Research Document 2020/039

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

By Nichol, L.M., Doniol-Valcroze, T., Watson J.C., and Foster, E.U.

Abstract

The BC sea otter population is estimated at 8,110 sea otters, based on a counts of 7,696 sea otters during surveys in 2017 and an additional 414 sea otters estimated to account for animals missed in three survey segments that could not be surveyed or surveyed completely as a result of weather conditions. The sea otter population in British Columbia (BC) has continued to grow in numbers and range in recent years, following patterns that are typical of recovering sea otter populations. Two main processes explain observed trends in BC: demographic growth within occupied areas, and colonization of new areas. Estimates of growth rates at smaller geographical scales than the entire BC range were obtained by fitting deterministic population models to time series of sea otter counts grouped at sub-regional scales to explore growth trends at smaller spatial scales in the population. Annual growth rates for the period 2013-2017 were lower in long occupied sub-regions of Vancouver Island and the central mainland coast (1.55% to 2.88% year-1) than in more recently occupied areas (7.52% to 24.51% year-1). For long-occupied sub-regions, the selection of logistic growth models confirms that density-dependent processes are acting on sea otters as carrying capacity is approached. In more recently occupied sub-regions, exponential models were selected and growth rates were, in some cases, higher than the theoretical Rmax estimates for the species, indicating that immigration from other sub-regions was an additional factor in driving population trends of recently colonized areas. The average growth rate for the population as a whole during the period 2013-2017 was 5.26% year-1 (SE = 1.25). Range expansion since the previous assessment in 2013, was evident in Queen Charlotte Strait primarily. No range expansion was evident during extensive survey effort on the north coast of BC, confirming that as of 2017 no further range expansion north of Aristazabal Island had occurred. Potential Biological Removal (PBR) was calculated to be 534 sea otters.

Research Document 2020/039 (PDF, 3,513 KB)

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