Annual California Sea Otter Census: 2018 Census Summary Shapefile

Dates

Publication Date: 2018-09-24
Start Date: 2018-04-26
End Date: 2018-05-24

Citation

Hatfield, B.B., Yee, J.L., Kenner, M.C., Tomoleoni, J.A., and Tinker, M.T., 2018, Annual California sea otter census—2018 spring census summary: U.S. Geological Survey data release, https://doi.org/10.5066/P98012HE.

Summary

The GIS shapefile "Census summary of southern sea otter 2018" provides a standardized tool for examining spatial patterns in abundance and demographic trends of the southern sea otter (Enhydra lutris nereis), based on data collected during the spring 2018 range-wide census. The USGS spring range-wide sea otter census has been undertaken each year since 1982, using consistent methodology involving both ground-based and aerial-based counts. The spring census provides the primary basis for gauging population trends by State and Federal management agencies. This Shape file includes a series of summary statistics derived from the raw census data, including sea otter density (otters per square km of habitat), linear density (otters per km [...]

Summary

The GIS shapefile "Census summary of southern sea otter 2018" provides a standardized tool for examining spatial patterns in abundance and demographic trends of the southern sea otter (Enhydra lutris nereis), based on data collected during the spring 2018 range-wide census. The USGS spring range-wide sea otter census has been undertaken each year since 1982, using consistent methodology involving both ground-based and aerial-based counts. The spring census provides the primary basis for gauging population trends by State and Federal management agencies. This Shape file includes a series of summary statistics derived from the raw census data, including sea otter density (otters per square km of habitat), linear density (otters per km of coastline), relative pup abundance (ratio of pups to independent animals) and 5-year population trend (calculated as exponential rate of change). All statistics are calculated and plotted for small sections of habitat in order to illustrate local variation in these statistics across the entire mainland distribution of sea otters in California (as of 2018). Sea otter habitat is considered to extend offshore from the mean low tide line and out to the 60m isobath: this depth range includes over 99% of sea otter feeding dives, based on dive-depth data from radio tagged sea otters (Tinker et al 2006, 2007). Sea otter distribution in California (the mainland range) is considered to comprise this band of potential habitat stretching along the coast of California, and bounded to the north and south by range limits defined as "the points farthest from the range center at which 5 or more otters are counted within a 10km contiguous stretch of coastline (as measured along the 10m bathymetric contour) during the two most recent spring censuses, or at which these same criteria were met in the previous year". The polygon corresponding to the range definition was then sub-divided into onshore/offshore strips roughly 500 meters in width. The boundaries between these strips correspond to ATOS (As-The-Otter-Swims) points, which are arbitrary locations established approximately every 500 meters along a smoothed 5 fathom bathymetric contour (line) offshore of the State of California. References: Tinker, M. T., Doak, D. F., Estes, J. A., Hatfield, B. B., Staedler, M. M. and Bodkin, J. L. (2006), INCORPORATING DIVERSE DATA AND REALISTIC COMPLEXITY INTO DEMOGRAPHIC ESTIMATION PROCEDURES FOR SEA OTTERS. Ecological Applications, 16: 2293–2312, https://doi.org/10.1890/1051-0761(2006)016[2293:IDDARC]2.0.CO;2 Tinker, M. T., D. P. Costa , J. A. Estes , and N. Wieringa. 2007. Individual dietary specialization and dive behavior in the California sea otter: using archival time–depth data to detect alternative foraging strategies. Deep Sea Research II 54: 330–342, https://doi.org/10.1016/j.dsr2.2006.11.012

Contacts

Point of Contact :: Brian B Hatfield
Originator :: Brian B Hatfield, Julie L Yee, Michael C Kenner, Joseph A Tomoleoni, M. Tim Tinker
Metadata Contact :: U.S. Geological Survey, Western Ecological Research Center
USGS Mission Area :: Ecosystems
SDC Data Owner :: Western Ecological Research Center
Distributor :: U.S. Geological Survey - ScienceBase

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Census summary of southern sea otter 2018.xml Original FGDC Metadata	View	33.3 KB	application/fgdc+xml

Shapefile: Census_sum_2018.zip
Census_sum_2018.dbf		1.01 MB
Census_sum_2018.shp		646.4 KB
Census_sum_2018.prj		428 Bytes
Census_sum_2018.shx		15.62 KB

Purpose

The range-wide census is conducted to monitor trends in abundance of the southern sea otter, and thus provide State and Federal resource agencies with the information they need for effective management. Because the censuses represent uncorrected total counts (rather than sample-based surveys), they cannot be considered as accurate estimates of true population abundance. Instead, these data represent a valuable time-series of index counts, and provide the means of assessing spatial and temporal trends in relative abundance. The 3-year running average of the spring counts have been identified as the best index of trends and status of the population, because these averages decrease the influence of year-to-year sampling variance. The 3-year average counts are used by the US Fish and Wildlife Service in its listing determinations (de-listing or up-listing) for this legally threatened population.

Annual California Sea Otter Census: 2018 Census Summary Shapefile

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