HOBSON and CHESS: TROPHIC INTERACTIONS 



Figure 2.-The Dictyopteris field, 

 bordered by the Macrocystis forest. 



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Figure 3.-The study area at Santa Catalina Island. 



Family Labridae: wrasses 



Senorita, Oxyjulius californica Giinther) 

 Family Pomacentridae: damselfishes 



Blacksmith, Chromis punctipinnis (Cooper) 



Only two other species in the study area have 

 similar zooplanktivorous habits: the topsmelt, 

 Antherinops affinis (Ayres), and the shiner perch, 

 Cymatogaster aggregata Gibbons. These two, 

 however, are more characteristic of other habitats, 

 where the species composition of available prey is 

 different. Although for this reason they will be 

 described in separate reports later, their activities 

 are entirely consistent with what is reported and 

 discussed below. 



METHODS 



Direct Observations 



We used scuba and snorkeling (167 h under- 

 water) to observe activity of the fishes and tro- 

 phically related organisms during all periods of 

 day and night. Except when collecting specimens, 

 we tried to avoid influencing the organisms or 

 their environment. 



Collecting Zooplankters 



During the same period that we collected fishes 

 for the food-habit study, organisms in the water 

 column that might be their prey were sampled 

 with a 1-m plankton net (0.333-mm mesh) that we 

 pushed through the water for 5-min periods 

 (Figure 4). In this way, a series of paired collec- 

 tions sampled the waters above the Dictyopteris 

 field during September 1973, February 1974, and 

 May 1974. Of each pair, the first sampled the water 

 column midway between the water's surface and 

 the sea floor (in 10 to 15 m of water); and the 

 second, which followed immediately, sampled the 

 base of the water column to within about 10 cm 

 above the bottom. During each sampling month, 

 we made a set of eight collections-four at full 

 moon, and four at new moon. Each set included a 

 pair at night (between 2 and 4 h after last evening 

 light), and a pair the following day (between 1200 

 and 1400 h). In addition, we made one set of 

 collections in the kelp forest bordering the study 



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