HOBSON and CHESS: TROPHIC RELATIONS OF THE BLUE ROCKFISH 



Although a wide variety of plants were ingested, 

 S. mystinus seemed able to utilize only certain algal 

 tissues. These included zoospores of Nereoeystis leut- 

 keana and the two epiphytes, Porphyra nereocystis 

 and Smithora naidum. Probably this is because as 

 members of a carnivorous family they have only 

 limited abilities to digest plant material. 



The zoospores of A'', leutkeana are especially vul- 

 nerable to S. mystinus when the sori have dropped 

 from the plants' fronds, because at this time the sori 

 have lost their epidermis (an adaptation of the 

 mature sori that facilitates release of the zoospores 

 (David C. Walker^)). Zoospores may be appropriate 

 food for this largely carnivorous fish because they 

 are, as their name implies, animal-like: they have 

 cell membranes but not the cellulosic cell walls 

 (Wilson 1952) that preclude plants as food for many 

 fishes (e.g., Lobel 1981). The other algae apparent- 

 ly utilized— Porp%ra nereocystis and Smithora 

 naidum— may be appropriate forage for a fish with 

 limited herbivorous abilities because they are mono- 

 stromatic plants only 25-60 fi thick (Abbott and 

 Hollenberg 1976). So while Gotshall et al. (1965) 

 reported that algae in the guts of S. mystinus gen- 

 erally are undigested, some forms appear to be im- 

 portant foods. 



It is possible that plant materials are among items 

 of little or no food value that are ingested during 

 food shortages simply because at such times the 

 adult 5. mystinus become less discriminating in their 

 choice of drifting objects. On the other hand, it is 

 also possible that these fish have unusual herbiv- 

 orous abilities as a result of adaptations to a diet 

 rich in thaliaceans, which are among the few animals 

 with cellulosic tissues (Berrill 1961). This second 

 possibility is weakened, however, by the fact that 

 the cellulosic tunics of the thaliaceans appear to pass 

 through the gut undigested (Gotshall et al. 1965; our 

 observations). 



The Downwelling Season 



As the downwelling season developed in the fall, 

 S. mystinus experienced progressively poorer feed- 

 ing conditions. Offshore water flowing into the near- 

 shore habitats at these times tended to be poor in 

 phytoplankton (hence its transparency and blueness, 

 in contrast to the turbid greenness of a few months 

 before), and so generally lacked the herbivorous 



^David C. Walker, Department of Botany, University of British 

 Columbia, Vancouver, British Columbia V6T 1W5, Canada, pers. 

 commun. 10 May 1977. 



thaliaceans that were major prey during periods of 

 peak feeding. Relatively large gelatinous zooplank- 

 ters continued on occasion to be numerous in the 

 shoreward flow, but the species at this time tended 

 to be carnivores rather than herbivores, and S. 

 mystinus did not feed on some of them. For exam- 

 ple, while siphonophores made "Pelagic Hydrozoa" 

 the top-ranked food category (Table 4), the most 

 numerous pelagic hydrozoan in the water column, 

 Eutonina indicans, went virtually untaken. (The 

 small medusae visible throughout Figure 1, especial- 

 ly against the dark kelp, are of this species.) On the 

 other hand, another relatively large, gelatinous 

 zooplankter, the ctenophore Pleurobrachia bachei 

 (Fig. 10), was prominent both as prey and in the 

 plankton. Probably the presence of P. bachei ac- 

 counted for the gut-content occurrences of the 

 hyperiid Hyperoche medusarum, which is a parasite 

 of this ctenophore (Brusca 1970). 



As was true during late spring and summer, cer- 

 tain plant materials became major foods during the 

 fall when favored zooplankters were in short supply. 

 But unlike late spring and summer, when many 

 different plant forms were taken, virtually the only 

 plant materials consumed in the fall were the sori 

 of N. leutkeana. This reflected a decreasing abun- 

 dance of other plants in the en\dronment at the time. 

 Nevertheless, while sporophytes of A'', leutkeana 

 were fewer in fall than in summer, those present 

 were larger, more mature, and produced more sori. 



Although winter produced the poorest feeding 

 conditions of the year, with by far the highest in- 

 cidence of empty stomachs among the fish exam- 

 ined (Figure 13; also noted by Gotshall et al. 1965), 

 occasionally the waters flowing into the nearshore 

 habitats were rich in offshore zooplankters, in- 

 cluding thaliaceans, and at these times the fish fed 

 well. Our sampling of the highly varied winter con- 

 ditions was not frequent enough to recognize a pat- 

 tern, but they differed from fall conditions. In 

 general, the distinctive fall and winter conditions 

 off Mendocino matched the "oceanic" and "David- 

 son Current" oceanographic seasons defined by 

 Skogsberg (1936) and Bolin and Abbott (1962) for 

 Monterey Bay. 



Ekman Transport and the Distribution 

 of Sebastes mystinus 



Ekman transport may be important to the distri- 

 bution of S. mystinus. This is implicit in our finding 

 that alternations between seaward and shoreward 

 surface transport produce feeding opportunities for 



741 



