Diopatra ornata (polychaete), and Styela mon- 

 tereyensis (ascidian). 



DISCUSSION 



During the years of this study (1967-73) there 

 was a pronounced oscillation in the number of 

 adult Macrocystis in the study area. Partially 

 concurrent with these in situ observations has 

 been a fluctuation in the amount of kelp har- 

 vested in this area since 1940 (Figure 4). The 

 disappearance or detachment of adult Macroc^'s^ts 

 along the transect was usually related to physical 

 stress from an increase in water motion and/or 

 entanglement with detached, drifting kelp plants. 

 Storms, particularly during the months of October 

 through April, seem to be the major cause of 

 plant mortality in this bed. For example, during 

 a 2-mo period (December-February 1973), 46% 

 of the adult Macrocystis were lost along the tran- 

 sect (Figure 2). There is historical evidence of 

 severe storm mortality as Brandt (1923) reported 

 the La Jolla and Pt. Loma kelp beds were severely 

 diminished in area by storms in 1888-89 and 1915. 

 Doty (1971) also reports that storms have impor- 

 tant effects on the standing crop of algae 

 in Hawaii. 



Another cause of plant attrition was commer- 

 cial harvesting. Four tagged Macrocystis repre- 

 senting 21% of the marked plants in the study 

 area were detached from the substratum during 

 a kelp harvesting operation in January 1969. 

 However, this rate of mortality could be higher 

 than the harvesting attrition rate in other beds 

 because of the relatively unstable substratum of 

 the north San Diego County kelp beds. Certainly 

 a preharvest consideration should be given to the 

 actual cutting technique best suited to each 

 individual stand of kelp. 



The vulnerability of adult Macrocystis to en- 

 tanglement and detachment would appear to 

 result in the development of a multiage class 

 kelp bed in which there are patches of cohorts 

 or plants of the same age class. That is, the 

 drifting and entanglement is rather localized and 

 results in distinct patches of the canopy being 

 cleared. Thus, germination and recruitment take 

 place not only around the periphery of the bed, 

 but also within central portions. This spatial 

 heterogeneity of different aged Macrocystis may 

 add stability to a particular kelp bed as mature, 

 perhaps more secure plants are growing adjacent 



FISHERY BULLETIN: VOL. 72. NO 3 



to both young and senile plants. During February 

 1972, it was estimated that the Del Mar bed was 

 composed of at least four age classes of Macro- 

 cystis. 



The paucity of juvenile Macrocystis sporo- 

 phjd;es during the first 23 mo of the study was 

 believed to be related to the establishment of a 

 well-developed adult canopy, turbidity of the 

 water column, and the relatively unstable sub- 

 stratum. These factors are interrelated since they 

 contribute to the absorption and scattering of the 

 available light necessary for germination and 

 sporophyte development. A layer of fine sediment 

 (silt) remained along the bottom during the entire 

 study period, and this silt was usually suspended 

 by increased water motion. The Los Penasquitos 

 salt marsh located approximately 2 km south- 

 east of the study site could be a source of much 

 silt. Whatever the source, this sediment layer 

 seems to be a general feature of most north 

 San Diego County kelp beds, and as it contrib- 

 utes to a reduction in the submarine light and 

 physically scours the substratum, it probably has 

 a strong negative effect on Macrocystis recruit- 

 ment. Despite these limiting factors, young 

 Macrocystis sporophytes appeared in the Del Mar 

 bed during August 1969. The reduction of the kelp 

 canopy from natural attrition and harvesting 

 probably paved the way for the late summer 

 germination and recruitment. Following recruit- 

 ment of these young plants there continued to be 

 a strong attrition as only 4 of the original 387 

 sporophytes growing within the fixed quadrats 

 survived the 3.25 yr (to December 1972). 



Many of the same physical parameters that 

 influence the Macrocystis population at Del Mar 

 appear to effect the distribution, frequency of 

 occurrence, and abundance of the fauna. Quast 

 ( 1968a) determined from three flsh collection sites 

 off Del Mar, Bathtub Rock (San Diego County), 

 and Papalote Bay, Baja California that Del Mar 

 was lowest in fish species diversity, despite the 

 fact that it covered the greatest area. He con- 

 cluded that the differences between the three 

 areas were due to differences in the amount of 

 shifting sediment and the durability of the rocky 

 substratum. Furthermore, he found a positive 

 correlation between the degree of bottom relief 

 and the abundance and species diversity of fish. 



The epibenthic invertebrates are similarily 

 affected by the physical characteristics of the 

 habitat. The aggregated distribution patterns of 



682 



