HOBSON and CHESS: TROPHIC RELATIONS OF THE BLUE ROCKFISH 



Table a.— Continued. 



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

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

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



■■Many of the calanoids from the plankton included in this category were juveniles and other undetermined stages of the species 

 distinguished above Most were at the lower end of the size range indicated. 

 ^Digested beyond recognition. 



DISCUSSION 



It is clear that wind-driven movement of the sur- 

 face water profoundly influences feeding by Sebastes 

 mystinus off northern California. Water set in 

 motion by the wind can be tens of meters deep 

 (Bakun 1973; Barber and Smith 1981), and so carries 

 most of the foods of nearshore planktivores. The 

 movement is seaward (with upwelling) under north- 

 erly winds and shoreward (with downwelling) under 

 southerly winds. Thus, with winds along the Men- 

 docino coast being northerly or southerly about 

 80% of the time (based on records of the NOAA 

 weather station there), adult S. mystinus in that 

 area alternate between periods when planktonic 

 foods are being carried into their habitat and pe- 



riods when these foods are being carried away. 



This perception of alternations between upwell- 

 ing and downwelling is simplified, perhaps overly 

 so, to emphasize features we consider essential to 

 the feeding of S. mystinus, and also because details 

 of what clearly is a complex oceanographic system 

 remain unclear. In particular, we stress the impor- 

 tance of shoreward surface transport in carrying 

 prey to S. mystinus in nearshore habitats. The major 

 prey of adult 5. m^s^iwws— thaliaceans, pelagic 

 hydrozoans, and other relatively large, gelatinous 

 zooplankters— tend to be concentrated in areas of 

 oceanic convergence, and dispersed in areas of 

 oceanic divergence (e.g., Bakun and Parrish 1980). 

 Thus, when offshore surface waters converge on the 

 coast, the planktonic foods of S. mystinus become 

 concentrated near shore. 



735 



