HOBSON and CHESS: TROPHIC RELATIONS OF THE BLUE ROCKFISH 



Table 5.— Continued. 



'Value is estimated mean number per 100 m^ of water, based on water filtered (54.8 m^) during the 5-min collection. 

 ^Adult S. mystmus often were seen feeding on large individuals of Cyanea capillata, wfiicfi were avoided by us during plankton col- 

 lections because they would have made collections umanageable. 

 3NR = not recorded. The enumeration was either omitted or unfeasible. 

 'Velella velella floats on the water's surface, where it was not effectively sampled by our net. 

 ^Digested beyond recognition. 



planktivorous fishes. The following discussion con- 

 siders how the diet of S. mystinus, summarized in 

 Figure 13, is influenced by these alternations in sur- 

 face transport during distinct upwelling and down- 

 welling seasons. 



The Upwelling Season 



The spring-summer upwelling season produces op- 

 timal feeding conditions for S. mystinus. During this 

 period the combined effects of increased nutrients 

 (from strong upwelling) and increased daylight (from 

 longer days, higher sun-angle, and less storm-pro- 

 duced sediments in suspension) result in growth of 

 diatom populations that constitute the food-base of 

 the zooplankton community. 



Seaward Ekman transport in response to the 

 season's persistent northerly winds carries the up- 

 welled nutrients and increasing number of diatoms 

 offshore, where the response of zooplankters can be 



spectacular. Consider, for example, thaliaceans, 

 which are a major prey of adult S. mystinus. Re- 

 cent study has shown that populations of Salpa 

 fusiformis (a common thaliacean in the California 

 Current) normally are food-limited, but can grow 

 rapidly when diatoms are abundant (Silver 1975). 

 In response to a diatom bloom, Thalia democratica 

 (another salp common in the California Current), can 

 increase in size by up to 10%/hour and in numbers 

 up to 2.5 times/day, the highest rate recorded for 

 a metazoan animal (Heron 1972a, b). 



Zooplankters thus increased in size and number 

 are then carried to S. mystinus near shore by the 

 shoreward flow that develops with relaxation of up- 

 welling, or, more forcefully, with shoreward Ekman 

 transport under southerly winds. It remains uncer- 

 tain, however, whether the numbers of zooplankters 

 entering the Mendocino nearshore habitats are in 

 fact related to the productivity of local upwelling. 

 Wickett (1967) concluded that zooplankton abun- 



737 



