FISHERY BUI I.ETIN: VOL. 72, NO. 3 



along the 150-m transect. In addition to the har- 

 vesting effect, the kelp bed was aging and the 

 canopy was being reduced through the natural 

 attrition of older plants. More areas of the bed 

 were gradually opened to receive increased light 

 penetration. 



The first visible germination of Macrocystis 

 sporophytes occurred during August-September 

 1969, when plants approximately 1 to 3 cm in 

 height appeared on or near the survey lines. 

 Although juvenile plants were found in scattered 

 locations throughout the kelp bed, the most dense 

 concentrations appeared around the shoreward 

 (eastern) end of the transect, from about 100 m 

 to 150 m (Figure 1). Only two adult plants re- 

 mained within this 2 x 50 m band transect during 

 the summer of 1969 (Figure 3); therefore, shading 

 of the underlying sea floor was probably minimal. 

 Also, this shoreward edge of the bed is approxi- 

 mately 5 m shallower than the seaward portion 

 and there may have been more ambient light in 

 this part of the kelp bed. Juvenile sporophyte 

 density ranged from to 32 plants/m^ within 

 the fixed quadrats. Macrocystis recruitment was 

 especially pronounced in the 100- to 150-m area 

 on the transect. For this reason two additional 

 4-m2 quadrats (no. 13 and 14) were added in this 

 location when the first sporophytes appeared in 

 August 1969. In September 1969, a total of 387 

 single-bladed Macrocystis were counted within 

 the fixed quadrats (Figure 6). After the first 

 month only 156 plants survived, and by June 

 1970 the young sporophj^te population was re- 

 duced to only 5 survivors. 



Most of the juvenile plant mortality was be- 

 lieved to be caused by plant entanglement, fish 



grazing, physical scouring, and/or actual burial 

 of the plants by moving sand. Many of the young 

 sporophytes were damaged, primarily in the 

 region of the apical tip. These plants were in- 

 spected for invertebrate grazers, but none were 

 found. It seems likely that the grazers were 

 positioned in the water column because most of 

 the grazing was located around the terminal ends 

 of each blade. On two occasions the labrid Oxy- 

 julis californica and the embiotocid Embiotoca 

 jacksoni were seen nibbling on juvenile Macro- 

 cystis. However, because of marginal underwater 

 visibility in this area, it was generally not 

 possible to observe the grazing activities of fishes 

 directly. 



Physical scouring and burial is another cause 

 of juvenile plant mortality. For example, one 

 group of young plants (n = 36) growing within a 

 shallow depression along the sea floor became 

 completely covered with sand during the late fall 

 of 1969 and never reappeared. The stipes of 

 Macrocystis are quite sensitive to being enclosed 

 or covered (North, 1971), and apparently the 

 young plants can not tolerate burial for any 

 length of time. In kelp beds such as Del Mar, 

 the juvenile Macrocystis appear to be highly 

 vulnerable to this physical phenomenon. 



The five surviving Macrocystis plants that grew 

 within the fixed quadrats continued to mature 

 and eventually the stipes reached the sea surface. 

 One plant perished in September 1972; however, 

 the other four were still present within the 

 quadrats in December 1972, 3 yr and 3 mo after 

 germination. 



Plant Longevity 



IOOOt 

 500- 



A 



100^ 



so: 



(I9S9) 



\ 



(1970) 



(1971) 

 AGE IN MONTHS 



'1 ' 

 28 



40 



Figure 6. — Survivorship curve of individual Macrocystis which 

 were observed to recruit in September 1969. 



At the present time there is little information 

 available in the literature on the life span of 

 Macrocystis because most of the data have been 

 collected on the longevity of individual fronds. 

 The maximum life span of a Macrocystis frond 

 was found to be about 6 mo (Brandt, 1923; Cribb, 

 1954; North, 1961). North (1968) reported that 

 3 yr were required to totally eliminate tagged 

 mature plants within a stand of kelp off La Jolla, 

 Calif. Most of the plants were very large at the 

 beginning of the study, "so they probably were at 

 least 5 years of age when they vanished, or quite 

 Hkely older." (North, 1968:224). 



Of the 49 adult Macrocystis plants originally 

 marked along the Del Mar transect no survivors 



676 



