FISHERY BULLETIN; VOL. 72. NO. 3 



Fish damage to Macrocystis was minimal in 

 mature plants. Most of the grazing by fishes was 

 directed towards juvenile plants. Quast (1968b, 

 1971) investigated the stomach contents of fishes 

 collected at Del Mar and found Macrocystis frag- 

 ments in the stomachs of the labrid Oxyjulis 

 californica, the kyphosid Medialuna californien- 

 sis and the embiotocid Phanerodon furcatus. He 

 also found macroalgae as a predominant item in 

 terms of frequency and volume in the stomachs 

 of Girella nigricans. During daylight hours in 

 this area O. californica, Embiotoca jacksoni , and 

 G. nigricans were observed biting off portions of 

 adult and/or juvenile Macrocj'sft's. Some of the fish 

 grazing may have been directed at the inverte- 

 brates associated with the algae, but for what- 

 ever reason, the plants were damaged by these 

 feeding activities. 



Most adult Macrocystis attrition recorded over 

 the 5.7 yr was caused by detachment of the 

 holdfast and thereby elimination of the entire 

 plant. Plants with weakened or decaying hold- 

 fasts were particularly vulnerable to physical 



stress. Three factors probably accounted for most 

 of the plant mortality in this location: 1) storms 

 and strong surge, 2) entanglement of drifting 

 plants with attached Macrocystis, and 3) kelp 

 harvesting. Many of the kelp stands in San 

 Diego County have been greatly thinned or 

 almost torn away by the effects of storms (Brandt, 

 1923; ZoBell, 1971). Brandt (1923) reported that 

 the La Jolla and Pt. Loma kelp beds were 

 reduced nearly 40% in area by storms in the 

 late winter and early spring of 1915. When 

 Macrocystis plants become detached they drift 

 along in the water column, often becoming en- 

 tangled with other kelps (Figure 5). Drifting or 

 dislodged plants thus present a potential source 

 of mortality for attached Macrocystis. The stipes, 

 blades, and holdfasts of entangled plants become 

 so entwined that separation becomes almost im- 

 possible. We have observed as many as 18 adult 

 Macrocystis entwined in one cluster near the 

 transect. In almost every case the entanglement 

 resulted in the mortality of the attached plant. 

 This is a partial explanation why Macrocystis 



Figure 5. — A detached Macrocystis drifting through the kelp bed. 



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