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Black Abalone ( Haliotis cracherodii ) Population Density, Recruitment, Size Structure, and Population Growth at Naval Base Ventura County, San Nicolas Island, California, 2013–22

Michael C. Kenner, Julie L. Yee, M.C. Kenner, J.L. Yee, E.L. Korn, B.I. Graubard · U.S. Geological Survey

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Ecosystems Mission Area—Land Management Research Program and Species Management Research Program Prepared in cooperation with the U.S. Navy

Black Abalone (Haliotis cracherodii) Population Density, Recruitment, Size Structure, and Population Growth at Naval Base Ventura County, San Nicolas Island, California, 2013–22

Open-File Report 2025–1015

U.S. Department of the Interior U.S. Geological Survey

Cover. Black abalone (Haliotis cracherodii) adult and recruit, San Nicolas Island, California. Photograph by Michael Kenner, U.S. Geological Survey, February 16, 2023.

Black Abalone (Haliotis cracherodii) Population Density, Recruitment, Size Structure, and Population Growth at Naval Base Ventura County, San Nicolas Island, California, 2013–22 By Michael C. Kenner and Julie L. Yee

Ecosystems Mission Area—Land Management Research Program and Species Management Research Program Prepared in cooperation with the U.S. Navy

Open-File Report 2025–1015

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2025

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Kenner, M.C., and Yee, J.L., 2025, Black abalone (Haliotis cracherodii) population density, recruitment, size structure, and population growth at Naval Base Ventura County, San Nicolas Island, California, 2013–22: U.S. Geological Survey Open-File Report 2025–1015, 10 p., https://doi.org/10.3133/ofr20251015. ISSN 2331-1258 (online)

iii

Acknowledgments We would like to thank the U.S. Navy for funding and facilitating this research project. The analytical methods in this report were developed from constructive discussions with the Black Abalone Recovery Team established by the National Marine Fisheries Service under the Black Abalone Recovery Plan. Thanks also to Melissa Miner and Melissa Neuman for their constructive comments on an earlier draft of this report.

Contents Acknowledgments��iii Abstract��1 Introduction��1 Methods and Results��2 Habitat-Based Density��3 Recruitment��4 Size Structure��6 Population Trend��7 Discussion��10 Habitat-Based Density��10 Recruitment��10 Size Structure��10 Population Trend��10 References Cited��10

Figure 1. Map showing black abalone monitoring sites on San Nicolas Island, California, showing location, number of transects, and total sampling area at each site��2

Tables 1. Total number of 1-square meter quadrats, number of quadrats in each habitat quality category, and expected density for each of the nine black abalone monitoring sites at San Nicolas Island, California��3 2. Black abalone densities from 2018 to 2022 at the nine monitoring sites at San Nicolas Island, California��4 3. Number of recruits, defined as black abalone counted with a shell length less than or equal to 30 millimeters, calculated annually from 2013 to 2022 at the nine monitoring sites at San Nicolas Island, California��5 4. Size structure of black abalone from 2018 to 2022 at the nine monitoring sites at San Nicolas Island, California��6 5. Annual and average population growth trend for black abalone greater than 50 millimeters shell length over a 10-year period at the nine monitoring sites at San Nicolas Island, California��8

Conversion Factors International System of Units to U.S. customary units

Multiply

To obtain

Length centimeter (cm)

0.3937

inch (in.)

millimeter (mm)

0.03937

inch (in.)

Area square meter (m2)

Abbreviations CI

confidence interval

shell length

0.0002471

acre

Abstract The range of the endangered black abalone (Haliotis cracherodii) is divided into the North Central California region, the Central California region, the Southern California Mainland region, the Channel Islands region, and the Baja California region by the National Marine Fisheries Service for management purposes. San Nicolas Island is one of eight subregions of the Channel Islands region. The black abalone recovery plan establishes five demographic criteria for the possible delisting or downlisting of the species. The U.S. Geological Survey monitors nine long-term intertidal black abalone sites at San Nicolas Island, California, in cooperation with the U.S. Navy, which owns the island. This report uses data collected between 2013 and 2022 and the delisting criteria to analyze and describe the density, recruitment, size structure, and population trends at the nine U.S. Geological Survey monitoring sites at San Nicolas Island.

Introduction For management purposes, the Final Endangered Species Act Recovery Plan for Black Abalone (Haliotis cracherodii; National Marine Fisheries Service [2020] at https://repository.library.noaa.gov/view/noaa/27415; hereinafter referred to as the Recovery Plan) considers the range of the black abalone to be distributed among five regions. These regions are the North Central California region, the Central California region, the Southern California Mainland region, the Channel Islands region, and the Baja California region. San Nicolas Island is one of eight subregions of the Channel Islands region. The Recovery Plan

establishes five demographic criteria for the possible delisting or downlisting of the species. The criteria are similar for downlisting and delisting and differ only in the number of subregions within each region that should meet the conditions for each criterion. At the time that the Recovery Plan was developed, a test run analysis was done on data collected up to 2015 or 2016, depending on the subregion, to help determine and refine the thresholds for the criteria. This report updates the analysis and describes the density, recruitment, size structure, and population trends in the data collected through 2022 at the nine long-term monitoring sites at San Nicolas Island (fig. 1). As stated in the “Criterion 1: Geographic range occupied” section of the Recovery Plan, this criterion is met at the subregion level if the species is present in each of the last 5 years. At San Nicolas Island, black abalone were detected and counted annually at all the sites from 2001 to 2022 (Kenner and Yee, 2025). Data that can be used by managers to evaluate the remaining four population-based criteria are presented below. As context, we reiterate the Recovery Plan criteria for the subregion level here. We provide data related to these criteria but do not evaluate whether the criteria have been met because that evaluation is the purview of those managing the taxon and is beyond the scope of U.S. Geological Survey. The “Criterion 2: Habitat-based density” section of the Recovery Plan states that the criterion is met if the measured density of black abalone is at least the expected density, based on the quality of habitat, at 50 percent or more of study sites, for at least each of the past 5 years. The expected density for the different habitat quality categories has not yet been developed for the many geographic subregions of the black abalone population. Developing these expected densities is a recovery action task laid out in the Recovery Plan.

2 Black Abalone Population Density, Recruitment, Size Structure, and Population Growth at Naval Base, 2013–22

San Nicolas Island Black Abalone sites A LI FO R

Site 1: 6 transects (360 m 2)

Site 2: 6 transects (274 m 2)

N IA

Site 3: 7 transects (298 m 2) Site 9: 6 transects (198 m 2)

Site 8: 5 transects (278 m 2) Site 7: 5 transects (264 m 2) 0 0

2 2

Site 6: 4 transects (252 m 2)

Site 5: 3 transects (70 m 2)

Site 4: 2 transects (52 m 2)

4 MILES Coordinate System: WGS 1984 Web Mercator Auxiliary Sphere Michael Kenner 6/21/2018

4 KILOMETERS

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

Figure 1. Black abalone (Haliotis cracherodii) monitoring sites on San Nicolas Island, California, showing location, number of transects, and total sampling area at each site. Abbreviations: CNES/Airbus DS, National Centre for Space Studies/Airbus Defense and Space; IGN, Institut Geographique National; m2, square meters; USDA, U.S. Department of Agriculture; USGS, U.S. Geological Survey; WGS 1984, World Geodetic System of 1984.

The “Criterion 3: Recruitment” section of the Recovery Plan states that the criterion is met if black abalone recruit successfully, at 50 percent or more of study sites, determined by evidence of recruitment events observed in at least 2 non-consecutive years over the past 10 years. The “Criterion 4: Size structure” section of the Recovery Plan states that the criterion is met if black abalone populations are characterized by a broad distribution of size classes representing multiple cohorts that are stable at 50 percent or more of study sites over at least the past 5 years. The size distribution should consist of at least 40 percent small adults (from 50 to 100 millimeters [mm] shell length [SL]) and at least 10 percent large adults (greater than 100 mm SL). Finally, the “Criterion 5: Population trend” section of the Recovery Plan states that the criterion is met if population growth for reproductively mature individuals (greater than 50 mm SL) is stable or increasing when averaged across all study sites and at 50 percent or more of the individual study sites over at least the past 10 years, indicating that juveniles

are surviving to adulthood to reproduce and maintain or increase populations over time. Each of the study sites also must meet or exceed a minimum population growth rate. The minimum population rate proposed in the Recovery Plan is based on black abalone populations from Monterey County that have not suffered withering syndrome declines and exist in the presence of southern sea otters (Enhydra lutris nereis). This is a preliminary report with tables that provide data for San Nicolas Island black abalone populations with respect to these population criteria.

Methods and Results Information on the location of the nine San Nicolas Island sites and monitoring field methods is in Kenner and Yee (2025). Other methods used for this analysis are described in the following sections.

Methods and Results 3

Habitat-Based Density For Criterion 2: Habitat-based density, the population density for at least half the monitored sites within a subregion must meet the expected density for the habitat quality in each of the last 5 years (National Marine Fisheries Service, 2020). Therefore, the rating of the habitat quality of the nine San Nicolas Island sites needed to be determined, and the expected habitat-based density needed to be agreed upon. It is not clear how habitat quality at the San Nicolas Island sites was assessed for the test run analysis (appendix B of National Marine Fisheries Service [2020]), nor what quantitative mix of habitat quality each site was ultimately assigned. Three categories of habitat quality have been designated in habitat surveys completed along the California coast: (1) good, (2) moderate, and (3) poor (National Marine Fisheries Service, 2020). Good habitat is characterized by deep cracks or crevices greater than 10–15 centimeters (cm), or boulders. Moderate habitat is characterized by shallow or wide cracks, crevices, or depressions. Poor habitat is defined by flat, open, bare substrate, with no or minimal protection. In the 2022 field season, each of the 2,054 annually monitored permanent 1-square meter (m2) quadrats at San Nicolas Island was rated into one of the three habitat categories. Photographs of most of the transects are in Kenner (2020).

Although the Recovery Plan states that regional expected densities will eventually be developed, central California habitat-based densities were used as the expected densities for the test run analysis reported in the Recovery Plan because the central California populations were non-depleted but exposed to natural predators, including sea otters, and anthropogenic pressures (National Marine Fisheries Service, 2020). The values used in the test run and in this report are defined for each habitat quality category: Good habitat expected density is 1.58 abalone per m2; Moderate habitat expected density is 0.278 abalone per m2; and Poor habitat expected density is 0.0049 abalone per m2. To evaluate habitat-based density for each of the San Nicolas Island sites for this report, the proportion of each of the three habitat quality categories at a site was multiplied by the expected density for each category, and the products were summed, yielding an expected density for each site (table 1). The total number of abalone counted at each site in each of the last 5 years, 2018–22, was used to calculate a measured density for each site and year. The annual observed density for each site can be compared to the calculated expected density provided for each site. The measured density exceeded the calculated expected density at three of the sites (6, 7, and 8) during each of the last 5 years (2018–22). These values are presented in table 2.

Table 1. Total number of 1-square meter (m2) quadrats, number of quadrats in each habitat quality category (good, moderate, poor), and expected density (abalone per m2) for each of the nine black abalone (Haliotis cracherodii) monitoring sites at San Nicolas Island, California. [Good habitat is characterized by deep cracks or crevices greater than 10–15 centimeters (cm), or boulders. Moderate habitat is characterized by shallow or wide cracks, crevices, or depressions. Poor habitat is defined by flat, open, bare substrate, with no or minimal protection. The proportion of each of the three habitat quality categories at a site was multiplied by the expected density for each, and the products were summed. The expected densities used were Good, 1.58 abalone per m2; Moderate, 0.278 abalone per m2; Poor, 0.0049 abalone per m2]

Site

Total quadrats

Good

Moderate

Poor

Expected density of site

1 2

360

199

0.56

274

132

139

0.90

300

162

0.53

0.56

0.44

252

178

0.13

270

136

0.51

278

121

131

0.82

198

138

0.49

4 Black Abalone Population Density, Recruitment, Size Structure, and Population Growth at Naval Base, 2013–22 Table 2. Black abalone (Haliotis cracherodii) densities from 2018 to 2022 at the nine monitoring sites at San Nicolas Island, California.

Table 2. Black abalone (Haliotis cracherodii) densities from 2018 to 2022 at the nine monitoring sites at San Nicolas Island, California.—Continued

[To calculate the expected density of a site, the proportion of each of the three habitat quality categories observed at a site was multiplied by the expected density for each, and the products were summed. The expected densities used were Good, 1.58 abalone per square meter (m2); Moderate, 0.278 abalone per m2; Poor, 0.0049 abalone per m2]

Year

Total abalone

Measured density (abalone per m2)

Expected density of site (abalone per m2)

Year

Total abalone

Measured density (abalone per m2) Site 7

Site 1 2018

0.27

2018

611

2.26

2019

0.19

2019

664

2.46

2020

0.22

2020

728

2.70

2021

141

0.39

2021

772

2.86

2022

144

0.40

2022

812

3.01

0.56

2018

973

3.50

0.13

2019

1017

3.66

0.20

2020

1103

3.97

0.20

2021

706

2.54

0.17

2022

787

2.83

2018

0.16

2018

0.20

2019

0.14

2019

0.15

2020

0.16

2020

0.39

2021

0.14

2021

0.23

2022

0.17

2022

0.34

0.19

2019

2020

2021 2022

0.51

Site 8

Site 2 2018

Expected density of site (abalone per m2)

0.90

0.82

Site 9

Site 3 0.53

0.49

Site 4 2018

0.96

2019

0.52

2020

1.04

2021

0.92

2022

1.29

2018

0.46

2019

0.34

2020

0.46

2021

0.80

2022

0.63

0.56

Site 5 0.44

Site 6 2018

112

0.44

2019

114

0.45

2020

168

0.67

2021

156

0.62

2022

136

0.54

0.13

Recruitment For Criterion 3: Recruitment, it must be evident in at least 2 non-sequential years in a 10-year period at 50 percent or more of the sites in a subregion. Evidence of recruitment is defined in the Recovery Plan as the presence at a site of at least 10 individuals less than or equal to 30 mm in length (National Marine Fisheries Service, 2020). In the San Nicolas Island black abalone sampling protocols, we measure all accessible abalone on a site until at least 200 have been measured, if that many are counted. Beginning in 2019, for individuals that were not measured because of accessibility problems or excess sample size, classifications of abalone as recruits (30 mm or less in length) or non-recruits (greater than 30 mm) were estimated visually. For the recruitment data presented here, estimated recruits are included as part of the measure of recruitment, but they are noted separately because they represent a change in methods starting in 2019 (table 3). Sites 3, 5, 7, 8, and 9 had two or more non-sequential recruitment events in the last 10 years.

Methods and Results 5 Table 3. Number of recruits (R), defined as black abalone (Haliotis cracherodii) counted with a shell length less than or equal to 30 millimeters (mm), calculated annually from 2013 to 2022 at the nine monitoring sites at San Nicolas Island, California. [R measured are those measured in the field. R estimated are those that appeared to be recruitment size but were not measured. R total is the sum of measured recruits and estimated recruits. Years with no recruits at a site are not listed. Abbreviation: —, recruits were not estimated]

Year

R measured

R estimated

Table 3. Number of recruits (R), defined as black abalone (Haliotis cracherodii) counted with a shell length less than or equal to 30 millimeters (mm), calculated annually from 2013 to 2022 at the nine monitoring sites at San Nicolas Island, California.— Continued [R measured are those measured in the field. R estimated are those that appeared to be recruitment size but were not measured. R total is the sum of measured recruits and estimated recruits. Years with no recruits at a site are not listed. Abbreviation: —, recruits were not estimated]

R total Year

Site 1 2014

—

2015

—

2016

—

2017

—

2018

—

2019

2020

—

2021

2022

R measured

R estimated

R total

Site 5—Continued 2018

—

2019

2020

2021

2022

Site 6 2015

—

2016

—

2017

—

2018

—

2020

2021

2022

Site 2 2014

—

2015

—

2016

—

2019

2020

2022

2013

—

2014

—

2015

—

2016

—

2017

—

2018

—

2019

2020

105

2021

2022

Site 3 2013

—

2014

—

2015

—

2017

—

2018

—

2020

2021

2022

Site 7

Site 8

Site 4 2013

—

2014

—

2015

—

2016

—

2017

245

—

245

2018

—

2019

125

195

2020

223

299

2021

105

2022

105

178

283

2014

—

2017

—

2020

2021

2022

2 Site 5

2013

—

2014

—

2015

—

2016

—

2017

—

6 Black Abalone Population Density, Recruitment, Size Structure, and Population Growth at Naval Base, 2013–22 Table 3. Number of recruits (R), defined as black abalone (Haliotis cracherodii) counted with a shell length less than or equal to 30 millimeters (mm), calculated annually from 2013 to 2022 at the nine monitoring sites at San Nicolas Island, California.— Continued

Table 4. Size structure of black abalone (Haliotis cracherodii) from 2018 to 2022 at the nine monitoring sites at San Nicolas Island, California. Counts and percentages of small (50–100 millimeter [mm] shell length [SL]) and large (greater than 100 mm SL) abalone for each site are displayed.

[R measured are those measured in the field. R estimated are those that appeared to be recruitment size but were not measured. R total is the sum of measured recruits and estimated recruits. Years with no recruits at a site are not listed. Abbreviation: —, recruits were not estimated]

[Counts and percentages of small (50–100 millimeter [mm] shell length [SL]) and large (greater than 100 mm SL) abalone for each site are displayed. At sites 7 and 8, only the counts from the first three transects are listed and used to calculate percentages. Abbreviation: >, greater than]

Year

R measured

R estimated

R total

Site 9 Year

Number measured

2015

—

2016

—

2017

—

2019

—

2018

2020

2019

2021

2020

2022

2021 2022

Size Structure Criterion 4 states that the size structure of the populations at 50 percent or more of the sites in a subregion must favor young adults such that at least 40 percent of the population at these sites must be 50–100 mm SL (small adults) and at least 10 percent must be more mature individuals that are greater than 100 mm SL (large adults). This analysis is to be based on a sample size of at least 50 measured individuals and is to be calculated as the most recent value of a running cumulative proportion over a 5-year period (National Marine Fisheries Service, 2020). Sites 7 and 8 have several hundred abalone, and at these sites, the measuring of SLs was curtailed after at least 200 measurements were made. Because most or all the reachable abalone on the first three transects were measured, this analysis uses only these transects at these two sites. The population structure conditions are presented in table 4. Only sites 6, 7, and 8 had more than 50 individuals every year over the past 5 years, and they had 64.5, 41.0, and 25.4 percent, respectively, in the small adult size category. All three sites had more than 10 percent in the large adult category.

Running average percentage of

Count of 50–100 mm

>100 mm

50–100 mm

>100 mm

40.6

46.9

33 31.7

61.0

48.8

31.2

69.4

18.1

32.3

5.2

Site 1

Site 2 2018

2019

2020

2021

2022

Site 3 2018

2019

2020

2021

2022

Site 4 2018

2019

2020

2021

2022

Site 5 2018

2019

2020

2021

2022

Methods and Results 7 Table 4. Size structure of black abalone (Haliotis cracherodii) from 2018 to 2022 at the nine monitoring sites at San Nicolas Island, California. Counts and percentages of small (50–100 millimeter [mm] shell length [SL]) and large (greater than 100 mm SL) abalone for each site are displayed.—Continued [Counts and percentages of small (50–100 millimeter [mm] shell length [SL]) and large (greater than 100 mm SL) abalone for each site are displayed. At sites 7 and 8, only the counts from the first three transects are listed and used to calculate percentages. Abbreviation: >, greater than]

Year

Number measured

Running average percentage of

Count of 50–100 mm

>100 mm

50–100 mm

>100 mm

64.5

18.2

41.0

32.4

25.4

41.3

60.9

20.5

Site 6 2018

106

2019

108

2020

166

116

2021

148

2022

118

Site 7 2018

208

2019

181

2020

183

2021

176

2022

207

Site 8 2018

463

132

185

2019

505

164

216

2020

370

127

2021

200

110

2022

234

Site 9 2018

2019

2020

2021

2022

Population Trend Criterion 5 focuses on the population growth rate of reproductively mature individuals, defined in the Recovery Plan as having greater than 50 mm SL. The population growth rate for this size class measured over a 10-year period must be (1) stable or increasing when averaged over all sites in the subregion; (2) stable or increasing at 50 percent or more of the sites; and (3) greater than or equal to a minimum expected growth rate at all sites (National Marine Fisheries Service, 2020). The minimum expected growth rate suggested by the Recovery Plan is currently under review by the Black Abalone

Recovery Team but is expected to be the smallest growth rate calculated from apparently healthy populations in central California during 2009–16 (Susan Wang, National Marine Fisheries Service, oral commun., 2023). The protocol for collecting size data at the San Nicolas Island sites that have high numbers of abalone can yield misleading results when calculating population growth of a specified size class. The protocol at these sites is to measure at least 200 abalone, which captures a reasonable size distribution sample without using too much valuable field time during the low tide sampling window. The actual number measured can vary considerably between field seasons depending on the accessibility of abalone, tidal heights, time of day, sea state, and other limiting weather conditions and is not related to changes in the population size. Because the analysis for this criterion requires that population growth be based on numbers of abalone of a certain size class (greater than 50 mm SL), the number being used to calculate population growth is affected by the size of the sample of measured individuals, not the counted number of total individuals. Sites 7 and 8 are the only sites where measurements are limited by sample size rather than just accessibility. At these sites, transects 1–3 were counted first to prioritize consistency, and most abalone counted on transects 4–5 were rarely measured after reaching the sample size limit for measurements. To reduce sampling variations when evaluating changes across time, only data from transects 1 to 3 were used in this analysis for sites 7 and 8. The numbers reported for these sites represent extrapolated estimates for counts of abalone in the greater than 50 mm SL size class on transects 1–3. To account for sampling error, we calculated 95-percent confidence intervals (CI) for the true proportion of abalone in that size class among all abalone counted on these transects by using the “svyciprop” function in the “survey” package of R statistical software (Korn and Graubard, 1998; Lumley, 2004; R Core Team, 2023) and then multiplied the CI limits of the proportion with the total abalone counted to derive 95-percent CI for the number of abalone in this size class. To address (1) of Criterion 5, the average population growth rate at the nine San Nicolas Island black abalone monitoring sites from 2013 to 2022 was calculated from the slope over those 10 years of the Log10[Mean(Total count)+1], where Mean(Total count) is the total number of measured abalone in the adult size class (greater than 50 mm SL) for all the sites divided by the number of sites. The resulting average population growth rate for San Nicolas Island was −0.003 (table 5). Growth rates at the individual sites were calculated to address (2) and (3), of Criterion 5, described earlier, using the slope over the last 10 years of the Log10[(Total count)+1], where (Total count) is the total number of measured abalone in the adult size class at each site. Five sites (1, 2, 4, 6, and 9) had positive growth rates, and four sites (3, 5, 7, and 8) had negative growth rates (table 5). Site 6 had the highest calculated growth rate (0.097), and site 3 had the lowest calculated growth rate (−0.037).

8 Black Abalone Population Density, Recruitment, Size Structure, and Population Growth at Naval Base, 2013–22 Table 5. Annual and average population growth trend for black abalone (Haliotis cracherodii) greater than 50 millimeters (mm) shell length (SL) over a 10-year period at the nine monitoring sites at San Nicolas Island, California.

Table 5. Annual and average population growth trend for black abalone (Haliotis cracherodii) greater than 50 millimeters (mm) shell length (SL) over a 10-year period at the nine monitoring sites at San Nicolas Island, California.—Continued

[At sites 7 and 8, only size data from the first three transects were used. We calculated 95-percent confidence intervals (CI) for the true proportion of abalone greater than 50 mm among all abalone counted on these transects and then multiplied the CI limits of the proportion with the total abalone counted to derive 95-percent CI (shown in parentheses) for the number of abalone in this size class. Abbreviation: >, greater than]

Average growth slope of Log10(Count+1)

Year

Abalone >50 mm

−0.003

2013

1.857

1.988

2014

1.763

1.959

2015

1.771

76.33

1.883

2016

1.491

93.78

1.972

2017

1.663

2018

104.11

2.017

2018

1.643

2019

109.67

2.040

2019

1.544

2020

112.56

2.051

2020

1.544

2021

76.78

1.885

2021

1.431

2022

73.33

1.65

2022

1.556

2013

1.362

2013

0.778

2014

1.431

2014

1.146

2015

1.690

2015

1.176

2016

1.477

2016

1.431

2017

1.415

2017

1.568

2018

1.690

2018

1.556

2019

1.568

2019

1.000

2020

1.792

2020

1.653

2021

1.690

2021

1.531

2022

1.653

2022

1.663

Year

Abalone >50 mm

Annual growth Log10(Count+1)

2013

90.00

1.954

2014

97.33

2015

90.89

2016 2017

Annual growth Log10(Count+1)

Average growth slope of Log10(Count+1)

Site 3

All sites (mean count)

−0.037

Site 4

Site 1 0.033

0.071

Site 5

Site 2 2013

1.146

2013

1.322

2014

1.255

2014

1.230

2015

1.342

2015

0.954

2016

1.204

2016

0.903

2017

1.322

2017

1.114

2018

1.477

2018

1.279

2019

1.362

2019

1.114

1.146

0.027

2020

1.415

2020

2021

1.398

2021

0.903

2022

1.415

2022

1.041

−0.019

Methods and Results 9 Table 5. Annual and average population growth trend for black abalone (Haliotis cracherodii) greater than 50 millimeters (mm) shell length (SL) over a 10-year period at the nine monitoring sites at San Nicolas Island, California.—Continued

Year

Abalone >50 mm

Annual growth Log10(Count+1)

2013

1.301

2014

1.447

2015

1.415

2016

1.591

2017

1.663

2018

1.881

2019

1.982

2020

133

2.127

2021

130

2.117

2022

103

2.017

2013

213 (203, 222)

2.330

2014

268 (258, 277)

2.430

2015

188 (177, 198)

2.276

2016

152 (144, 160)

2.185

2017

181 (166, 194)

2.260

Average growth slope of Log10(Count+1)

Year

0.097

2013

417 (400, 432)

2.621

2014

426 (400, 450)

2.630

2015

428 (398, 456)

2.632

2016

368 (363, 372)

2.567

2017

452 (441, 462)

2.656

2018

481 (462, 499)

2.683

2019

548 (532, 564)

2.740

2020

453 (430, 475)

2.657

2021

214 (204, 223)

2.332

2022

180 (165, 195)

2.258

Abalone >50 mm

Site 6

171 (164, 178)

2.236

2019

202 (192, 210)

2.307

2020

189 (179, 198)

2.279

173 (166, 180)

2.241

175 (167, 182)

2.246

2021 2022

Average growth slope of Log10(Count+1)

Site 8, transects 1, 2, and 3 only

Site 7, transects 1, 2, and 3 only

2018

Annual growth Log10(Count+1)

−0.011

−0.028

Site 9 2013

1.342

2014

1.255

2015

1.301

2016

1.146

2017

1.301

2018

1.462

2019

1.322

2020

1.724

2021

1.447

2022

1.544

0.036

10 Black Abalone Population Density, Recruitment, Size Structure, and Population Growth at Naval Base, 2013–22

Discussion Habitat-Based Density Rating the habitat quality of each quadrat was subjective but not time-consuming. Because expected densities are different for the three habitat quality categories, varying interpretations of the categories may result in different site-specific expected densities. For example, preferentially rating habitat with low-quality ledges as moderate rather than poor will yield expectations of higher densities than warranted and could result in a lower or misleading rate of meeting the population density criterion. Some training between monitoring groups working in different regions and using different protocols could reduce this variability.

Recruitment Classifying non-measured abalone as recruits or not has only been done since 2019. Only once in this analysis did the change in classification make a difference in raising the number of recruits above 10 in a year (site 9 in 2020). This change in protocol results in more abalone being classified as recruits, which eventually could result in more sites exceeding the 10-recruit threshold as the analysis is repeated in future years.

Size Structure Out of the 9 sites at San Nicolas Island, 6 had less than 50 measured individuals in each of the last 5 years. Of these, only site 1 consistently had counts exceeding 50, but abalone were often inaccessible and could not be measured. Beginning in 2022, all abalone that were not measured were estimated into size bins. In a few years, these bins could be used to increase the number of sites with meaningful size-structure data. Several of the other sites are close to having 50 abalone counted and, if counts continue to increase, could contribute to this analysis in future years.

Population Trend This analysis is based on changes in the numbers of reproductive-size abalone and is affected by the proportions of counted populations that are sized. Therefore, size samples

that are incomplete because of inaccessible or numerous abalone can affect interpretations of the data. The size binning of non-measured abalone that began in 2022 could eventually alleviate this problem, but because the growth rate trend examines a 10-year period, data collected using the new protocol cannot be used to examine population trends for several years.

References Cited Kenner, M.C., 2020, Black abalone surveys at Naval Base Ventura County, San Nicolas Island, California—2019, annual report: U.S. Geological Survey Open-File Report 2020–1047, 41 p., accessed December 20, 2023, at https://doi.org/10.3133/ofr20201047. Kenner, M.C., and Yee, J.L., 2025, Black abalone surveys at Naval Base Ventura County, San Nicolas Island, California—2022 annual report: U.S. Geological Survey Open-File Report 2025–1014, 34 p. [Available at https://doi.org/10.3133/ofr20251014.] Korn, E.L., and Graubard, B.I., 1998, Confidence intervals for proportions with small expected number of positive counts estimated from survey data: Survey Methodology, v. 24, no. 2, p. 193–201. Lumley, T., 2004, Analysis of complex survey samples: Journal of Statistical Software, v. 9, no. 8, p. 1–19. [Available at https://doi.org/10.18637/jss.v009.i08.] National Marine Fisheries Service, 2020, Final Endangered Species Act (ESA) recovery plan for black abalone (Haliotis cracherodii): Long Beach, Calif., National Oceanic and Atmospheric Administration, National Marine Fisheries Service, West Coast Region, Protected Resources Division, 112 p., accessed January 9, 2025, at https://repository.library.noaa.gov/view/noaa/27415. R Core Team, 2023, R—A language and environment for statistical computing: Vienna, Austria, R Foundation for Statistical Computing, accessed December 20, 2023, at https://www.R-project.org/.

For more information concerning the research in this report, contact the Director, Western Ecological Research Center U.S. Geological Survey 3020 State University Drive East Sacramento, California 95819 https://www.usgs.gov/centers/werc Publishing support provided by the Science Publishing Network, Sacramento Publishing Service Center

Kenner and Yee—Black Abalone Population Density, Recruitment, Size Structure, and Population Growth at Naval Base, 2013–22—OFR 2025–1015

ISSN 2331-1258 (online) https://doi.org/10.3133/ofr20251015

Black Abalone ( Haliotis cracherodii ) Population Density, Recruitment, Size Structure, and Population Growth at Naval Base Ventura County, San Nicolas Island, California, 2013–22

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