Spring 2010 Mainland California Sea Otter Survey Results
The spring 2010 sea otter count began on 3 May. All of the ground-counted areas and half of the aerial sections were finished by 26 May, but poor weather conditions and plane availability precluded the completion of the southern aerial portions until 7 July. The 66-day span of this survey is of record duration. Overall viewing conditions this year were more favorable than those during the spring 2009 survey (2.4 vs. 2.0, where 0=poor, 1=fair, 2=good, 3=very good, and 4=excellent). And whereas the surface canopies of Macrocystis were unusually abundant and dense during last year’s spring count, kelp canopies were very low in most areas of the otter’s central range. Seasoned observers estimated the kelp canopy this May to be about 10% of normal this year between Monterey and Cambria. The region surveyed spans the coastline from Point San Pedro, San Mateo County in the north to Rincon Point at the Santa Barbara/Ventura County line.
Based on this year’s survey, the 3-year running average of the total count is 2,711, representing a drop of 3.6% from last year’s value of 2,813 (Table 1). The 3-year average is the metric the U.S. Fish and Wildlife Service’s Southern Sea Otter Recovery Plan recommends using to reduce the influence of anomalously high or low counts from any particular year. This is the second year in a row that the 3-year average has dropped (Figure 1), and indicates that the southern sea otter population is in a period of decline. The raw count for this year’s survey, 2,719, is 2.4% higher than the 2009 count, 1.5% lower than the 2008 count, and 11.6 % lower than the 2007 count; however, it should be noted that raw counts are uncorrected and subject to considerable year-to-year variance due to counting conditions, sea otter distribution and other factors. The difference in numbers between this year’s census and 2009 can be attributed primarily to higher aerial counts in three segments: Capitola to Seaside, Cayucos to Hazard Canyon, and Pismo to Point Sal (Figure 2a). All of these segments encompass male-dominated areas (Figure 3), and all were counted under more favorable conditions this year than last. These aerially-counted, male-dominated areas exhibit greater variability than any other areas within the range, and because last year’s aerial counts were unusually low it is likely that the difference reflects better counting conditions and more males being concentrated in these three areas, rather than in male groups at the northern and southern range extremities (both of which showed lower counts). All other coastal segments but one (San Simeon Pt. to Cayucos) showed a drop in numbers as compared to the previous two years (Figure 2a), resulting in a negative trend in abundance overall, but particularly in the center of the range (Figure 4).
The number of pups counted this year, 267, is the lowest number counted in a spring survey since 2003 (Table 1), and the resulting trend in the 3-year average pup count is also now negative (Figure 1). There were fewer pups counted (relative to the previous survey) in 10 coastal segments (Figure 2b), including all areas from Pigeon Point to San Simeon Point, while just 2 segments at the south end of the range showed slight increases in pup numbers, including the area that encompasses the southern-most established breeding area just north of Point Conception (Figure 3). No pups were observed north of Pigeon Point or south of Point Conception during the survey (Figure 2b). This low pup count came after a relatively severe winter in terms of storms and associated periods of high surf. It is worth noting that the sea otter stranding network recovered the highest number of pups and immature otters in the January – April period in at least 5 years. We strongly suspect pup mortality is positively correlated with the number and severity of storms in winter/spring (as occur during El Niño conditions). It is possible that the sparse kelp canopy earlier this year also could have contributed to higher than usual pup mortality and the low pup count.
Fewer animals were counted this year at the northern and southern extremities of the sea otter’s range along the mainland coast (Figure 2). The limits of the sea otters range along the mainland coast has contracted as compared to the previous two years: the northern boundary has moved from about Tunitas Creek to a point 2 km southeast of Pigeon Pt., and the southern boundary has moved from approximately Coal Oil Point to Gaviota State Park (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.
Further data summaries, in both tabular and GIS format, are available at the USGS-WERC web site: http://www.werc.usgs.gov/seaottercount. The annual survey is a cooperative effort between USGS-BRD-Western Ecological Research Center, CDFG–Marine Wildlife Veterinary Care and Research Center, Monterey Bay Aquarium, University of California-Santa Cruz, and many experienced and dedicated volunteers. Assistance was also received from staff of the U.S. Fish and Wildlife Service and the U.S. Minerals Management Service.