Census_sum_08

The GIS layer "Census_sum_08" 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 2008 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 2008). 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.

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.

1. Census Methods
During each census, the entire mainland range of the sea otter in coastal California is counted by one of two methods: aerial surveys or shore-based counts. The latter method is used in all areas that are accessible by ground-based observers, except in a few regions where otters often move far off shore (such as shallow, sandy embayments) and are therefore difficult to count reliably from the shore. For the majority of the sea otter's range, however, ground surveys are practical, and are considered the more reliable means of censusing. It has been estimated that shore-based observers generally detect about 90-95% of the otters located in a given area (Estes and Jameson 1988). The ground survey area is divided into sections and each section is assigned to a team of observers. Each team consists of two individuals, a primary and secondary observer. The primary observer in every case is an individual with considerable experience counting and observing sea otters. Generally, the secondary observer has less experience than the primary, but in some cases the teams consisted of equally skilled individuals. Occasionally, small areas are counted by a single individual, but this occurs infrequently, and in every instance the individual is highly skilled with many years of experience and intimately familiar with the area to be surveyed. Each team is equipped with a high resolution 50-80X telescope and each member has binoculars (10X).

Shore-based procedures are as follows: the team starts at one end of their assigned section and selects an observation point that provides good viewing of a "viewable area of habitat", which generally consists of 100-300m of coastline and all waters out to approximately 1.5 km from shore. In most cases observers use the same counting locations year after year, for consistency. The observers scan the area with unaided eye and binoculars for otters or objects that are suspected to be sea otters. Large groups and suspicious objects are scanned by the primary observer with the aid of the telescope while the secondary observer continuously scans the area with binoculars for foraging or resting otters missed during the initial scan. After having taken sufficient time (15-30 minutes) to make a thorough count of all otters within this first area of habitat, the observers move down the coast to another location that provides good viewing of the next area of habitat, contiguous with the first area. This process is continued until the entire section is counted.

All sea otter observations are marked as points directly onto field maps (scale 1:6000) that show all major features of the area (including offshore rocks and kelp beds) for reference. These features allow data to be precisely placed on the maps, and reduce the chances of recounting or undercounting when the team moves to the next observation point. In addition to the otter locations, associated data recorded directly onto the maps include the following: number of independent otters, number and relative age of pups (pups are classed as either small or large depending primarily on the presence or absence of the natal pelage, but sometimes on relative size or behavior), behavior (resting, foraging, or "other"), group size, and micro-habitat type (open water, kelp, or hauled out). Time and general counting conditions are also recorded. Viewing conditions are rated from excellent to poor (coded 4 to 0). Teams are instructed to not begin surveying if conditions are "poor", or to abort the survey if conditions deteriorate to "poor". Because each team is headed by an experienced observer, it is left to that individual to determine if conditions are suitable for counting.

For those portions of the range where ground counting is impossible or impractical, aerial surveys are conducted using a Partenavia PN68 "Observer" fixed-wing plane. The plane carries three observers and a pilot, and flies at an air speed of approximately 167 kilometers per hour (90 knots) at an altitude of approximately 60 meters (200 feet). Pilot and data recorder/observer occupy front seats; principal observers occupy middle seats viewing out through bubble-type viewing windows. The flight path is a predetermined track line constructed using GIS software and loaded into a GPS for the pilot to follow. Transects are oriented parallel to the coastline, with the nearshore transect line centered approximately 300 meters from shore. At survey section boundaries, the plane turns offshore, reverses direction and flies parallel to the first transect, continuing to follow pre-established tracklines on the pilot's GPS. The distance moved offshore is dependent of habitat type: transects over kelp habitat are spaced 600 meters apart (300 meter viewing area to each side of the plane), while transects over open water are spaced 800 meters apart (400 meter viewing area to each side of the plane). The survey transects are established to cover all waters within 60m of depth (30 fathoms). In some areas, such as Pismo Beach, this includes over 10 transects spaced at 800 meters. The survey track line and waypoints for otter sightings are recorded on a separate handheld GPS unit. Additional information on each sea otter sighting is recorded onto data sheets which are later transferred to a spreadsheet. The data fields include group size (with adults and pups tallied separately), observer, and viewing conditions (ranked on a 1-10 scale).

2. Data Processing
Census data recorded on paper survey maps are digitized directly into a GIS system using a standard digitizing tablet. All paper survey maps have 4 geo-reference calibration points marked on them to ensure spatial accuracy. A custom GIS data capture interface program was designed for this purpose (using ESRI ArcView 3.2 software and Avenue programming language) by Brian Fulfrost, Department of Environmental Studies, UC Santa Cruz. After completing the digitizing of all survey maps from the ground-based count, all spatial data collected during the aerial survey (including waypoints and associated attribute data in spreadsheet form) are imported into the program. Rigorous error checking is then conducted to ensure that the totals for each paper survey map and each aerial survey section are consistent with the data in the GIS database.

3. Data Analysis and Summarization
After raw census data have been entered into the GIS database, they are further classified by location: this is accomplished by conducting a spatial join between the census data layer and the Shape file "ATOS_polygon_teale83". The result of this join is that every sea otter sighting location is assigned to a habitat polygon (identified by the field "POLY_ID"), where each polygon represents a sub-division of the total sea otter range. The north-south boundaries between habitat polygons 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. Habitat polygons are further divided based on depth/distance to shore, resulting in 4 possible depth-zone categories: b = in-shore bays or sloughs; n = "nearshore" (low tide line out to the 30m depth isobath); o = "offshore" (30m isobath out to the 60m isobath or 10km from shore, whichever is closer); and w = "way offshore" (depths up to 60m but farther than 10km from shore). Having assigned survey sighting locations to habitat polygons, numbers of independent animals and pups are then summed for each polygon. To account for both temporal and spatial variability in survey conditions, and for the mobility of sea otters, these sums are smoothed temporally by taking the 3-year average for each polygon, and then spatially smoothed using a 10km "moving window" average. The smoothed data are used to calculate 4 statistics for each polygon: density (number of animals per square km), linear density (otters per km of coastline), relative pup abundance (ratio of pups to independent animals) and 5-year population trend. The last statistic is calculated by taking the natural logarithm of the 3-year average counts from the last 5 years and fitting a linear regression: the slope of this regression represents the exponential rate of change, with negative values representing local population decreases and positive values representing local population decreases.


References
Estes, J. A., and R. J. Jameson. 1988. A double-survey estimate for sighting probability of sea otters in California. Journal of Wildlife Management 52(1):70-76.
Tinker, M. T., D. P. Costa, A. Estes James, 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.
Tinker, M. T., J. A. Estes, K. Ralls, T. M. Williams, D. Jessup, and D. P. Costa. 2006. Population Dynamics and Biology of the California Sea Otter (Enhydra lutris nereis) at the Southern End of its Range. MMS OCS Study 2006-007. Page 253. Coastal Research Center, Marine Science Institute, University of California, Santa Barbara, California. MMS Cooperative Agreement Number 14-35-0001-31063.