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Abstract
The U.S. Geological Survey monitors a suite of intertidal black abalone (Haliotis cracherodii) sites at San Nicolas Island, California, in cooperation with the U.S. Navy, which owns the island. The nine rocky intertidal sites were established in 1980 to study the potential effect of translocated sea otters on the intertidal black abalone population at the island. The sites were monitored from 1981 to 1997, typically annually or biennially. Monitoring resumed in 2001 and has been completed annually thereafter. Since 2018, the work has been carried out by the U.S. Geological Survey Western Ecological Research Center. The study sites became particularly important, from a management perspective, after a virulent disease decimated black abalone populations throughout southern California beginning in the mid-1980s. The disease, withering syndrome (Candidatus Xenohaliotis californiensis), was first observed on San Nicolas Island in 1992 and over the next few years, withering syndrome reduced the black abalone population on San Nicolas Island by more than 99 percent. In 2009, the black abalone subsequently was listed as endangered under the Endangered Species Act.
The subject of this report is the 2023 survey of the sites and the status of the measured population in comparison to long-term patterns (based on data collected since 1981) at San Nicolas Island. Between the years 2000 and 2023, the total monitored black abalone population at the island has grown from roughly 200 to more than 2,500 abalone following disease-related decline. Since it was first consistently measured in 2005, the average distance between adjacent black abalone has decreased substantially from approximately 50 centimeters to less than 15 centimeters, indicating that abalone are sufficiently close together at several of the sites to reproduce successfully. The total abalone count in 2023 was 2,570, which was 19.2 percent higher than in 2022 and the highest count since 1993. All nine sites had higher counts in 2023 than in the previous year. Over 25 percent of the black abalone counted in 2023 were classified as recruits, defined as having a shell length of 3 centimeters or less.
Plain Language Summary
The 2023 monitoring of black abalone at San Nicolas Island shows strong signs of population recovery following the severe declines caused by withering syndrome in the 1990s. The island-wide summed count from the study sites reached 2,570 individuals—the highest since 1993—and increased nearly 20 percent from 2022, with higher numbers recorded at all nine study sites. Recruitment was particularly strong, with over a quarter of individuals classified as young abalone, and densities and spacing between individuals indicate increasing likelihood of successful reproduction. Although one historically important transect at Site 8 continues to show reduced numbers of larger adults despite high recruitment, the overall population trend across the island remains positive. Continued monitoring is important to track long-term recovery, habitat conditions, and potential risks.
Suggested Citation
Kenner, M.C., and Yee, J.L., 2026, Black abalone surveys at Naval Base Ventura County, San Nicolas Island, California—2023 annual report: U.S. Geological Survey Open- File Report 2026–1015, 39 p., https://doi.org/10.3133/ofr20261015.
ISSN: 2331-1258 (online)
Study Area
Table of Contents
- Acknowledgments
- Abstract
- Plain Language Summary
- Introduction
- Methods
- Results and Discussion
- Conclusion
- References Cited
- Appendix 1. Sites
| Publication type | Report |
|---|---|
| Publication Subtype | USGS Numbered Series |
| Title | Black abalone surveys at Naval Base Ventura County, San Nicolas Island, California—2023 annual report |
| Series title | Open-File Report |
| Series number | 2026-1015 |
| DOI | 10.3133/ofr20261015 |
| Publication Date | June 03, 2026 |
| Year Published | 2026 |
| Language | English |
| Publisher | U.S. Geological Survey |
| Publisher location | Reston, VA |
| Contributing office(s) | Western Ecological Research Center |
| Description | viii, 39 p |
| Country | United States |
| State | California |
| Other Geospatial | Naval Base Ventura County, San Nicolas Island |
| Online Only (Y/N) | Y |
| Additional Online Files (Y/N) | N |