Annual California Sea Otter Census: 2017 Census Summary Shapefile

The GIS shapefile "Census summary of southern sea otter 2017" 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 2017 range-wide census. The USGS range-wide sea otter census has been undertaken twice a year since 1982, once in May and once in October, using consistent methodology involving both ground-based and aerial-based counts. The spring census is considered more accurate than the fall count, and 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 2017). 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 behaviour 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

地理信息系统(Geographic Information System)形状文件（shapefile）"2017年南部海獭普查汇总"为分析南部海獭（*Enhydra lutris nereis*）的种群丰度空间格局与种群动态趋势提供了标准化分析工具，其数据源自2017年春季开展的全分布范围普查。

美国地质调查局（United States Geological Survey, USGS）自1982年起每年开展两次全分布范围海獭普查，分别于5月和10月进行，采用统一的地面计数与航空计数相结合的调查方法。春季普查结果被认为比秋季普查更为精准，是州级与联邦管理机构评估海獭种群动态的核心依据。

本形状文件包含从原始普查数据中提取的一系列汇总统计量，具体包括：海獭栖息地密度（单位：栖息地每平方千米海獭数量）、线性密度（单位：海岸线每千米海獭数量）、幼崽相对丰度（幼崽与独立个体的数量比）以及5年种群趋势（以指数变化率计算得出）。所有统计量均针对小块栖息地单元进行计算与可视化，以展现2017年加州海獭本土全分布范围内各项统计量的局域变异特征。

海獭栖息地范围被定义为从平均低潮线向海延伸至60米等深线的海域：根据无线电标记海獭的潜水深度数据（Tinker等，2006、2007），该深度区间覆盖了海獭99%以上的觅食潜水活动。加州海獭的本土分布范围被认为是沿加州海岸延伸的这片潜在栖息地带，其南北边界由以下分布极限标准界定："在最近两次春季普查中，沿10米等深线测量的10千米连续海岸线段内记录到5只及以上海獭的最远离分布中心的点位；或在前一年度满足该计数标准的点位"。

与该分布范围定义对应的多边形随后被划分为宽度约500米的近岸/远岸条带。这些条带之间的边界对应于ATOS（As-The-Otter-Swims，即海獭游动路径）点位，这些点位为沿加州近岸经平滑处理的5英寻等深线（线要素）每隔约500米设置的任意参考点位。

参考文献：
1. Tinker, M. T., Doak, D. F., Estes, J. A., Hatfield, B. B., Staedler, M. M. 与 Bodkin, J. L. (2006). 将多样数据与现实复杂性纳入海獭种群统计估算流程. 《生态应用》, 16: 2293–2312, https://doi.org/10.1890/1051-0761(2006)016[2293:IDDARC]2.0.CO;2
2. Tinker, M. T., Costa, D. P., Estes, J. A. 与 Wieringa, N. (2007). 加州海獭的个体食性特化与潜水行为：利用归档式时间深度数据探测差异化觅食策略. 《深海研究II辑》, 54: 330–342, https://doi.org/10.1016/j.dsr2.2006.11.012