FISHERY BULLETIN: VOL. 78, NO. 4 



wooden frame covered with galvanized chicken 

 wire, which allowed fairly free passage of water 

 through the enclosure. For experiments using 

 larger fish (>125 mm SL, standard length) the 

 mesh measured 2.5 x 3.7 cm; mesh size was 

 halved to 1.3 x 1.9 cm when smaller fish were 

 used. Seine floats were attached to the top of the 

 cages to make them positively buoyant, and the 

 cages were suspended in midwater at a depth of 5 

 m by a length of rope anchored to the substratum. 

 The cages were located in the same areas on the 

 reef from which plankton were sampled. One cage 

 was placed at the eastern margin of the kelp bed, 

 60 m east of the reef. The other was placed over the 

 reef top, 50 m from the extreme west end of the 

 reef (Figure 1). When half of the experiments were 

 completed, the cages were translocated. 



Blacksmith were captured while in shelters 

 with the anesthetic quinaldine, or while attracted 

 to chum stations (broken sea urchins). Fish were 

 placed in holding tanks on the boat and haphaz- 

 ardly assigned to a cage. One to five fish were 

 captured for each cage. Before each experiment, 

 the cages were scrubbed with a brush to remove 

 encrusting organisms. When fish were placed in 

 the cage, they swam to the bottom; by the next 

 day, they had usually gathered in the middle. 

 At night, the fish nestled in the bottom corners 

 of the cage. 



Each experiment lasted 7-10 d. Preliminary 

 experiments indicated that at least 5 d were 

 required before caged fish began to feed. Fish 

 were removed from cages in the midafternoon and 

 fixed in buffered Formalin. On three occasions, I 

 speared free-living fish in the vicinity of the 

 incurrent cage at the same time that caged fish 

 were removed, to see if caging affected the fish's 

 diet. Since smaller blacksmith do not undergo 

 large-scale movements and are abundant through- 

 out the reef, I also compared gut contents of free- 

 living juveniles speared at the incurrent and 

 excurrent ends. For all collections, blacksmith at 

 the incurrent end were removed first; those at the 

 excurrent end within an hour later. Thus, positive 

 differences in gut fullness between incurrent and 

 excurrent fish are conservative because excurrent 

 fish were able to forage for a longer period of time. 



In the laboratory, fish were measured for stan- 

 dard length, blotted weight, and displaced volume. 

 The gut was removed and divided into stomach 

 and intestine. The contents of each were measured 

 for displaced volume, then examined under a 

 dissecting microscope and identified into broad 



taxonomic categories. Finally, contents were 

 washed onto a preweighed filter and dried at 60° C 

 to a constant weight. 



To standardize for differences in fish sizes, the 

 volumes and weights of the gut contents were 

 expressed as relative measures: 1) volumetric 

 fullness as (Vg/Vf) x 10^ where Vg is the dis- 

 placed volume (in milliliters) of the gut contents, 

 and Vf is the displaced volume (in milliliters) of 

 the intact fish; 2) gravimetric fullness as: (Wg/Wf) 

 X 10"*, where Wg and Wf represent weights (in 

 grams) of the gut contents and intact fish. Finally, 

 for each experiment, I scaled volumetric and 

 gravimetric fullness from 1.0 for the largest value, 

 down toward 0, then averaged the two scaled 

 values to obtain an overall estimate of gut full- 

 ness. This enabled me to pool data among experi- 

 ments to increase sample size. 



RESULTS 



Physical Measurements 



I made 39 surveys between December 1975 and 

 July 1977. However, since water visibility directly 

 influenced the volume of water in which I counted 

 fish, I excluded from further analysis those 

 surveys when mean visibility was significantly 

 greater at the incurrent end of the reef (^-tests; 

 P<0.05) or when variances in visibility at the 

 incurrent and excurrent ends were heterogeneous 

 (F-tests, P<0.05). In the remaining 27 surveys, 

 water flowed over the reef from the east in 23; on 4 

 occasions (twice in April 1976, once in August 

 1976, and once in January 1977), the current was 

 reversed and flowed from the west. Mean monthly 

 water visibility ranged from 4.5 m in March to 

 8.7 m in December. Visibility was slightly greater 

 at the excurrent end of the reef. When the current 

 flowed from the east, visibility averaged 7.1 m at 

 the east end and 7.4 m at the west end; when the 

 current reversed, visibility averaged 6.0 m and 

 5.1 m at the east and west ends. 



Spot checks on the movements of small particles 

 indicated that the net flow of water was roughly 

 unidirectional over most of the reef; this general 

 flow was confirmed on 2 d when pieces of kelp were 

 followed as they drifted the length of the reef 

 (Bailey^). Nonetheless, variances in velocity were 

 significantly heterogeneous between reef ends for 



^Thomas Bailey, Marine Science Institute, University of 

 California, Santa Barbara, CA 93106, pers. commun. June 1977. 



832 



