Spring 2013 California Sea Otter Census Results
USGS Western Ecological Research Center
Santa Cruz Field Station
Brian Hatfield and Tim Tinker
Email: firstname.lastname@example.org or email@example.com
12 September 2013
Data referenced in this report can be downloaded here.
This summary is intended for educational purposes. For official reports on southern sea otter population trends and management, please consult the U.S. Fish and Wildlife Service Southern Sea Otter Recovery Plan webpage.
NOTE: For the 2013 effort, the high count from the previous year (2012) at SNI was added to the mainland 3-year running average to derive the range-wide southern sea otter abundance index. This method has been updated in 2014 to standardize SNI with mainland calculations. Now, the 3-year running average of spring counts at SNI are added to the mainland 3-year running average to obtain this range-wide index. This has resulted in the 2013 range-wide abundance index being retroactively revised from 2,941 to 2,939.
The spring 2013 sea otter count began on 3 May. All of the ground-counted areas and half of the aerial sections were finished by 16 May, but poor weather conditions and plane availability precluded the completion of the southern aerial portions until 4 June. Overall viewing conditions this year were slightly less favorable than those during the 2012 spring census (2.5 vs. 2.7, where 0=poor, 1=fair, 2=good, 3=very good, and 4=excellent). The surface canopies of Macrocystis
were considered to be about normal for this time of year in most areas of the otter’s central range. The region surveyed spans the mainland coastline from Point San Pedro, San Mateo County in the north to Rincon Point at the Santa Barbara/Ventura County line. The small population at San Nicolas Island is monitored by a separate survey effort, details of which are provided below.
The total count of the mainland population was almost unchanged from the 2012 census (
Table 1). The number of pups counted this spring represents a record high, while the number of independent sea otters (juveniles and older) was actually down slightly, with the result that the number of pups per 100 adults (18.6) was the highest since 1990. It should be emphasized that there is a considerable degree of uncertainty (random variation due to sampling and measurement error) in any one census count, and thus longer-term trends are far more informative than year-to-year differences. The U.S. Fish and Wildlife Service’s Southern Sea Otter Recovery Plan specifies that the 3-year running average of the total count be used as the official metric for monitoring trends of the mainland population, thereby reducing the influence of anomalously high or low counts from any particular year. The trend in the mainland population over the past 5-7 years remains essentially flat (
Figure 1), with a mean annual growth rate of just 0.16%; however, regional trends vary considerably (
Figure 2). In the north part of the range (Pigeon Pt. – Seaside) the number of independent otters counted was down this year but the longer-term trend remains positive. In the center portion of the range (Seaside to Cayucos), where sea otter densities are highest (
Figure 3), the number of independent sea otters counted was up this year compared to last, and there is a slightly positive trend here too. In contrast, the number of independent sea otters counted south of Cayucos was down for the third year in a row and there is a relatively strong negative trend, particularly between Pismo Beach and Pt. Sal (
Figure 4). This latter trend is consistent with the record number of carcasses recovered from the southern portion of the range, over half of which were associated with shark-bite mortality, and suggests that the increased rate of shark attacks in recent years is in large part responsible for declining numbers in this area.
Only one sea otter was observed north of Pigeon Point and that was an animal seen approximately 3 km north of the Point. At the south end of the range, 68 otters were counted southeast of Point Conception (11 fewer than last spring), with only one animal spotted southeast of Gaviota State Beach (near Naples Reef). The limits of the sea otter’s range along the mainland coast remain approximately unchanged from 2012: the northern boundary is considered to be approximately 2 km southeast of Pigeon Pt., while the southern boundary is considered to be near the Agua Caliente Creek mouth approximately 3 km west of Gaviota State Beach (
Figure 5). The mainland range limits are defined as the points farthest from the range center (to the north and south) 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.
At San Nicolas Island, the most remote of the Channel Islands in southern California, there is a small colony of sea otters resulting from translocation efforts in the late 1980’s that remains demographically distinct from the mainland population. This population has been surveyed 3-4 times per year since the mid 1990’s, with the high count from the last complete calendar year being used as the official index of population abundance. The high count for San Nicolas Island in 2012 was 59 sea otters (
Table 1), including 51 independents and 8 pups. This continues a positive trend in the population at San Nicolas, with a mean annual growth rate of approximately 8% over the past 5 years (
and 4). Because San Nicolas Island is no longer classified an experimental population, these index counts will henceforth be added to the 3-year running average of the mainland range to arrive at an official California-wide index of abundance for southern sea otters, the current value of which is 2,941
Additional data summaries, in both tabular and GIS format
, are available at the USGS-WERC web site: http://www.werc.usgs.gov/seaottercount
. The annual census is a cooperative effort between USGS-Ecosystems-Western Ecological Research Center, CDFW–Marine Wildlife Veterinary Care and Research Center, Monterey Bay Aquarium, University of California-Santa Cruz, the US Fish and Wildlife Service, and many experienced and dedicated volunteers. Assistance was also received from staff of the Bureau of Ocean Energy Management.