Census_sum_15

The GIS layer "Census_sum_15" 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 2015 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 2015). 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.地理信息系统（Geographic Information System, GIS）图层“Census_sum_15”是一款标准化分析工具，可依托2015年春季全范围普查采集的观测数据，研究南部海獭（Enhydra lutris nereis）的丰度空间分布格局与种群动态变化趋势。美国地质调查局（United States Geological Survey, USGS）自1982年起每年开展两次全范围海獭普查，分别于5月和10月进行，采用统一的调查方法，涵盖地面计数与航空计数两种手段。春季普查被认为较秋季普查结果更为精准，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（Shapefile）包含一系列从原始普查数据衍生得到的汇总统计量，具体包括海獭密度（每平方千米栖息地内的海獭数量）、线性密度（每千米海岸线内的海獭数量）、幼崽相对丰度（幼崽与独立个体的数量比值）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分区计算并可视化呈现，用以展示2015年加州海獭整个本土分布范围内各项统计量的局域差异。海獭栖息地被划定为平均低潮线向海延伸至60米等深线的海域：基于无线电标记海獭的潜水深度数据（Tinker等，2006、2007），该水深范围覆盖了99%以上的海獭觅食潜水活动。加州本土范围内的海獭分布被界定为沿加州海岸延伸的潜在栖息地带，其南北边界以如下标准划定：在最近两次春季普查中，或在前一年满足相同标准的情况下，沿10米等深线连续10千米海岸线范围内被记录到至少5只海獭的最远点位，即为分布范围的极限。基于该范围定义生成的多边形随后被划分为宽度约500米的近岸/远岸条带。这些条带的边界对应海獭游动路径（As-The-Otter-Swims, ATOS）点位，该类点位是沿加州外海经过平滑处理的5英寻等深线每隔约500米设置的基准参考点。