80- 



ra 



60 



50- 



40- 



30- 



20- 



10 



I' 



H 



Station 2 



Seaward edge of kelp 



J S J S 

 1974 1975 



J S 

 1976 



Strongylocentrotus froncisconus 



I 



Station 3 

 50 m seaward 

 of kelp 



J S 

 1974 



J S 

 1975 



J s 

 1976 



I' 



Station 4 

 100 m seaward 

 of kelp 



J S 

 1974 



J S 

 1975 



J S 

 1976 



FIGURE 2. — Densities of Strongylocentrotus franciscanus at 

 three fixed stations off Point Santa Cruz as estimated in June 

 and September 1974, 1975, and 1976. Each station encompassed 

 an area of 2,500 m 2 and the density estimates are based on counts 

 from 12 randomly selected 10 m 2 quadrats. Station 1 was located 

 50 m inshore from Station 2 within the kelp forest and always 

 contained very low densities of sea urchins, <1/10 m 2 . Figure 

 shows mean number of animals per 10 m 2 and the standard er- 

 ror of the mean. The arrows indicate the period of the mass 

 mortality. 



offshore. This represents a decrease of about 60% 

 of the dense population of animals at the kelp 

 forest edge and about 95% of the animals farther 

 offshore. The area of each study station was about 

 2,500 m 2 . In absolute terms, the decrease in 

 number of animals within the study station at the 

 edge of the kelp forest was about 9,000 animals, 

 while in each of the two study stations 50 and 100 

 m farther offshore, about 5,500 animals were lost. 



About 10% of the animals remaining in our 

 Point Santa Cruz study site in June 1976 had large 

 conspicuous portions of the test covered with 

 regenerating spines only 1-5 mm long, contrast- 

 ing noticeably with the surrounding normal- 

 appearing areas. Since we did not detect any ab- 

 normalities in January 1976, the mass mortality 

 probably followed its full course in less than 6 mo, 

 as did the one described by Johnson (1971), and 

 probably the one we observed at Ario Nuevo 

 Island. 



During August-October 1976 we (M. B. Y. and 

 C. R. A.) surveyed the 35-km coastline between 

 Point Santa Cruz and Aho Nuevo Island at 2-km 

 intervals. Most of the kelp forests along this 

 coastline have dense populations of S. francis- 

 canus along their seaward edge, similar to condi- 

 tions found at Point Santa Cruz before 1976. No 

 evidence of mass mortality of these populations of 

 sea urchins was found, either as large numbers of 



dying animals or unusually low numbers of ani- 

 mals. However, diseased animals with partially 

 denuded tests were found occasionally all along 

 the coastline with estimated incidences of 1 in 

 1,000 animals. These observations suggest that 

 potential outbreaks of localized mass mortalities 

 could occur in many places under suitable condi- 

 tions. 



The mass mortality of S. fransicanus at Point 

 Loma in 1970 and those at Aho Nuevo Island and 

 Point Santa Cruz in 1976 were all relatively small 

 and localized in both space and time. Moreover, all 

 the animals in the populations were not killed. 

 Rather, within less than 6 mo low numbers of 

 normal and healthy-appearing animals were 

 present and there was little trace of the mass 

 mortalities — no piles of empty tests remained. 

 Small localized mass mortalities might occur in 

 other areas and not be noticed or reported. If they 

 do, such mass mortalities could be important in 

 regulating the distributions and densities of sea 

 urchin populations. Moreover, since a major por- 

 tion of the recruitment of juveniles of S. francis- 

 canus occurs under adult animals (Tegner and 

 Dayton 1977), near complete mass mortalities, 

 such as that in our study stations 50 and 100 m 

 seaward of the kelp forest off Point Santa Cruz, 

 could have long lasting effects. Such a source of 

 mortality could have practical importance both as 

 means of minimizing overgrazing of kelp by sea 

 urchins (North and Pearse 1971) and as a threat to 

 the developing sea urchin fishery in California 

 (Kato 1972). 



Acknowledgments 



We appreciate discussions and critical readings 

 of the manuscript by V. A. Gerard, A. H. Hines, 

 and V. B. Pearse. We are grateful to R. Buchsbaum 

 for the photograph used in Figure 1 . This work was 

 supported by NOAA Office of Sea Grant, U.S. De- 

 partment of Commerce, under Grant No. 04-6- 

 1584402 and the Marine Mammal Commission, 

 Contract No. MMCAC029. 



Literature Cited 



JOHNSON, P. T. 



1971. Studies on diseased urchins from Point Loma. Kelp 

 Habitat Improvement Project, Annual Report, 1970- 

 1971, p. 82-90. Calif. Inst. Technol., Pasadena. 



KATO, S. 



1972. Sea urchins: A new fishery develops in California. 

 Mar. Fish. Rev. 34(9-10):23-30. 



647 



