Science Advisory Report 2020/036

Trends in the growth of the sea otter (Enhydra lutris) population in British Columbia 1977 to 2017

Summary

• During a range-wide survey in 2017, 7,696 sea otters were counted and 414 sea otters were estimated for three areas not completed due to weather, for total of 8,110 sea otters in British Columbia (BC).
• The survey provides an index of abundance and a report of range expansion. The data allow for analysis of population growth trends in the region overall, as well as at a sub-regional scale.
• Extensive reconnaissance survey effort in 2017 along the north coast of BC found no evidence of expansion north of the previously documented range limit (i.e. Aristazabal Island).
• The annual rate of increase across the whole BC region was 6.4% per year from 2008 to 2012 and 5.2% per year for the period 2013 to 2017.
• Sea otters occupy small home ranges and exhibit limited dispersal with the result that density-dependent processes, linked to food availability, operate at sub-regional scales. Therefore, growth trends vary geographically across the population with lower growth rates in areas that have been occupied for longer periods of time.
• Annual rates of increase for the period 2013-2017 in long-occupied areas were low (e.g. 1.6% to 2.9% per year), suggesting these sub-regions were approaching carrying capacity. In more recently occupied areas, growth was exponential with several sub-regions exhibiting growth rates of 20.4% to 24.5% per year; rates that are within the theoretical maximum for this species in the absence of density dependent factors.
• In some sub-regions, growth rates have been in excess of the theoretical maximum indicating that immigration had contributed to the observed growth rates.
• Potential Biological Removal (PBR) for the entire population was estimated to be 534 animals. Recognition of the sub-regional spatial scale at which the population is structured has implications for conservation and management of this species. For instance, the population-wide PBR may not be precautionary if applied without taking this structure into account.

This Science Advisory Report is from the October 21-26, 2019 National Peer Review on Trends in population growth of sea otters (Enhydra lutris) in British Columbia 1977 to 2017. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2020/036 (PDF, 837 KB)

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