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NOAA Technical Memorandum NMFS-AFSC-361

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Qualitative and quantitative assessment of use of offshore oil and gas platforms by the California sea lion (Zalophus californianus)


There are 23 oil and gas production platforms in federal waters offshore southern California. These platforms provide haul-out 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. 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 6-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.

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