Characterizing and Quantifying California Sea Lion 

(Zalophus californianus) Use of Offshore Oil and Gas Platforms 

in California

US Department of the Interior

Bureau of Ocean Energy Management

Pacific OCS Region

OCS Study

BOEM 2016-009
Characterizing and Quantifying California Sea Lion 

(Zalophus californianus) Use of Offshore Oil and Gas Platforms 

in California

Authors

Anthony J. Orr1*, Jeffrey D. Harris1, Kari A. Hirschberger1, Jeffrey L. Laake1,

Robert L. DeLong1, and Gregory S. Sanders2

Prepared under Inter-Agency Agreement Number M12PG00027

by

1 Marine Mammal Laboratory

Alaska Fisheries Science Center

NOAA Fisheries

National Oceanic and Atmospheric Administration

7600 Sand Point Way NE

Seattle, WA 98115

2 Bureau of Ocean Energy Management

Pacific OCS Region

760 Paseo Camarillo, Suite 102

Camarillo, CA 93010

US Department of the Interior

Bureau of Ocean Energy Management

Pacific OCS Region

July 2016

OCS Study

BOEM 2016-009
ii

Disclaimer

This study was funded, in part, by the US Department of the Interior, Bureau of Ocean 

Energy Management, Environmental Studies Program, Washington, DC, through InterAgency Agreement Number M12PG00027 with the NOAA Fisheries. This report has been 

technically reviewed by BOEM and it has been approved for publication. The views and 

conclusions contained in this document are those of the authors and should not be 

interpreted as representing the opinions or policies of the US Government, nor does 

mention of trade names or commercial products constitute endorsement or 

recommendation for use.

Report & Data Availability

To download a PDF of this report, go to the US Department of the Interior, Bureau of Ocean 

Energy Management, Environmental Studies Program Information System (ESPIS) website 

at http://marinecadastre.gov/espis/#/ and search on OCS Study BOEM 2016-009. The 

report may also be downloaded from BOEM’s Recently Completed Environmental Studies –

Pacific webpage at http://www.boem.gov/Pacific-Completed-Studies/.

Metadata for this study is provided in InPort NMFS Enterprise Data Management Program 

(https://inport.nmfs.noaa.gov); Dataset ID 26353. Data for this study are archived at NOAA 

National Centers for Environmental Information (https://nodc.noaa.gov); Package 

Reference ID R1B85T.

Citation

Orr, A.J., J.D. Harris, K.A. Hirschberger, J.L. Laake, R.L. DeLong, and G.S. Sanders. 2016. 

Characterizing and quantifying California sea lion (Zalophus californianus) use of 

offshore oil and gas platforms in California. US Department of the Interior, Bureau of 

Ocean Energy Management, Pacific OCS Region, Camarillo, CA. OCS Study BOEM 

2016-009. 35 p.

Contacts

Anthony J. Orr: Gregory S. Sanders:

Email: tony.orr@noaa.gov Email: greg.sanders@boem.gov

Phone: 206.526.4512 Phone: 805.384.6396

Fax: 206.526.6615 Bureau of Ocean Energy Management

California Current Ecosystem Program Pacific OCS Region

Marine Mammal Laboratory 760 Paseo Camarillo, Suite 102

Alaska Fisheries Science Center Camarillo, CA 93010 USA

NOAA Fisheries

National Oceanic and Atmospheric Administration

7600 Sand Point Way NE

Seattle, WA 98115 USA
iii

Contents

List of Figures ............................................................................................................................................................iv

List of Tables...............................................................................................................................................................v

List of Acronyms.........................................................................................................................................................v

ABSTRACT ..................................................................................................................................................................vi

INTRODUCTION..........................................................................................................................................................1

METHODS ....................................................................................................................................................................1

Platforms & camera systems .................................................................................................................................. 1

Data collection......................................................................................................................................................... 5

Data analyses........................................................................................................................................................... 7

RESULTS.......................................................................................................................................................................8

Intra-platform (temporal) comparisons ............................................................................................................. 10

Platform Elly...................................................................................................................................................... 10

Platform Gina..................................................................................................................................................... 14

Platform Habitat................................................................................................................................................ 14

Platform Heritage.............................................................................................................................................. 15

Platform Harvest............................................................................................................................................... 16

Inter-platform (spatial) comparisons.................................................................................................................. 17

DISCUSSION...............................................................................................................................................................19

Platforms & camera systems ................................................................................................................................ 19

Data Analysis Issues.............................................................................................................................................. 20

Pinniped presence................................................................................................................................................. 22

Intra-platform (temporal) comparisons ............................................................................................................. 23

Inter-platform (spatial) comparisons.................................................................................................................. 25

Notables ................................................................................................................................................................. 26

Conclusions............................................................................................................................................................ 32

ACKNOWLEDGEMENTS ...........................................................................................................................................33

LITERATURE CITED .................................................................................................................................................34
iv

List of Figures

Figure 1. Several of the 23 oil and gas production platforms in federal waters offshore 

southern California that were visited during October and November 2012 to evaluate for 

the installation of time-lapse camera systems to document the use of the platforms by 

pinnipeds. 

Figure 2. Locations of gas and oil platforms located in the Pacific Outer Continental Shelf 

Region offshore southern California where time-lapse camera systems were installed to 

document the use of the platforms by pinnipeds between January 2013 and January 2015. 

The offshore platforms included (from south to north): Platform Elly (Beta Offshore; San 

Pedro Bay), Platform Gina (DCOR; Point Hueneme Unit), Platform Habitat (DCOR; Pitas 

Point Unit), Platform Heritage (Exxon Mobil; Santa Ynez Unit), and Platform Harvest (Plains 

Exploration and Production Company (PXP)/Freeport McMoran; Point Arguello Unit). 

Figure 3. Digital automated time-lapse camera systems (developed by Trail Watcher Game 

Cameras; Monticello, Georgia) that were installed on five gas and oil platforms offshore 

southern California and programmed to take images between January 2013 and January 

2015. The systems consisted of a timing circuit, a camera, and battery bank. They were 

placed in locations where most areas where pinnipeds could be viewed were covered.

Figure 4. Pinniped species observed at or near the gas and oil platforms besides the 

California sea lion (Zalophus californianus) included: (a) Steller sea lion (Eumetopias 

jubatus), and (b) Pacific harbor seal (Phoca vitulina richardii). 

Figure 5. Monthly median (top) and maximum (bottom) counts of California sea lions

(Zalophus californianus) on gas and oil platforms located in the Pacific Outer Continental 

Shelf Region offshore southern California from January 2013 to January 2015. Platforms 

are listed in order from south to north. 

Figure 6. Hourly median (top) and maximum (bottom) counts of California sea lions 

(Zalophus californianus) on gas and oil platforms located in the Pacific Outer Continental 

Shelf Region offshore southern California during 2013 and 2014. Platforms are listed in 

order from south to north. 

Figure 7. Proportion of California sea lions (Zalophus californianus) using gas and oil 

platforms that were identified to a specific age/sex class [from south (top) to north 

(bottom)] by month from January 2013 through January 2015. Numbers within bars 

indicate total number of animals identified during that month. 

Figure 8. California sea lions (Zalophus californianus) feeding in waters adjacent to gas and 

oil platforms; sometimes feeding with other animals including cetaceans and sea birds.

Figure 9. Adult female California sea lions (Zalophus californianus) nursing pup (left) and 

juvenile (right) while hauled out on gas and oil platforms during the study period.
v

Figure 10. Marked (i.e. (a) tagged, (b) branded, or (c) instrumented) California sea lions 

(Zalophus californianus) were seen on and around the gas and oil platforms during the 

study area. Marked individuals included those that were: (a) were rehabilitated and tagged 

at The Marine Mammal Center, Sausalito, California; and (b) tagged and/or branded by the

Marine Mammal Laboratory, Seattle, Washington for demographic studies on San Miguel 

Island, Washington or in the Pacific Northwest. 

Figure 11. California sea lions (Zalophus californianus) hauled out on gas and oil platforms 

exhibiting signs of encounters with (a) fisheries (e.g. monofilament fishing line) and (b) 

sharks (e.g. lesions).

List of Tables

Table 1. Tasks and dates of visitation to oil and gas platforms in federal waters off the 

southern California Bight. Camera systems were deployed on the platforms to document 

their usage by pinnipeds between January 2013 and January 2015.

Table 2. Months during which at least one camera system located at a focal gas and oil 

platform was not operational, or not set up (i.e. January 2013, Platform Harvest). Data were 

not analyzed during those months. 

Table 3. Crew boat departure to (Platforms Elly, Gina, and Habitat) or arrival at (Platform 

Heritage) platform times. Actual arrival times to platforms were dependent on weather, 

sea-state conditions, and activities of the crew but usually within 1.5 hours. Crew 

exchanges on Platform Harvest were primarily conducted by helicopter.

List of Acronyms

BOEM Bureau of Ocean Energy Management

BSEE Bureau of Safety and Environmental Enforcement

MML Marine Mammal Laboratory

MMPA Marine Mammal Protection Act

NOAA National Oceanic and Atmospheric Administration

OCS Outer Continental Shelf

PXP Plains Exploration and Production Company
vi

ABSTRACT

There are 23 oil and gas production platforms in federal waters offshore southern 

California. These platforms provide haulout space near foraging areas for California sea 

lions (Zalophus californianus), a species protected under the Marine Mammal Protection 

Act. Information on abundance, and age-, sex-, and seasonal-use patterns of California sea 

lions on oil and gas platforms is useful for the environmental review of ongoing activities 

and the eventual removal of platforms when oil and gas production ceases (i.e. 

decommissioning). Quantitative estimates of potential harassment, injury, or mortality for 

future activities may be derived from data collected during this study. 

For this study, 5 of the 23 federal platforms were selected as focal sites based on 

their geographical location and relative accessibility by sea lions. Time-lapse camera 

systems were deployed on the platforms from January 2013 to January 2015. Photos were 

taken every 30 minutes during day and night. A subsample of images was randomly 

selected from six-hour blocks of time throughout the day, and during randomly selected 

days throughout each month. Individuals were counted and identified to a particular 

age/sex-class, when possible. Counts of animals by month and hour were conducted to 

examine intra-platform (temporal) and inter-platform (spatial) comparisons. A subsample 

of images (ntotal = 12,489; Platform Elly = 1,981, Platform Gina = 1,960, Platform Habitat = 

4,742, Platform Heritage = 2,551, Platform Harvest = 1,255) was used in data analyses.

There were no consistent spatial trends (i.e. south-to-north) in numbers of sea lions using 

the platforms. Platform Habitat (central) had the highest counts of sea lions (Median: 

2013range = 32 – 134; 2014range = 40 – 110), whereas Platform Gina (south) had the lowest

(Median: 2013range = 1 – 21; 2014range = 3 – 22) throughout the study. Also, there were no 

consistent seasonal trends in numbers across all platforms. At some platforms (i.e. 

Platforms Gina and Habitat), the animals primarily hauled out at night and were away 

during the middle of the day, whereas at others (i.e. Platforms Heritage and Harvest) the 

opposite trend was apparent. At Platform Elly, the sea lions used the platform relatively 

consistent throughout the day.

Additional observations made during this study included: (1) pinniped-use on all 

platforms was dominated by California sea lions, however Steller sea lions (Eumetopias 

jubatus) were occasionally present at Platforms Elly and Habitat; (2) California sea lion 

adult females were observed nursing at platforms; and (3) adult and subadult California 

sea lion males were seen at the platforms year-round. These observations have enhanced 

our understanding about the distribution and behavior of California sea lions and their use 

of offshore platforms.

Keywords: California sea lion, oil and gas platform, Pacific Outer Continental Shelf,

pinniped, time-lapse camera system, Zalophus californianus.
1

INTRODUCTION

As of 2016, there are 27 oil and gas production platforms offshore southern 

California. Twenty-three of the platforms are in federal waters offshore San Pedro Bay (n = 

4), the Santa Barbara Channel (n = 15), and in the Santa Maria Basin (n = 4). These 

platforms are under leases issued by the Bureau of Ocean Energy Management (BOEM) and 

administered by the Bureau of Safety and Environmental Enforcement (BSEE). The other 

four platforms are in state waters (less than 5 km from shore) and operate under leases 

issued by the California State Lands Commission. 

The federal platforms were erected between 1967 and 1989. Currently, they 

produce approximately 63,000 barrels of oil and 130 million cubic feet (Mmcf) of natural 

gas per day (BOEM Studies Plan). At this rate, production could be sustained into the next 

decade. However, eventually all of these facilities will be decommissioned at the end of 

their productive life times. 

Many of the oil and gas platforms provide haulout space near foraging areas for 

pinniped species like the California sea lion (Zalophus californianus), the Steller sea lion 

(Eumetopias jubatus), and the Pacific harbor seal (Phoca vitulina). Analyses of on-going 

activities and decommissioning of platforms will benefit from the information collected 

during this study.

Decommissioning of platforms falls under the jurisdiction of BSEE, and BOEM

conducts environmental studies and environmental review support for BSEE actions. 

During 2012, BOEM entered an inter-agency agreement with the National Oceanic and 

Atmospheric Administration’s (NOAA) Marine Mammal Laboratory (MML; Seattle, 

Washington) to collect information in support of analyses pursuant to the National 

Environmental Policy Act (NEPA) and Marine Mammal Protections Act (MMPA) permitting 

requirements. Specifically, the objectives of this study were to: (1) characterize and 

quantify California sea lion use of oil and gas platforms offshore southern California area of 

the Pacific OCS Region; and (2) assess the abundance, seasonal use patterns, and age/sex 

class structure of animals using the offshore facilities.

METHODS

Platforms & camera systems

All 23 federal platforms were visited between 30 October and 7 November 2012

(Fig. 1). During these visits, the number and age/sex class of each individual pinniped 

hauled out on the platforms were recorded. Additionally, each platform was evaluated for 

the installation of digital automated time-lapse camera systems including; number and size 

of landing decks, height of railing above pinniped-occupied decks, and diameter of rails.
2

Figure 1. Several of the 23 oil and gas production platforms in federal 

waters offshore southern California that were visited during October and 

November 2012 to evaluate for the installation of time-lapse camera 

systems to document the use of the platforms by pinnipeds. 

Camera systems could not be placed on all the platforms due to time and monetary 

constraints. After the initial assessments were completed, 5 of the 23 platforms were 

selected as focal study sites. Selection criteria included: (1) high usage by pinnipeds, (2) 

camera placement constraints, and (3) a “good” representative within a particular 

geographical area. The five platforms selected were (from south to north): Platform Elly 

(Beta Offshore; San Pedro Bay), Platform Gina (DCOR; Point Hueneme Unit), Platform

Habitat (DCOR; Pitas Point Unit), Platform Heritage (Exxon Mobil; Santa Ynez Unit), and 

Platform Harvest (originally Plains Exploration and Production Company (PXP) but 

ownership transferred to Freeport McMoran during the study; Point Arguello Unit; Fig. 2). 
3

Figure 2. Locations of gas and oil platforms located in the 

Pacific Outer Continental Shelf Region offshore southern 

California where time-lapse camera systems were 

installed to document the use of the platforms by 

pinnipeds between January 2013 and January 2015. The 

offshore platforms included (from south to north): 

Platform Elly (Beta Offshore; San Pedro Bay), Platform 

Gina (DCOR; Point Hueneme Unit), Platform Habitat 

(DCOR; Pitas Point Unit), Platform Heritage (Exxon Mobil; 

Santa Ynez Unit), and Platform Harvest (Plains 

Exploration and Production Company (PXP)/Freeport 

McMoran; Point Arguello Unit).

±

0 30 60 120 180 240

Kilometers

115°0'0"W

120°0'0"W

120°0'0"W

40°0'0"N

40°0'0"N

35°0'0"N

35°0'0"N

California

Mexico

Elly

Gina

Habitat

Heritage Harvest

San Miguel Is.

San Nicolas Is.

Los Angeles

Santa Barbara Is.

San Clemente Is.
4

The camera systems (Trail Watcher Game Cameras; Monticello, Georgia; Fig. 3)

were composed of a timing circuit, a small point-and-shoot camera (Sony Cyber-shot DSCWX50, 16.2 megapixels), and battery bank (Energizer Ultimate Lithium AA and Duracell 

Procell C), which were all mounted within a small Pelican® waterproof case (Fig. 3).

Cameras systems were attached to custom fabricated aluminum backing plates with a 

hood. The backing plates containing the camera systems were attached to railings on the 

platform using clamps and stainless steel fastings (Fig. 3). Initially, the time-lapse camera

systems were programmed to take a photo every 15 minutes between the hours of 0300h 

and 1900h. 16GB memory cards were installed in each camera. Cameras were deployed on 

Platforms Elly, Gina, Habitat and Heritage in January 2013 and Platform Harvest in 

February 2013. The number of cameras required, location, and viewing angle to maximize 

coverage of pinniped-use areas were all considered in determining the placement of the 

camera systems. The number of camera systems installed on each platform were as 

follows: Platform Elly = 3, Platform Gina = 3, Platform Habitat = 4, Platform Heritage = 4, 

and Platform Harvest = 3.

Figure 3. Digital automated time-lapse camera systems (developed by Trail 

Watcher Game Cameras; Monticello, Georgia) that were installed on five 

gas and oil platforms offshore southern California and programmed to take 

images between January 2013 and January 2015. The systems consisted of 

a timing circuit, a camera, and battery bank. They were placed in locations 

where most areas where pinnipeds could be viewed were covered.
5

In February 2013, the four platforms that had camera systems were revisited so that 

the conditions of the camera systems could be inspected and maintenance performed (e.g. 

batteries replaced, battery voltage measured) and memory cards containing digital images 

were collected and replaced with new/empty cards. It was determined that 16GB memory 

cards did not have sufficient space to store two-months’ worth of images at a rate of 1 

image/15 min. The sampling protocol was changed to 1 image/30 min throughout the 

entire day and 16GB cards were replaced by 32GB cards. We also encountered problems of 

failing battery voltage on a few of the camera systems [i.e. Platform Elly (n = 2) and 

Platform Gina (n = 1)], which resulted in the cameras operating for less than a month. 

During this month, three camera systems were deployed on Platform Harvest. 

In April 2013, all five of the platforms were revisited so that batteries and memory 

cards could be exchanged and other maintenance (e.g. cleaning viewing window) could be 

performed following a two-month deployment. A couple of the camera systems [i.e. 

Platform Elly (n = 1) and Platform Habitat (n = 1)] were determined not be working 

properly, resulting in failures to take photos at programmed times since the last visit. The 

failing units were replaced with functional ones and subsequently repaired or replaced.

After April 2013, we visited the platforms every two to three months for data 

retrieval and maintenance of the camera systems, which included replacing the batteries, 

measuring battery voltage, and checking the functionality of the cameras (Table 1). All 

camera systems were removed from the platforms in January 2015.

Data collection

Images were uploaded from memory cards to a laptop computer. Some of the digital 

images were examined to determine if there were any issues with camera performance or 

the camera system housing (e.g. unclear viewing window, faulty programming, alteration of 

housing unit). Organizational and nomenclature changes of images were made using the 

freeware Total Commander [C. Ghisler, Ver. 8.51a, 32 bit (2014)]. Image file names were 

changed to a Year Month Day, Hour Minute Second, original file name, and Camera Number 

format: “YYYYMMDD_HHMMSS_original file name_CAM #”. 

A Microsoft Access database was created to archive and analyze the time-lapse 

images. The database contained fields to input metadata for each image (e.g. visibility 

quality, environmental conditions, distance of camera from haulout sites, species of 

pinniped seen), as well as to identify and count animals. To identify and count individuals, 

an analyst would view the image files for a given platform corresponding to a specific date 

and time. Landmarks on images that had overlapping areas taken from two or more 

cameras were marked. For each image file, the analyst used their computer mouse to place 

a marker on each animal they saw hauled out on the platform. The analyst chose one of 

seven different age/sex class (individuals that generally share the same morphological 

characteristics) or unclassified markers and a blank category indicating no animals were 

seen (the first four categories are descriptors for California sea lions only): (1) “mature 

males” – included subadult and adult males; approximately 6 or more years old, 1.5 to 2 m 

long, brown to black in coloration, sagittal crest partially or fully developed, thick neck; (2) 

“adult females” – 5 or more years old, 1.5 to 2 m long, light brown or cream colored, no 

sagittal crest present; (3) “juveniles” – of both sexes, approximately 1 to 4 years old, from 

<1 to 1.5 m long, light to dark brown coloration; (4) “pups” – of both sexes, <1 year old, ≤ 1 
6

Table 1. Tasks and dates of visitation to oil and gas platforms in federal waters off the southern California Bight. Camera 

systems were deployed on the platforms to document their usage by pinnipeds between January 2013 and January 2015.

Trip 

#

Purpose Dates Platforms Notes

1 Reconnaissance 30 October - 7 

November 2012

All 23

2 Camera system 

installation

7-10 January 2013 Elly, Gina, Habitat, 

Heritage, Harvest

3 Data retrieval, 

maintenance

7-12 February 2013 Elly, Gina, Habitat, 

Heritage, Harvest

4 Data retrieval, 

maintenance

9-14 April 2013 Elly, Gina, Habitat, 

Heritage, Harvest

Camera 1 removed from 

Harvest

5 Data retrieval, 

maintenance

4 - 7 and 25 June 2013 Elly, Gina, Habitat, 

Heritage, Harvest

6 Data retrieval, 

maintenance

20 - 24 August 2013 Elly, Gina, Habitat, 

Heritage, Harvest

7 Data retrieval, 

maintenance

12 - 22 November 

2013

Elly, Gina, Habitat, 

Heritage, Harvest

8 Data retrieval, 

maintenance

24 February - 2 and 

23 March 2014

Elly, Gina, Habitat, 

Heritage, Harvest

10 Data retrieval, 

maintenance

20 - 24 May 2014 Elly, Gina, Habitat, 

Heritage, Harvest

11 Data retrieval, 

maintenance

18 - 31 August 2014 Elly, Gina, Habitat, 

Heritage, Harvest

12 Data retrieval, 

maintenance

1 - 6 December 2014 Elly, Gina, Habitat, 

Heritage, Harvest

13 Data retrieval, 

camera system 

removal

12 - 17 January 2015 Elly, Gina, Habitat, 

Heritage, Harvest
7

m long, dark brown coloration. It was often difficult to discern yearlings (categorized as 

juveniles) from older pups, therefore 1 July was set as the demarcation date in which 

individuals that looked like pups were categorized as juveniles; (5) “unknown” –

individuals that could not be identified to a specific age/sex class or could only see a body 

part (e.g. flipper, rump); (6) “cannot count” – for various reasons (e.g. blurred images, too 

dark); and (7) “blank”. Tallies of identifications were automatically computed using coding 

embedded in the database. Analysts also noted if an image contained marked (e.g. branded, 

tagged, instrumented) individuals, animals with shark bites or lesions, evidence of fisheryinteractions (e.g. individual entangled with monofilament line, bands, fish hooks), human 

disturbance (e.g. humans working on the platform, crew boat exchange), or any other 

unusual occurrence (e.g. birthing, nursing). Analysts were conservative with their 

identifications. If there were any uncertainties in assigning an age/sex class to an animal, 

then the analyst would consult with other analysts or label animal as “unknown”. 

Additionally, individuals that were difficult to differentiate between being a pup or juvenile 

were marked as “juvenile”. Individuals that were questionable to identify as juvenile, adult 

female, or subadult male were labeled as “unknown”.

Quality control

To ensure that identifications and counts were done correctly, the analyst would 

open the image a second time and view the assigned markers. Images were viewed at least 

twice to make sure all animals were accounted for. A subset of images was viewed by at 

least two analysts to determine if the counts and identifications were consistent across 

analysts. Additionally, a subset of images was re-analyzed after all images for a platform 

were processed in order to determine if analysts were consistent with their counts and 

identifications from the beginning through the end of their image counting processing.

Data analyses

Due to time constraints, it was not possible to analyze all the images obtained 

during the study, so a subsampling protocol was developed. The dates when all the 

cameras on a given platform were operational was split into two-week blocks. From each 

block, two days were selected at random. Within each day, two images were analyzed from 

the following blocks of time: 0001h/earliest hour – 0659h, 0700h – 1259h, 1300h – 1859h, 

1900 – 0000h/latest hour. Months when a particular camera-system was not operational 

were not included in the subsampling protocol (Table 2). If an image from one or more 

camera-systems on a platform was not able to be analyzed (e.g. too dark, out of focus) then 

discernable photos from the nearest in time (prior in time was given preference) to the 

time period selected in the subsampling protocol were used.

Total counts were calculated by summing the number of individuals (identified to a 

specific age/sex class, if possible) counted from images taken from all cameras for the same

time and date. Only individuals on the platform, including supporting beams, landing decks, 

and stairs were counted. Individuals in the water beneath the platform or around landing 

decks were not. Counts were used to generate hourly, daily, and monthly median numbers

of animals using the platforms. Inter-platform comparisons were made to examine spatial 

trends in animal usage. Intra-platform comparisons were examined to determine if there 

were any temporal (i.e. monthly/seasonal) changes. 
8

Statistical differences in counts of California sea lions were assessed using 

generalized linear models (GLMs) with the glm function (quasipoisson family) in R (R Core 

Team 2015). GLMs included response variables platform, year, month, hour, and 

interaction terms. GLMs were used because response variables had non-normal 

distributions. Significance level was set at α = 0.05. No statistical comparisons were made 

among age/sex classifications because most identifications were “unknown” and 

individuals were not assigned to an age/sex class on Platform Habitat.

Table 2. Months during which at least one camera system located at a focal gas and oil 

platform was not operational, or not set up (e.g. January 2013 for Platform Harvest). Data 

were not analyzed during those months. 

Platform 2013 2014 2015

Elly January, March, 

April, May

February, March, 

April

Gina January October, November, 

December January

Habitat January February January

Heritage September, October

Harvest January September, October, 

November

RESULTS

Of the 464,174 images obtained from all the platforms during this study, 12,489

(Platform Elly = 1,981, Platform Gina = 1,960, Platform Habitat = 4,742, Platform Heritage = 

2,551, Platform Harvest = 1,255) were used in data analyses. There were periods of time 

when one or more of the camera-systems were not functioning properly. Images were not 

analyzed during those months (Table 2). 

Steller sea lions were observed in photographs on only two of the platforms: 

Platform Elly on 22 February 2013 and 3 January 2014, and Platform Habitat on 22 January 

2014 (Fig. 4a). Harbor seals were occasionally seen in waters adjacent to the platforms

during our visits, but none were actually seen hauled out on the platforms (Fig. 4b).
9

Figure 4. Pinniped species observed at or near

the gas and oil platforms besides the California 

sea lion (Zalophus californianus) included: (a)

Steller sea lion (Eumetopias jubatus), and (b)

Pacific harbor seal (Phoca vitulina richardii). 

(a)

(b)
10

Intra-platform (temporal) comparisons

Platform Elly

Counts could not be conducted during several months on Platform Elly because of 

camera-system malfunctions (Table 2). Therefore it was not possible to examine complete 

inter- and intra-annual comparisons of California sea lion use on this platform. However, 

using data that were collected and analyzed indicated that significantly more sea lions used 

the platform during 2013 compared to 2014 (GLM, p < 0.05; Fig. 5).

Figure 5. Monthly median (top) and maximum (bottom) counts of 

California sea lions (Zalophus californianus) on gas and oil platforms 

located in the Pacific Outer Continental Shelf Region offshore southern 

California from January 2013 to January 2015. Platforms are listed in 

order from south to north.

0

25

50

75

100

125

150

Median

Elly Gina Habitat Heritage Harvest

0

100

200

300

400

500

J F M A M J J A S O N D J F M A M J J A S O N D J

Maximum

Month

2013 2014 2015
11

Monthly comparisons: Monthly median counts fluctuated throughout the study 

and differed significantly (GLM, p < 0.05). During 2013, monthly median counts ranged 

from 13 (February) to 86 (October). The range of monthly maximum counts was 81 

(December) to 137 (October; Fig. 5). During 2014, monthly median counts were more 

consistent and ranged from 15 (November) to 58 (May). The range of monthly maximum 

estimates was 32 (November) to 123 (December; Fig. 5). Intra-monthly counts/trends 

were not consistent and differed significantly between years (GLM, p < 0.05). For example, 

the highest number of animals were observed during October 2013, whereas October 2014

had some of the lowest counts (Fig. 5).

Hourly comparisons: Overall, hourly median counts differed significantly (GLM, p 

< 0.05). During 2013, hourly median numbers ranged from 33 (0300h) to 83 (1500h; Fig. 

6). Hourly maximum counts ranged from 64 (0100h) and 137 (1800h). During 2014, 

hourly median counts ranged from 13 (1800h) to 54 (0200h). Hourly maximum estimates

ranged from 25 (1200h) to 123 (2200h; Fig. 6). Intra-hourly comparisons of median counts 

indicated that in 2013 there was a gradual increase in numbers of sea lions hauled out from 

early morning to the afternoon (0300h – 1500h) then a decrease in numbers during the 

remainder of the day, whereas median counts were relatively consistent throughout the 

day during 2014 (Fig. 6).

Age/sex class comparisons: The monthly mean percentage (± standard error) of 

individuals identified to an age/sex class was 47.3% (SE = 4.0) during 2013 and 49.8% (SE 

= 3.4) during 2014. 

Juvenile California sea lions comprised the highest proportion of identified animals 

on Platform Elly during every month of the study (Fig. 7). They represented a higher 

proportion of animals using the platform during 2013 [range: 0.75 (September) – 0.92

(July)] compared to 2014 [0.73 (November, December) – 0.86 (May)].

In general, adult females had the second highest proportions of identified 

individuals, depending on month (Fig. 7). Their monthly proportions were higher in 2014 

[range: 0.09 (May) – 0.18 (December)] compared to 2013 [range: 0.04 (July, November) –

0.14 (February)]. 

Mature males had higher proportions than adult females during five months of the 

sampling period (i.e. August, September, and December 2013; January and July 2014); 

however, overall they represented the third most assigned age/sex class. During 2013, the 

proportion of mature males ranged from 0.06 (February, November) to 0.14 (September; 

Fig. 7). During 2014, they constituted 4.4% (May) to 12.8% (January) of the classified

population (Fig. 7). The mean annual proportion of mature males was the same during both

years (mean = 0.08, standard deviation = 0.03). 

Pups were only seen on Platform Elly during February 2013 (0.5%, n = 4; Fig. 7).

There did not appear to be a consistent seasonal trend in the proportion of individuals 

within any age/sex class during any year of the study (Fig. 7).
12

Figure 6. Hourly median (top) and maximum (bottom) counts of 

California sea lions (Zalophus californianus) on gas and oil 

platforms located in the Pacific Outer Continental Shelf Region 

offshore southern California during 2013 and 2014. Platforms are 

listed in order from south to north. 

0

50

100

150

Median

Elly Gina Habitat Heritage Harvest

0

100

200

300

400

500

0 2 4 6 8 10121416182022 0 2 4 6 8 10121416182022

Maximum

Hour

2013 2014
13

Figure 7. Proportion of California sea lions (Zalophus californianus) using gas and oil platforms that were 

identified to a specific age/sex class [(from south (top, left) to north (bottom, right)] by month from 

January 2013 through January 2015. Numbers within bars indicate total number of animals identified 

during that month. 

Pup Juvenile Adult female Mature male

Proportion

Month

0.0

0.2

0.4

0.6

0.8

1.0

J F M A M J J A S O N D J F M A M J J A S O N D J J F M A M J J A S O N D J F M A M J J A S O N D J

0.0

0.2

0.4

0.6

0.8

1.0

2013 2014 2015 2013 2014 2015 16121043

742273128348401516708122

126

167522452508171702858264

2434

809

1161

677

1458

788422195272749422419696275228638

338

122602142470780385651613521491634308

497149122459704718292304345220

233

956612286372838724323634880399791

481402

1086

700969892451

1462

487

220

Elly

Harvest

Gina

Heritage
14

Platform Gina

Data were not obtained during January 2013 nor any month after September 2014 

from Platform Gina because of camera-system failures. Therefore, complete inter- and 

intra-annual comparisons could not be made. However, general patterns in the number of 

sea lions using Platform Gina could be ascertained from the months that were sampled.

Monthly comparisons: During 2013, monthly median numbers ranged from 1 

(April) to 21 (November); and maximum monthly counts ranged from 3 (April) to 59 

(October; Fig. 5). During 2014, monthly median counts ranged from 3 (February) to 22 

(August). The range for monthly maximum estimates was 24 (June) to 79 (May; Fig. 5). 

There was a general upward trend in the number of California sea lions using Platform Gina 

from February to November 2013 (Fig. 5). During 2014, the trajectory was similar to 2013 

until June, after which counts fluctuated (Fig. 5). 

Hourly comparisons: General trends of hourly median data indicated that 

California sea lions hauled out less during “daytime” hours (~0600h-2000h) compared to 

“nighttime” hours in both 2013 and 2014 (Fig. 6). During 2013, hourly median counts 

ranged from 2 (1000h, 1700h, and 1800h) to 21 (0100h; Fig. 6). Hourly maximum counts 

ranged from 15 (0800h and 1700h) to 59 (0500h). During 2014, hourly median numbers

ranged from 2 (1700h) to 35 (0500h). The range of hourly maximum counts was 18 

(1800h) to 79 (0400h; Fig. 6). General patterns of intra-hourly and inter-annual counts

were similar (Fig. 6). 

Age/sex class comparisons: The monthly mean percentage of individuals 

identified to an age/sex class was 59.2% (SE = 5.2) during 2013 and 73.0% (SE = 4.7) 

during 2014. 

Juveniles were the most numerous age/sex class during every month for which data 

were available (Fig. 7). Overall, juveniles had approximately the same proportions during 

2013 [range: 0.52 (June) to 0.92 (March)] and 2014 [range: 0.48 (January) to 0.94 (July, 

August)]; however, their proportions were not consistent by month (Fig. 7).

Adult females were the second most common age/sex class counted. Their

proportions were similar between years [range 2013: 0.07 (November, December) to 0.23 

(May); range 2014: 0.02 (August) to 0.28 (February)] although intra-monthly comparisons 

were not (Fig. 7). 

Pups were only seen during May to June 2013. Their proportions ranged from 0.14

(May) to 0.20 (April, June; Fig. 7). During 2014, pups were sighted as early as January. Their 

monthly proportions ranged from 0.07 (February) to 0.29 (June; Fig. 7).

Mature males monthly proportions ranged from 0 (March) to 0.25 (February) 

during 2013, and 0.03 (July) to 0.20 (January) during 2014 (Fig. 7). 

There did not appear to be a consistent seasonal trend in the proportion of 

individuals within any age/sex class during either year of the study (Fig. 7).

Platform Habitat

Data were obtained during all months on Platform Habitat except January 2013, 

February 2014, and January 2015. Using data that were collected and analyzed indicated 
15

that California sea lions used Platform Habitat significantly more during 2014 compared to 

2013 (GLM, p < 0.05; Fig. 5).

Monthly comparisons: During 2013, monthly median counts ranged from 32 

(July) to 134 (October). Maximum monthly counts ranged from 65 (April) to 234 (October; 

Fig. 5). During 2014, monthly median numbers ranged from 40 (November) to 110 (April); 

and maximum monthly counts ranged from 73 (June) to 493 (September; Fig. 5). Intramonthly median counts were different between years (GLM, p <0.05), however some 

trends were apparent. For example, prior to July, there was a general downward trend in 

median counts, followed by an upward trend in counts until November during both years

(Fig. 5). 

Hourly comparisons: There were significant differences in hourly median counts 

in both years (GLM, p < 0.05). During 2013, hourly median counts ranged from 33 (2200h) 

to 96 (1400h); and hourly maximum counts ranged from 65 (0100h) to 234 (1100h; Fig. 6). 

During 2014, hourly median numbers ranged from 25 (1500h) to 161 (0200h); and hourly 

maximum counts ranged from 67 (1500h) to 493 (0600h; Fig. 6). Intra-hourly trends were 

similar during some periods of the day. For example, there was a decreasing trend in the 

number of sea lions using the platform from 0300h to approximately 1000h, although the 

decrease in numbers was more pronounced during 2014 (Fig. 6). Intra-hourly trends were 

inconsistent during other periods of the day (Fig. 6). 

Age/sex class comparisons: Individuals were not identified to a particular 

age/sex class on Platform Habitat during the study, primarily because animals were at a 

distance from the camera systems or tightly aggregated that made classifying difficult.

Platform Heritage

Although images were obtained during every month on Platform Heritage except 

September and October 2014, it was not possible to examine complete inter- and intraannual comparisons of California sea lion usage of the platform. From data that were 

available, it was apparent that California sea lions used the platform significantly more 

during 2013 compared to 2014 (GLM, p < 0.05; Fig. 5).

Monthly comparisons: During 2013, monthly median counts ranged from 26 

(July) to 64 (February and May). Monthly maximum estimates ranged from 53 (July) to 116 

(September; Fig. 5). During 2014, monthly median counts ranged from 14 (July) to 42 

(May). Monthly maximum counts ranged from 50 (November) to 108 (January; Fig. 5). 

Although the median number of animals were different between years, the inter-annual 

pattern of median numbers between April through August were similar (Fig. 5).

Hourly comparisons: During 2013, hourly median counts ranged from 19 (1700h) 

to 63 (1000h). Hourly maximum counts ranged from 42 (0400h) to 116 (1200h; Fig. 6). 

During 2014, hourly median counts ranged from 13 (2100h) to 45 (1400h); and maximum 

hourly counts ranged from 20 (0000h) to 108 (0800h; Fig. 6). Intra-hourly trends were 

similar during both years (Fig. 6). Hourly median data indicated that California sea lions 
16

were hauled out on Platform Heritage more during late morning through afternoon 

(~0900h – 1500h) compared to other periods of the day (Fig. 6). 

Age/sex class comparisons: The monthly mean percentage of individuals 

identified to an age/sex class was 50.9% (SE = 3.0) during 2013 and 42.8% (SE = 3.9) 

during 2014.

Juveniles represented more than 50% of classified animals during every month. 

They represented a greater proportion of individuals during 2014 [range: 0.60 (December) 

to 0.94 (June); Fig. 7] compared to 2013. During 2013, proportion of juveniles ranged from 

0.58 (October) to 0.85 (May; Fig. 7). 

Adult females were the second-most classified sea lions. They had higher monthly 

proportions during 2013 compared to 2014 (GLM, p < 0.05). During 2013, adult females 

represented between 13.5% (May) to 37.5% (October) of animals identified during each 

month (Fig. 7). During 2014, the monthly proportion of adult females ranged from 0.05 

(May, June) to 0.30 (December; Fig. 7).

Proportions of mature males were similar in 2013 and 2014 (Fig. 7). During 2013, 

their monthly proportions ranged from 0.01 (January) to 0.12 (December; Fig. 7). Mature

males comprised between 1.1% (June) to 9.5% (December) of the classified population 

during 2014 (Fig. 7). 

Pups represented the age/sex class least identified on Platform Heritage. They were 

seen during two months during 2013 and their monthly proportions ranged from 0.01 –

0.02 in December and April, respectively (Fig. 7). Pups were seen during spring 2014. Their 

monthly proportions ranged from 0.01 (March, May) to 0.02 (April; Fig. 7). 

There did not appear to be any other consistent monthly or annual trends in the 

proportion of individuals within any age/sex class (Fig. 7).

 

Platform Harvest

Data were not obtained in January 2013, and September – November 2014 on 

Platform Harvest due to camera-system operational issues; therefore, complete inter- and 

intra-annual monthly and hourly comparisons of California sea lion numbers on this 

platform were not possible. However, using data that were collected and analyzed

indicated that there were slightly more California sea lions using Platform Harvest during 

2014 compared to 2013 (Fig. 5).

Monthly comparisons: During 2013, monthly median counts ranged from 19 (June 

and November) to 42 (October). Monthly maximum counts ranged from 37 (April) to 78 

(October; Fig. 5). During 2014, monthly median numbers ranged from 22 (December) to 54 

(June); and monthly maximum counts ranged from 50 (February) to 107 (June; Fig. 5). 

Intra-monthly median count trends were not consistent between years (Fig. 5). Monthly 

median counts fluctuated during 2013, whereas in 2014 they steadily increased during 

January through June followed by a steady decrease until August (Fig. 5). 

Hourly comparisons: During 2013, hourly median counts ranged from 7 (0300h) 

to 42 (1400h); and hourly maximum estimates ranged from 11 (0200h) to 78 (1600h; Fig. 

6). During 2014, hourly median numbers ranged from 18 (0400h) to 67 (1000h). Hourly 
17

maximum counts ranged from 47 (0400h) to 107 (1200h and 1600h; Fig. 6). Intra-hourly

comparisons from both years indicated that sea lions were most often present on Platform 

Harvest during late morning through the afternoon (~1000h – 1400h) compared to other 

hours of the day (Fig. 6). Hourly platform-use patterns were similar during both years (Fig. 

6). 

Age/sex class comparisons: The monthly mean percentage of individuals 

identified to an age/sex class was 73.3% (SE = 1.1) during 2013 and 82.8% (SE = 2.3) 

during 2014.

Similar to other platforms, juveniles comprised the majority of individuals classified

during each month on Platform Harvest (Fig. 7). Their monthly proportions ranged from 

0.55 (November) to 0.80 (May, July) during 2013 (Fig. 7). During 2014, their monthly 

proportions ranged from 0.53 (February) to 0.90 (June; Fig. 7). 

Adult females had the second highest proportions of classified animals, however 

they were outnumbered by mature males during a few months of the study period (i.e. 

April 2013, May 2014, and January 2015; Fig. 7). During 2013, adult female monthly 

proportions ranged from 0.05 (May) to 0.25 (February, November, and December; Fig. 7). 

Their monthly proportions ranged from 0.08 (May, June) to 0.30 (January, February) 

during 2014 (Fig. 7).

Mature males had higher monthly proportions during 2013 [range: 0.06 (July) to 

0.23 (April); Fig. 7] compared to 2014 [range: 0.03 (June) to 0.18 (December); Fig. 7]. 

Fewer pups were seen on Platform Harvest compared to other age/sex classes. 

Additionally, they were only seen during three months in both 2013 and 2014 (Fig. 7). 

When present, their monthly proportions ranged from 0.01 (May) to 0.03 (April) during 

2013; and 0.01 (March) to 0.04 (April) during 2014 (Fig. 7). 

Using data that were available indicated that there were inter-annual and intramonthly trends of proportions of juveniles and mature males. The proportion of juveniles 

increased during summer and decreased during winter. The opposite trend was apparent 

for mature males (Fig. 7).

Inter-platform (spatial) comparisons

Monthly comparisons

There were monthly data that were not analyzed because of issues with the camera 

systems. The months of camera system inoperability were not identical for all platforms. 

Comparisons here were made using data that were available.

There were no consistent spatial (south-to-north) trends in the number of California 

sea lions using the offshore platforms by month (Fig. 5). However, the number of sea lions 

increased during October 2013, which was consistent across platforms. During the first five 

months of 2013, Platforms Habitat and Heritage had the highest median number of 

animals. Median numbers decreased for Platform Heritage after May, increased until 

September and subsequently decreased the remainder of 2013, whereas numbers of 

animals increased at Platform Habitat until August, after which they declined (Fig. 5). 

Median counts on Platform Elly (southernmost platform) increased during the summer 

2013 and remained one of the most used platforms the rest of the year. Median counts at 
18

Platform Gina were the lowest throughout 2013 with the exception of November (Fig. 5). 

Platform Harvest was the second least used platform during 2013 (Fig. 5). 

In 2014, more animals were counted at Platform Habitat during each month except 

March when no counts were available and June (Fig. 5). Platform Gina remained the least 

used platform with the exception of July and August during which time counts were similar 

to those at other platforms. Median numbers of sea lions at Platform Harvest were slightly 

higher than at Platform Heritage, with the exception of January, December, and January 

2015 (Fig. 5). Patterns of median numbers at Platform Elly were similar to those at 

Platform Harvest (Fig. 5). After pooling all monthly data, the order of mean medians (from 

lowest to highest) was Platform Gina, Platform Harvest, Platform Heritage, Platform Elly, 

and Platform Habitat.

Contrary to monthly data analyzed separately for each year, averaged monthly data 

from all years (e.g. mean January2013-2015, February2013-2014, etc.) indicated that there was a 

general increase in California sea lion numbers at Platforms Elly, Gina, and Habitat from 

January to at least May, and a corresponding decrease in numbers at Platforms Heritage 

and Habitat. There was another increase in numbers of individuals at Platforms Gina, 

Habitat, and Heritage in August and September, corresponding with no change at the other 

platforms. However, from examining the count data by individual dates, it did not appear 

that individuals from one of the selected platforms moved to another of the selected ones 

because of a disturbance (including change in accessibility). However, animals could have 

moved to any one of the adjacent facilities during such events.

Hourly comparisons

As with monthly comparisons, there were no distinct, consistent spatial (south-tonorth) patterns of time of day when California sea lions hauled out on the platforms (Fig. 

6). During 2013, the general trend for Platforms Elly, Heritage, and Harvest was a gradual 

increase in numbers from morning to midday, followed by a gradual decrease from midday 

to night (Fig. 6). The exact opposite trend was exhibited at Platform Habitat. Median 

numbers at Platform Gina gradually decreased until midday, but then remained relatively 

the same during the remaining hours of the day (Fig. 6). Median count trends were more 

variable during 2014 across platforms. Numbers increased after 2200h, which also 

happened during 2013, with the exception of Platform Elly (Fig. 6). 

Age/sex class comparisons 

In general, Platforms Elly and Gina had a higher proportion of juveniles than

Platforms Heritage and Harvest. Platform Heritage had a higher proportion of adult females 

compared to the other platforms, and Platform Harvest had a higher proportion of adult 

males (Fig. 7). Proportion of juveniles tended to increase during late spring/early summer 

on Platforms Heritage and Harvest (Fig. 7). Only four pups were counted during just one 

month (February 2013) on Platform Elly, whereas more pups were counted on the other 

platforms (esp. Platform Gina) during multiple months (Fig. 7).

Using all age/sex class data pooled together, there were some noticeable spatial 

south-to-north patterns with age/sex classes. There tended to be a decrease in the number 

of immature animals (pups and juveniles) starting from the southern-most platform 

(Platform Elly) northward to Platform Harvest. Contrarily, the northern platforms 
19

(Platforms Heritage and Harvest) had more adult females than those more southerly (i.e. 

Platforms Gina and Elly).

DISCUSSION

Platforms & camera systems

The five platforms selected for this study met the selection criteria of being used by 

pinnipeds (California sea lions, in particular) and were located in areas that encompassed 

the distribution range of the 23 oil and gas platforms administered by BOEM in the 

Southern California Planning Area. Some of the other offshore facilities had physical 

barriers up that limited access to pinnipeds. Although other platforms were viable options

to conduct this study at, we were limited in the number of camera systems that could be 

installed. We decided to allocate enough camera-systems on each selected platform as to 

best cover areas where animals potentially could be located rather than just cover some 

areas on more platforms. Additionally, rather than change platforms during the different 

years of the study (environmental conditions were not the same during both years, which 

is addressed later under the “Intra-platform (temporal) comparison” section), we decided to 

retain the same sites so that temporal comparisons in pinniped-use of the platforms could 

be examined in addition to the spatial comparisons. Although the chosen platforms were 

selected as focal sites to represent other adjacent platforms, it might not be possible to

extrapolate our findings to those other sites because of differences in structure, exposure, 

or activities on each facility.

Prior to the onset of this project, there were concerns that the time-lapse camera 

systems might not be useful because they might have been affected by the salt spray 

common on the lower levels of the platforms where the animals hauled out. Failure to 

obtain satisfactory images could have occurred due to salt spray covering the lens or

penetrating the electrical control panel of the camera system. Although there is experience 

with remote digital camera systems in marine mammal research, what has been done was

based on terrestrial applications at sites near the sea (e.g. Burkanov and Altukhov 2014, 

2015) but not on artificial platforms poised just above the sea surface. Despite the 

conditions that the camera systems experienced, for the most part the salt spray did not 

adversely affect the cameras’ ability to obtain quality images. This is not to say that some of 

the images were not compromised, but the vast majority were usable. 

There were other issues with the camera systems on the platforms at different times 

throughout the study period. On several occasions, we noticed that batteries apparently 

shifted from their proper positions within the battery bank. The probable cause of battery 

displacement was from vibrations on the rails on which the camera systems were mounted

that were caused by platform activities. Batteries were strapped and taped into their 

proper position, but upon examination during the subsequent maintenance trip, a battery 

occasionally was not in proper alignment. It is not clear if this was a cause of camerasystem failure. A couple of the camera systems were malfunctioning and were 

subsequently replaced and sent back to the manufacturer. Regardless of the reason why a 

camera system was not operating properly, we could not detect the problem until the 

following maintenance trip. This resulted in an inability to capture photos for a period of 
20

time. We chose to only include times when all camera systems were functional to assure 

that data were comparable throughout the study.

Despite our best efforts, not all areas where animals potentially could haul out on 

the platforms were covered by the camera systems. Additionally, some of the camera 

systems were partially obstructed, placed in locations where it was difficult to ascertain the 

age/sex class of the animals, or were altered by activities conducted on the platforms. 

Data Analysis Issues

Except for the first two months, the time-lapse camera systems were programmed 

to take a photo every 30 minutes and over 400,000 photos were produced. Three people 

analyzed the photos. Without additional help it would have taken a couple of years to 

analyze all of the images. Because of time constraints and lack of additional analyzers, it 

was not possible to examine every image to count and identify the animals. The 

subsampling protocol encompassed all time periods of the day and month so that a 

reasonable amount of information could be examined to address the objectives of this 

study. All of the images are retained in a database and in a public repository [NOAA 

National Centers for Environmental Information (https://nodc.noaa.gov); Package 

Reference ID R1B85T] so that data are available for additional analysis, as needed. 

Because of issues with the time-lapse camera systems, and understanding that only 

animals actually on the platforms were counted, it is important to recognize that the counts 

represent a minimum estimate of animals using these offshore facilities. There may have 

been animals that were present but not seen in areas not covered by the camera systems. 

Individuals jugging (i.e. resting) or swimming underneath or around the platform may have 

used the platform prior to or after the photo was acquired. However, this was not known 

with certainty and we defined “use” as physically being on the platform. There were other 

factors that impeded pinnipeds from hauling out on the platforms at different times of the 

study, including: tidal height, physical barriers, and platform activities. Additionally, 

animals were disturbed at times and actively encouraged into the water during the 

arrival/departure of the crew boats (Table 3). The end result was that there were fewer 

animals on the platforms during those periods. However, some of the animals may have 

been in nearby waters waiting to get back onto the platforms when it was possible to do so. 

It should be noted that hourly and monthly numbers of animals do not necessarily reflect 

different individuals. Many of the same individuals could have been photographed in 

multiple images.

Identifying individuals to a particular age/sex class was often challenging. 

Difficulties included (but not limited to): only seeing part of an animal, inadequate light 

during the night to see clearly, blurry image, animal(s) at a distance, animals congregated 

too tightly to discern each individual, and morphology of animal not distinct enough to 

assign classification. Because of distinct morphological characteristics, the easiest animals 

to be assigned a particular age/sex class were pups and adult males. However, it should be 

noted that because of the smaller size of some yearlings (here, categorized with older 

juveniles) during the past few years, it was challenging to differentiate some juveniles from 

pups. Often it was difficult to distinguish large juveniles, adult females, and subadult males, 

so they were labeled “unknown”. Analyzers did their best to identify individuals and be 

consistent with their identifications; however, the age/sex class information provided here 

is incomplete and undoubtedly is not consistent with the actual use by individuals within 
21

specific age/sex categories. It should not be assumed that the proportions of classified 

individuals are reflective of those unidentified.
22

Table 3. Crew boat departure to (Platforms Elly, Gina, and Habitat) or arrival at (Platform 

Heritage) platform times. Actual arrival times to platforms were dependent on weather and 

sea-state conditions, and activities of the crew but usually within 1.5 hours. Crew 

exchanges on Platform Harvest were primarily done by helicopter.

Platform Mon Tue Wed Thr Fri Sat Sun

Elly 0500h 0500h 0500h 0500h 0500h 0500h 0500h

(Depart to) 0700h 0700h 0700h 0700h 0700h 0700h 0700h

1000h 1000h 1000h 1000h 1000h 1000h 1000h

1600h 1600h 1600h 1600h 1600h 1600h 1600h

Gina 0645h 0645h 0645h 0530h 0645h 1400h 1400h

(Depart to) 1400h 1000h 1400h 1400h 1000h

1400h 1730h 1730h 1400h

Habitat 0630h 0630h 0630h 0630h 0630h 0630h 0630h

(Depart to) 0830h 0830h 0830h 0830h 0830h 0830h 0830h

1430h 1430h 1430h 1430h 1430h 1430h 1430h

1830h 1830h 1830h 1830h 1830h 1830h 1830h

Heritage 0900h 0900h 0900h 0715h 0900h 0900h 0900h

(Arrive) 1830h 1200h 1200h 0900h 1830h 1830h 1830h

1830h 1830h 1200h

2300h 1830h

Harvest Helicopter

Pinniped presence

California sea lions were the dominant pinniped species on the platforms. This was

expected because the California sea lion is the most abundant pinniped within the 

California Current ecosystem and their use of manmade structures is well documented.

California sea lions primarily breed in the United States at several of the California Channel 

Islands, including Santa Barbara (33.48°N, 119.04°W), San Clemente (32.90°N, 118.50°W), 

San Miguel (34.03°N, 120.38°W), and San Nicolas (33.25°N, 119.50°W) Islands, which are 

all relatively near the oil and gas platforms.

Steller sea lions were seen at two of the platforms (i.e. Platforms Elly and Habitat) 

during winter months. This species also is present in the California Current ecosystem, 

although not as abundant as California sea lions. In U.S. waters, they are distributed from 

Alaska to central California. They used to breed as far south as the California Channel 

Islands, however no pups have been seen there in decades (although mature males have 
23

been sighted at San Miguel Island). There are a few Steller sea lion rookeries in California 

including Año Nuevo Island (37.11°N, 122.34°W), the Farallon Islands (37.72°N, 

123.03°W), and St. George Reef (Southwest Seal Rock; 41.80°N, 124.35°W); however, all

are located well north of the study area.

Pacific harbor seals in the eastern Pacific also occur in the California Current 

ecosystem. They haulout and breed on rocky islets, mud flats, reefs and mainland beaches 

along the North American coast from San Quintin Bay, Baja California, Mexico to Nome, 

Alaska (including the Channel Islands; Reeves et al. 1992). They exhibit strong site fidelity, 

however they are capable of making long-distance movements (Reeves et al. 1992). Harbor 

seals were seen at the Carpinteria Harbor Seal Preserve and Rookery, Carpinteria, 

California, which is near several platforms, including Platform Habitat. Although harbor 

seals were seen in waters near platforms, they were never seen actually on the platforms, 

which may be due to an inability to get onto the platforms, a propensity to return to a 

preferred haulout, or other reasons. 

Intra-platform (temporal) comparisons

Because California sea lions were the dominant pinniped species using the 

platforms, we will restrict our discussion to this species. The largest differences in counts 

among months were observed at Platforms Gina and Habitat. These platforms also 

exhibited the greatest disparity in counts either among hours throughout the day or 

between years. Platforms Heritage and Harvest exhibited similar use patterns by hour 

during both years of the study with animals hauling out more frequently during the day 

and leaving at night; whereas, Platform Gina exhibited the opposite pattern. 

Seasonal variability in numbers of animals using the platforms can be expected due 

to reproductive, foraging, and physiological characteristics of California sea lions, as well as 

changes in regional prey availability, oceanic conditions, and sea state. Their breeding 

season starts in May, when adult males arrive at rookeries to establish and defend 

territories. Adult females return to rookeries during May and June, and give birth four to

five days after coming ashore. Postpartum females remain on land nursing their pups for 

approximately seven to ten days, and then alternate between foraging trips of one to three 

days and nursing periods of one to two days (Heath 1989, Antonelis et al. 1990, Ono 1991, 

Melin 2000). They continue this pattern of feeding and nursing until their pup is weaned 

(~11 months). Lactating females are central place foragers and are constrained in duration 

by their pups’ fasting limitations to forage near rookeries during the reproductive season 

(Melin et al. 2000). Although lactating females can go on longer foraging trips because their 

pups start supplementing their diet by feeding at sea as early as 6 months-of-age (Orr et al. 

2012), they, along with other adult females, generally remain near rookeries year-round 

(Odell 1975, Antonelis et al. 1990, Melin 2000, Kuhn 2006, Kuhn and Costa 2014). Adult 

females may have been using the offshore platforms as resting sites while traveling to and

from foraging areas, some of which were near the platforms. Although the distribution and 

at-sea behavior of adult females from different rookeries (e.g. San Miguel and San Nicolas 

Islands) may differ by geographically – individuals from San Nicolas Island forage 

predominately in the Los Angeles Bight (offshore Los Angeles county; Kuhn, 2006, Kuhn 

and Costa 2014), and those from San Miguel Island feed in the Santa Barbara Channel, 

northward in the Santa Maria Basin, and farther north to Monterey Bay (Antonelis et al. 

1990, Melin and DeLong 2000; Melin et al. 2008) – these locations are near all of the 
24

platform sites. Adult females may not have been as abundant on the platforms as juveniles 

because of their need to return and nurse their young or because of our inability to classify 

many of them as “adult females”. 

Adult and subadult male California sea lions usually depart from the Channel Island 

rookeries during late July and August and migrate north to northern California, Oregon, 

Washington, and British Columbia, Canada (Peterson and Bartholomew 1967, Weise 2006, 

Wright et al. 2010, Gearin unpubl. data). This species reduces intraspecific competition by 

habitat partitioning and niche divergence. Therefore, mature males would normally only be 

present in the southern California OCS from April/May (when they are migrating to 

rookeries) through August (when they are making their return migrations). However, they 

were seen on the platforms during every month of the study, though in relatively low 

numbers during the breeding season. This finding was not expected because there are 

relatively few mature males at the rookery islands during the non-breeding season. Their 

presence during the breeding season might be attributable to non-territorial bulls using the 

platforms as resting sites while foraging (either at or away from the platform). Mature 

males present at the offshore facilities during the non-breeding season may be individuals 

from the Channel Islands, or they might have been animals migrating from rookeries off 

west side of Baja California, Mexico. They were not as abundant on the platforms as adult 

females or juveniles because of their inconsistent attendance in the southern California 

Bight, or that many of them were at the islands defending territories. 

Juvenile California sea lions were the most abundant age/sex class on the platforms. 

Unlike adult females, they are not necessarily central place foragers. Once weaned, 

juveniles are not obligated to return to the rookery until they are sexually mature. They 

only require energy for self-maintenance and growth. Although they need to alter their 

behaviors in response to prey movements, theoretically they can afford to be more 

selective in prey choice or follow migrating prey for greater distances than adult females, if 

needed. However, physiological factors that limit dive duration and traveling distance are 

positively correlated with body size and age (Feldkamp et al. 1989, Horning and Trillmich 

1997, Burns 1999, Baker and Donohue 2000, Fowler et al. 2007a,b). Therefore, the 

morphological, physiological, and energetic requirement changes that these immature 

individuals undergo may influence their distribution, at-sea behaviors, and use of the 

platforms throughout the year. Orr et al. (2012) reported that yearlings and juveniles 

instrumented with satellite tags at San Miguel Island were primarily distributed in areas 

around the northern California Channel Islands, and on the continental shelf just north and 

south of Point Conception, California, which are locations in close proximity to the oil and 

gas platforms in the Santa Barbara Channel. Seasonal changes in prey availability [e.g. 

spawning of market squid (Doryteuthis opalescens)] also may strongly influence where 

juveniles fed and rested. 

California sea lion pups were seen the least of the age/sex classes. During their first 

6-months of life, they are dependent exclusively on their mothers for nutrition (Orr et al. 

2012). As pups grow and develop, they begin to attain the necessary morphological and 

physiological characteristics, as well as the motor skills to swim and transition from 

dependence to foraging in the marine environment. After approximately 6-months of age, 

they begin to supplement their milk diet with prey acquired at sea (Orr et al. 2012). This 

was reflected by the presence of pups at Platform Gina as early as January. Their numbers 
25

increased during the spring until June when they were weaned and considered juveniles 

after their first birthday. 

Apart from the natural history aspects of California sea lions, and aforementioned 

factors (e.g. tidal heights, sea state, disturbance from the arrival/departure of crew boats or 

crew activities), their numbers may have fluctuated temporally because of environmental 

conditions. Environmental conditions strongly affect the annual and seasonal variability in 

prey distribution, availability, and abundance. These attributes of prey may also be 

influenced by potentially longer- or larger-scale periodic anomalies or perturbations (e.g. 

“The Blob”, El Niño). “The Blob” was a warm-water mass that started during fall 2013 when 

the usual winter storms in the Gulf of Alaska did not occur to cool down the North Pacific 

Ocean. That resulted in an expanse of warmer than usual water that by summer 2014 

spanned 3,200 km from Alaska to Baja, Mexico, and stretched 800 km wide (Almasy et al. 

2015, Bond et al. 2015, Kintisch 2015). It was observed that many individuals of some 

species died and others behaved abnormally (Almasy et al. 2015). Even prior to “The Blob” 

and the current El Niño (2015-2016), the National Marine Fisheries Service declared an 

Unusual Mortality Event in March 2013, in response to high numbers of stranded California 

sea lion pups on southern California Beaches (Melin and DeLong 2014). Pups at San Miguel 

and San Nicolas Islands (the two largest rookeries of California sea lions) exhibited low 

growth with pups averaging at least 10 kg lower than normal at 7 months of age (Melin and 

DeLong 2014). One hypothesis for the poor condition of pups was that prey distribution 

and abundance changed such that it was difficult for their mothers to find enough food for 

self-maintenance and nutrition for their pups. The stranding of pups on mainland 

California beaches indicated that the pups weaned early and sought to forage at sea 

because their mothers were not able to support them nutritionally (Melin and DeLong 

2014). Warmer, less productive waters have persisted in the eastern Pacific Ocean during 

the past few years and the compromised condition of pups (indicated by their lower 

weights) has reflected that. Therefore, the presence of pups observed on the platforms as 

early as January might reflect that many these individuals weaned early and foraged at sea 

because they had to for survival. 

Inter-platform (spatial) comparisons

There did not appear to be a spatial (south-to-north) pattern in the use of the 

platforms by California sea lions. Rather, size or space on the platform (i.e. the amount of 

structure that was available and accessible to use), location, and human activities (e.g. 

maintenance) appeared to be more important factors. Fencing and platform activities 

severely limited haulout space on all platforms except Platform Habitat at various times 

during the study, which had a direct effect on the numbers of California sea lions counted. 

Some of the fencing was a permanent barrier; however, some of it was set up temporarily.

Platform Habitat was not necessarily one of the larger facilities, but it had many large pipes 

and supporting structures that provided areas where sea lions could rest. Sea lions were 

observed occupying most of the lower-level deck. Platform Gina, on the contrary, was one 

of the smaller platforms where camera-systems were deployed. Although there were areas 

where sea lions potentially could have used, they predominantly used the landing decks, 

stairs, and periphery of the lower-level deck. Overall, the animals used Platform Heritage 

just slightly more than Platform Harvest. Whereas both platforms are north and relatively 

close to San Miguel Island, Platform Heritage is south of Point Conception within the 
26

southern California Bight, which provides a lee from the prevailing north-west winds. Wind 

and sea-state conditions are calmer at Platform Heritage compared to Platform Harvest. 

California sea lions were abundant on Platform Elly as well. It is east of Santa Barbara, San 

Clemente, and San Nicolas Islands. Individuals from these islands may have been using 

Platform Elly as a resting or foraging site.

Notables

During travels to/from the platforms, while installing and servicing the camera 

systems, and from examining the images obtained during the study, we observed California 

sea lions with characteristics or were engaged in activities that enhanced our 

understanding about the ecology of these animals or found qualities about the offshore 

platforms that were deemed noteworthy.

Above the water surface, the offshore platforms provided structure and space for 

animals like pinnipeds, gulls (Larus spp.), brown pelicans (Pelecanus occidentalis), and 

cormorants (Phalacrocorax spp.) to rest, and peregrine falcons (Falco peregrinus) to nest 

and hunt. Below the water surface, the gas and oil platforms provided structure and habitat 

for various invertebrate and fish communities. Consequently, areas beneath and around 

the platforms were habitats that California sea lions (among other species) could exploit 

for foraging. Love et al. (2006) noted that the offshore platforms harbor three fish 

assemblages: (1) ones that inhabit the shell mound area surrounding the base of the 

platform; (2) those that utilize the areas adjacent to the platform bottom; and (3) ones that 

occupy the midwater. Rockfishes (Sebastes spp.) predominate these assemblages (Love et 

al. 2006). Rockfishes are known prey of California sea lions (Antonelis et al. 1984, Lowry et 

al. 1991, Melin et al. 2008, Orr et al. 2011) and were seen being consumed by sea lions 

during our visits to the platforms (Fig. 8). Spiny dogfish (Squalus acanthias) were also seen 

being eaten by the sea lions during the study (Fig. 8). We observed schools of fish within a 

kilometer of the platforms being attacked by several predators, including: California sea 

lions, common dolphins (Delphinus delphis or D. capensis), minke whales (Balaenoptera 

acutorostrata), gulls, brown pelicans, cormorants, among others (Fig. 8). 
27

Figure 8. California sea lions (Zalophus californianus) feeding in waters 

adjacent to gas and oil platforms; sometimes feeding with other animals 

including cetaceans and sea birds.

Adult female California sea lions were observed nursing their young (including 

pups, yearlings, and juveniles; Fig. 9) on the offshore platforms. This is notable because 

Melin et al. (2000) examined the at-sea distribution and diving behaviors of adult female 

California sea lions and observed that adult female-pup pair attendance patterns indicated 

that the pups did not accompany their mothers to sea on foraging trips. Our observations 

indicated that some pups and older conspecifics follow their mothers from rookeries (e.g. 

Santa Barbara, San Clemente, San Miguel, and San Nicolas Islands) to foraging areas. 

Dispersal of mother-pup Steller sea lion pairs from natal rookeries has been documented as 

well (Calkins and Pitcher 1982, Merrick et al. 1988, Raum-Suryan et al. 2002, 2004). Dead 

premature pups were also seen on the offshore platforms during the study period. 

Premature birthing can occur at any place including the water, so we did not believe that 

the adult females were selecting the platforms to give birth. Rather they delivered the fetus 

prior to arriving at their rookery.
28

Figure 9. Adult female California sea lions (Zalophus californianus) nursing pup 

(left) and juvenile (right) while hauled out on gas and oil platforms during the 

study period.

California sea lions that were marked (branded and/or flipper-tagged) for 

demographic studies on San Miguel Island or in the Pacific Northwest conducted by the 

MML were spotted using the gas and oil platforms or adjacent waters (Fig. 10). Individuals 

that were rehabilitated, flipper-tagged, and released by one of the rehabilitation centers in 

central or southern California also were seen using the platforms (Fig. 10). Additionally, 

pups that were instrumented with satellite instruments by researchers at Hubbs-Sea World 

Research Institute (San Diego, California) during the 2013 Unusual Mortality Event were 

observed on the offshore facilities. Further examination of additional images obtained 

during this study may provide insight about the dispersal and movements of marked 

California sea lions for these various studies.
29

Figure 10. Marked (i.e. (a) tagged, (b) branded, or (c) instrumented) California sea 

lions (Zalophus californianus) were seen on and around the gas and oil platforms 

during the study area. Marked individuals included those that were: (a) were 

rehabilitated and tagged at The Marine Mammal Center, Sausalito, CA; and (b) 

tagged and/or branded by the Marine Mammal Laboratory, Seattle, WA for 

demographic studies on San Miguel Island, WA or in the Pacific Northwest. 

(a)

(b)

(c)
30

California sea lions (and harbor seals) depredate fish caught by anglers using line or 

gillnets. The animals may follow fishing vessels and subsequently take the fish that has 

been caught and being reeled in. While trying to capture the fish, these predators may get 

hooked with the fishing gear. Gillnets may be difficult for these animals to detect; 

subsequently, they might swim into or through the nets. Those that are not caught in the 

nets might still be entangled with pieces of the net around their neck or body (Fig. 11a). As 

the animal grows, the net can cut into their body and eventually kill the animal. Many sea 

lions on the platforms had fishing line or remnants of gill nets wrapped around them. 

Further examination of additional images collected during this study may provide

information about the type of gillnets being used and which animals primarily are being 

entangled.

The main predators of California sea lions include killer whales and sharks. The 

prevalence of shark predation or attacks by great white (Carcharodon carcharias) and 

mako sharks (Isurus oxyrinchus) on California sea lions have appeared to increase during 

the past several years, perhaps partly due to the recovery of these shark species. We 

observed several sea lions on the platforms that exhibited evidence of interactions with 

sharks (i.e. bite or teeth-rake lesions; Fig. 11b). With further examination of the images 

collected during this study, one may gain insight about which species of shark are attacking 

sea lions, what part of the body they are striking, and the age/sex class of sea lions they are 

targeting. 

California sea lions generally are skittish around humans. On the spectrum of fightor-flight responses, they will usually flee from humans until they are far enough away that 

they feel comfortable, or if they are in the water. This behavior was not observed on the 

platforms. The sea lions often had to be coaxed to move. Therefore, it should be noted that 

crew activities may not have necessarily adversely affected the sea lions.
31

Figure 11. California sea lions (Zalophus 

californianus) hauled out on gas and oil 

platforms exhibiting signs of encounters with 

(a) fisheries (e.g. monofilament fishing line)

and (b) sharks (e.g. lesions).

(a

)

(b

)
32

Conclusions

The platforms provide benefits to pinnipeds (esp. California sea lions). Whereas

some human activities may be disruptive to resting pinnipeds, the overall benefit of having 

a structure to rest in areas where pinnipeds feed or along foraging routes likely 

outweighed any negative impacts of human disturbance. The temporal and spatial 

variability in number of California sea lions using the platforms was pronounced for some 

platforms and minimal at others. They were observed on the platforms year-round and 

during all hours of the day. Most of the identified animals were juveniles; however, 

proportionally relatively few of the animals were identified to a particular age/sex class in 

comparison to all counted individuals. Therefore, caution should be taken when using or 

interpreting findings. 

We only analyzed images during periods when all camera systems were operational 

on a given platform for consistency in interpreting results. Many images remain to be 

examined and analyzed, which may result in a better resolution of our understanding of 

usage of the offshore oil and gas platforms by pinnipeds. Analyses of additional images may 

also be useful for other projects. For example, we now have empirical evidence that some 

California sea lion pups and juveniles travel with their mothers to areas a good distance 

away from their rookeries. Marked individuals were found using the platforms. It may be 

possible to examine their residency times and general movement patterns if they are seen 

at different platforms or other haulouts where either camera systems or observers are 

located. From examining images of individuals entangled in fishing gear, it may be possible 

to get an idea of the quantity of entanglement and information about the fishing 

gear/fishery for management purposes. Useful information may also be obtained from 

images with individuals with shark bite lesions, such as age/sex class of sea lion, species of 

predator, and geographical location information if there is a pattern. During this study, we 

have obtained information about the use of oil and gas platforms by pinnipeds in support of 

analyses pursuant to the NEPA and MMPA permitting requirements by owners of the 

offshore facilities when they are decommissioned. However, we also gained a greater 

understanding of the ecology of these animals in pursuant to scientific knowledge. 
33

ACKNOWLEDGEMENTS

We would like to thank the owners and operators of the platforms, including: Beta 

Offshore (Platform Elly), DCOR (Platforms Gina and Habitat), Exxon Mobil (Platform 

Heritage), and Plains Exploration and Production Company (PXP; Platform Harvest). We 

also extend our gratitude to the Bureau of Safety and Environmental Enforcement for their 

knowledge and assistance when accessing platforms by helicopter, the boat operators for 

providing the majority of the transportation to/from the platforms, and the platform crews 

who helped us perform our work in a safe and expeditious manner. We thank Dr. Alexey 

Altukhov for his help and guidance in the design of the database and camera systems. We 

also thank Trail Watcher Game Cameras for designing and building the camera systems. 

This study was funded by the Bureau of Ocean Energy Management. 
34

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