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Sea otters in a California estuary: Detecting temporal and spatial dynamics with volunteer monitoring

Cite this dataset

Wasson, Kerstin et al. (2022). Sea otters in a California estuary: Detecting temporal and spatial dynamics with volunteer monitoring [Dataset]. Dryad.


Volunteer monitoring can support conservation of imperiled wildlife, by providing higher resolution data in space and time than those available from professional scientists. However, concerns have been raised that data collected by amateurs are inaccurate or inconsistent and thus do not allow for robust detection of spatial or temporal trends. We evaluated the rigor and value of volunteer monitoring data for one iconic wildlife species, the southern sea otter (Enhydra lutris nereis), in Elkhorn Slough estuary in central California, USA, and explored whether volunteer monitoring could provide added value to complement limited professional surveys. First, we compiled and analyzed sea otter counts taken on daily ecotourist boat trips along the estuary, and then compared temporal patterns to data collected by professional scientists tasked with monitoring this federally listed species. Second, we analyzed data on sea otter abundance, habitat use, and behavior collected by a team of trained volunteers, the Elkhorn Slough Reserve Otter Monitoring Program. Overall, we demonstrated the ability to detect important ecological patterns relevant to sea otter conservation and wetland habitat management using volunteer-derived datasets. Long-term trends and inter-annual variability were similar between professional agency monitoring data and volunteer datasets. Moreover, the much higher frequency of volunteer observations allowed for seasonal and tidal dynamics to be detected that could not be revealed by less frequent professional monitoring. We found higher sea otter abundance in the estuary in spring–summer, indicating seasonality in use of the estuary. We detected differences in habitat use of the estuary between higher and lower tides, and greater frequency of foraging at low tide and in certain areas. Volunteer observations revealed fine-scale differences in habitat use: eelgrass beds were used much more heavily than adjacent areas only a few meters away. Volunteer data can thus provide critical information about coastal habitat use and behavior that can improve conservation strategies for threatened wildlife species.


Four CSV files with data from basic field surveys of southern sea otters in Elkhorn Slough estuary, central California, USA are provided.  The data almost entirely consist of otter counts conducted by visual surveys; one file also contains data on the percentage of otters observed foraging.  A complete description of the study objectives, methods, field sites, and uses of these data for analyses and interpretations can be found in the associated paper, Eby et al. 2022.

The Otter_Elkhorn_LongTerm dataset consists of data from three sources, 1) ecotourist counts from the Elkhorn Slough Safari boat, 2) volunteer observations from the Elkhorn Slough Reserve Otter Monitoring Program, and 3) annual range-wide surveys coordinated by USGS.  For the latter, we used only otter counts from the lower Elkhorn Slough estuary. USGS southern sea otter data are made available annually, e.g. for 2019 here:  These publically available data emphasize the value of 3-year running averages for larger regions, while the dataset archived here has annual counts just for the local Elkhorn Slough estuary. The other three datasets represent data collected by volunteers for the Reserve Otter Monitoring Program. 

Usage notes

These are CSV files; no special programs or software required.


NOAA's Office for Coastal Management