HOBSON and CHESS: TROPHIC RELATIONS OF THE BLUE ROCKFISH 



Plant material ranked as the top food-category 

 during upwelling episodes (Table 2) and as the 

 second-ranked food during downwelling episodes 

 (Table 3). In both cases, certain algae dominated the 

 diet on days when offshore zooplankters were in 

 short supply during the latter part of the upwelling 

 season (upwelling episodes on 8 August 1978, 12 

 June 1979, and 17 June 1981; downwelling episodes 

 on 24 July 1979 and 27 August 1979; see Table 1). 

 Algae were not taken earlier in the season, however. 

 Thus, while the six individuals collected during a 

 plankton-poor upwelling episode of 8 August 1978 

 (Table 1) were full of algae (90% of gut contents) 

 and epibenthic crustaceans (10%), all eight collected 

 during a plankton-poor upwelling episode of 9 April 

 1981 (Table 1) were empty. Similarly, the occurrence 

 of algae in the diet during downwelling episodes 

 (Table 3) is based mostly on 20 adults collected dur- 

 ing two days (24 and 27 August 1979) when en- 

 vironmental conditions indicated downwelling but 

 an absence of offshore zooplankters was noted 

 (Table 1). There was no indication of plants being 

 ingested for epiphytic animals. 



Of the wide variety of plant materials in the diet, 

 only the three most frequently ingested forms ap- 

 peared to some extent digested. These were the sori 

 of N. leutkeana, Porphyra nereocystis (an epiphyte 

 on A^. leutkeana), and Smithora naidum (an epiphyte 

 on certain seagrasses). The sori of A'^. leutkeana 

 developed as variably sized areas (typically about 

 50-150 mm long) in fronds near the water's surface. 

 They dropped from the fronds when mature, and 

 we saw several ingested by 5. mystinus as they 

 drifted toward the bottom. Sori recovered from the 

 intestines of S. mystinus were more translucent 

 than sori from the stomach, but comparison under 

 magnification of sectioned material from both 

 regions of the gut (Fig. 7) indicated that only zoo- 

 spores were digested. Ingested fragments of P. 

 nereocystis and S. naidum, both species having 

 monostromatic thalli, ranged from intact, but flaccid 

 and blanched, to disintegrating. Ingested S. naidum 

 often were attached to pieces oiZostera marina or 

 Phyllospadix torreyi, but these seagrasses never 

 evidenced digestion. Nor did other plant forms pres- 

 ent among the gut contents appear to be digested, 

 including D. ligulata and A^. leutkeana (vegetative 

 tissue), along with various unidentified phaeophytes 

 and rhodophytes. Thus, plants contributed less food 

 than is indicated by the tables and histograms, 

 where their rank is inflated by undigested materials. 



Zooplankters taken during upwelling episodes dif- 

 fered from early to late in the season, whereas those 



taken during downwelling episodes remained much 

 the same throughout (as listed in Table 3). Thus, of 

 the food categories listed for the upwelling condi- 

 tion (Table 2), pelagic hydrozoans, mysids, and 

 scyphozoans were taken only during the spring, 

 whereas euphausiids and caprellids were taken only 

 in summer. The only notable departure from the 

 downwelling condition depicted in Table 3 occurred 

 on 24 April 1979 (Table 1), when conditions were 

 at the time judged to be mixed. Upwelling had been 

 unusually weak during the first month of that up- 

 welling season, and there was no wind at the time 

 of sampling. The sea was calm and, at 10°C, warm 

 for April. Although these conditions usually indicate 

 downwelling (which is why the data are assigned to 

 that category), we neither collected nor saw organ- 

 isms typical of downwelHng conditions. Further- 

 more, of the nine adult S. mystinus (224-313, x 

 = 277.7 mm SL) collected, eight (89%) were empty. 

 The ninth contained one sorus from A'^. leutkeana 

 (an unusually large number of A'', leutkeana had per- 

 sisted through the previous winter, which had been 

 exceptionally mild) and also organisms not seen or 

 collected by us in the environment at the time: two 

 Corolla spectabilis (a pelagic gastropod typical of 

 downwelling conditions) and six Velella velella (a 

 pelagic hydrozoan typical of upwelling conditions). 

 The latter float on the water's surface (Fig. 8), and 

 often we saw adult S. mystinus break the surface 

 to feed on them. 



The Downw^elllng Season 



Between late August and mid-September it 

 became evident that transition to the downwelling 

 season was under way. Winds had become light and 

 variable (but generally remained either northerly or 

 southerly), and for a growing number of days at a 

 time, the water was notably blue and transparent. 

 Records of sea temperatures and wind for the 

 1980-81 downwelling season illustrate how occur- 

 rences of downwelling and upwelling related to 

 prevailing wind during that period (Fig. 9). 



Habitat Conditions 



The downwelling season developed with less wind 

 from the north and more wind from the south, but 

 either way with winds that tended to be light, so 

 that relatively tranquil conditions prevailed. Flow- 

 ing into the nearshore habitat with offshore surface 

 waters was a rich supply of relatively large, mostly 

 gelatinous zooplankters that were major prey of 5. 



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