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

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.