FISHERY BULLETIN: VOL. 82, NO. 1 



mysid species present differed in their patterns of 

 vertical distribution. Acanthomysis sculpta essen- 

 tially was confined to the canopy (no. /kg ± width of 

 95% C.I./2 = 73.6 ± 56.4 [C], 4.0 ± 2.2 [M], 7.7 ± 5.2 

 [B|), and accounted for 80.57c (by number) of all 

 mysids in the zone, whereas S.pacifica was less abun- 

 dant in the canopy (17.8 ± 6.3 [C], 147.6 ± 39.1 [M], 

 99.2 ± 46.9 [B]), but was dominant in the middle 

 (97.2%) and bottom (92.2%). An unidentified 

 erythropinid rarely was encountered and, when pre- 

 sent, found only in the lower zones (0 [C], 0.2 ± 0.2 

 [M|, 0.7 ± 0.5 [B]). Nearly all (99.9%) of the shrimps 

 associated with the kelp fronds were Hippolyte clarki, 

 and this species was most abundant in the canopy 

 (Table 3, Fig. 3). 



Throughout most of the study, gammarid sizes were 

 largest in the canopy, intermediate in the bottom, 

 and smallest in the middle (K-S test; C-M, C-B, MB: 

 P < 0.01; Table 4, Fig. 4). Mysids and shrimps also 

 were largest in the canopy, but were smallest in the 

 bottom (K-S test; C-M, C-B, M-B:P < 0.001; Table 

 4, Fig. 4). Among the mysids, S. pacifica was more 

 slender (mm, mg= 6.5,2.9 [C], 4.7, 1.2 [M], 4.5, 1.2 

 |B|) thanA sculpta (6.2, 3.7 |C], 4.2, 1.7 [M], 3.1, 1.1 

 [B]). Combined size distributions of the four major 

 taxa for the 19-mo study (weighted according to 

 mean monthly abundance) revealed proportionately 

 greater numbers of large individuals present in the 

 canopy than in either the middle or the bottom (K-S 

 test; C-M, C-B: P < 0.001; M-B: ns; Fig. 5) 



Seasonal Patterns of Species, 

 Abundances, and Sizes 



No seasonal patterns were apparent for total num- 

 ber of invertebrates in the canopy; however, total 

 biomass increased dramatically (from 1,696 to 6,315 

 g/kgkelp) during winter 1975-76 (Fig. 6). In the lower 

 zones, both numbers and biomass were highest dur- 

 ing winter 1975-76 and the following spring (Fig. 6). 



Seasonal patterns of abundance for the major 

 species were evident only for the shrimp H. clarki, 

 which displayed maximum abundance during both 

 winters of the study (Fig. 3). The canopy mysid, A. 

 sculpta, was abundant (1 13. 2-395. 5/kg kelp) during 

 winter 1975 and early spring 1976, but was uncom- 

 mon (6. 5/kg kelp) during the following winter (Fig. 3). 

 Single monthly samples collected in winter 1974-75 

 also indicated high numbers (79. 2-169. 8/kg kelp) of 

 the canopy mysid. Seasonal patterns were not evi- 

 dent for the three most common gammarids (Fig. 2). 

 As a group, copepods were most abundant during 

 winter and early spring in the lower zones, but no 

 seasonal pattern was apparent (Fig. 2). 



Seasonal variations in the sizes of gammarids, 

 mysids, and shrimps were frequently observed (Fig. 

 4). Gammarid sizes were largest during winter and 

 spring in the canopy (2.76-3.85 mm), but no seasonal 

 patterns were present in the lower zones. Carapace 

 lengths of S.pacifica were largest during winter in the 

 canopy (1.68-1.89 mm) and middle (1.24-1.45 mm) 

 with 1976 measurements greater than 1975. 

 Smallest sizes were present during summer in both 

 the canopy (1.28-1.39 mm) and middle (0.92-1.22 

 mm). No seasonal patterns were present in the bot- 

 tom. The shrimp H. clarki was largest in the canopy 

 during winter-spring of both years (1.55-2.08 mm) 

 and during spring 1 975 and winter 1976 in the middle 

 (1.56-1.64 mm) and bottom (1.36-1.43 mm). 

 Smallest shrimps were present during fall in all three 

 zones (1.30-1.42 mm [C]; 1.01-1.17 mm [MJ; 1.02- 

 1.03 mm [B]). No pattern was observed for A. 

 sculpta. 



DISCUSSION 

 Kelp Density 



Elevated temperatures and/or low nutrients may have 

 caused the Habitat Reef kelp forest to decline in late 

 1976. Kelp forests in southern California deteriorate 

 when the water temperature exceeds 20°C for substan- 

 tial periods (North 1971), and high temperatures often 

 are associated with low nutrients (Jackson 1977). Signif- 

 icantly, temperatures at Habitat Reef did not reach 

 20°C in 1975, but exceeded 20~C from mid-June to 

 November 1976. During the second half of 1976, other 

 areas of southern California also experienced warm 

 water and corresponding declines in Macrocystis stand- 

 ing crop (Southern California Edison Co. 1978 1 ). 



Preference of Macrocystis 

 as a Habitat 



Few of the 114 species associated with Macrocystis 

 fronds at Habitat Reef were restricted to the frond 

 habitat. Most were present, and many were more 

 abundant in Macrocystis holdfasts, understory algae, 

 or other habitats within or adjacent to Habitat Reef 

 (Hobson and Chess 1976; Hammer and Zimmerman 

 1979). A few, however, such as the gammarid M. 

 litotes, the shrimp H. clarki, and both species of 

 mysids, were more abundant in the Macrocystis 

 fronds than in other habitats. 



'Southern California Edison Company. 1978. Annual operating 

 report, San Onofre Nuclear Generating Station, Vol. IV. Biological, 

 sedimentological, and oceanographic data analyses. Southern 

 California Edison Co., Rosemead, CA 91770, 300 p. 



60 



