 constituted a downwelling season. The spring tran- 

 sition between downwelling and upwelling seasons 

 occurred over just a few days between late March 

 and early April, whereas the less distinct fall tran- 

 sition between upwelling and downwelling seasons 

 was extended over a month or more between mid- 

 August and late October. 



Despite this seasonal pattern, however, there 

 were short-term reversals of just a few days that 



had profound effects. Reactions of the environment 

 to major wind changes were virtually immediate. 

 Sharply reduced water temperatures signifying the 

 intrusion of upwelled water often followed within 

 hours of increased northerly winds, while warmer 

 water rich in such readily visible zooplankters as 

 ctenophores, hydrozoans, pteropods, thaliaceans, 

 and larvaceans, often flowed into the nearshore 

 habitats within a day after the onset of southerly 

 winds. Conditions during each sampling session are 

 listed in Table 1. 



Although downwelling conditions were most in- 

 tense under southerly winds, they also developed 

 during calms and with weak northerlies. We found 

 that generally it took northerlies of 10 knots (K) or 

 more to produce upwelling conditions, although as 

 little as 5 K were effective if held steady for several 

 days. In the absence of sufficient force, however, 

 upwelling ceased and warmer water rich in offshore 

 zooplankters entered the nearshore habitat, gen- 

 erally moving in a southerly direction along the 

 coast. 



The close relation between shifts in prevailing 

 wind and alternations between upwelling and down- 

 welling is illustrated by comparing the wind direc- 

 tion and velocity measured by the nearby NOAA 

 weather station to the sea temperatures recorded 

 by our thermographs. Although these data became 

 available only during the last year of the study, 

 subsequent years have produced similar profiles (ex- 

 cept 1982-83, when there w^as a strong El Nino). 

 Thus, the pattern of sea temperatures and wind dur- 

 ing the upwelling season of 1981 and during the 

 downwelling season of 1980-81, recounted in detail 

 below, are representative. 



The Upwelling Season 



The upwelling season began in late March or early 

 April with a precipitous drop in sea temperatures. 

 It was a drop of about 3 °C— typically from 

 11°-12°C to 8°-9°C-that coincided with the onset 

 of strong, persistent northerly winds characteristic 

 of this time of year. The pattern of sea temperatures 

 and wind during the 1981 upwelling season (Fig. 4) 

 illustrates how upwelling and downwelling related 

 to prevailing winds during that period. 



Habitat Conditions 



At the start of the upwelling season, the nearshore 

 habitats appeared barren. Storm seas during the 

 previous winter had carried away most of the bull 



719 



