Science Advisory Report  2015/043

Trends in the abundance and distribution of sea otters (Enhydra lutris) in British Columbia updated with 2013 survey results

Summary

  • BC sea otter surveys provide direct counts that represent an index of population abundance and allow for analysis of trends in population growth. Data collected also provide fine-scale detailed information about the distribution of sea otters, locations of rafts by sex and the timing of range expansion events.
  • As of 2013, population growth and range expansion continue to follow positive trends in British Columbia (BC), but show characteristics of density-dependent growth typical of sea otter populations. Annual rates of increase are lowest in long-occupied areas, presumably reflecting density dependence. However, in more recently occupied areas growth is still exponential, with rates of annual increase in excess of Rmax (17-25% per year), which is the maximum rate of increase for this species in the absence of density dependent factors, and due to immigration from other areas.
  • During a range wide survey in 2013, 6,754 sea otters were counted (5,612 in the Vancouver Island region and 1,142 in the central mainland coast region).
  • On Vancouver Island the annual rate of increase from 2004 to 2008 was 7.1% per year (SE = 1.05) and from 2009 to 2013 it was 6.8% per year (SE = 1.20). In the previous assessment, growth on Vancouver Island was estimated from a single exponential growth curve and a rate of 8.4% per year was estimated for the period 1995 to 2008. The lower rate in this assessment reflects the contribution of density-dependent growth in the long-occupied portions of Vancouver Island, detected by fitting population growth models to sub-regional time series of survey data.
  • On the central mainland coast, the annual rate of growth was 4.2% per year (SE = 1.62) from 2004 to 2008 and then 12.6% per year (SE = 1.65) during 2009 to 2013. The much higher rate of growth in the most recent period reflects the contribution of sub-regions that have only been occupied since 2007, where growth is best described by exponential growth models.
  • Since 2008, range expansion, signified by the observed presence of a raft of male sea otters in spring or summer in a previously unoccupied area, has occurred primarily in Queen Charlotte Strait and the adjacent mainland coast. On the central mainland coast, range expansion to Calvert Island occurred in summer 2013.
  • Analysis of population growth trends by sub-regions made possible the use of a larger data set of surveys and the examination of patterns of growth within the population associated with occupation time in this density dependent species. However, sub-regions do not represent distinct population units.
  • There are no estimates of the size of the sea otter population in BC prior to the maritime fur trade that commenced in 1778 and ended with the extirpation of the species in BC by 1930. However, even incomplete historical records, which only enumerate pelt landings over certain periods and for certain areas, suggest a much larger population than today. 
  • Habitat models suggest that the population is well below coast wide carrying capacity and is still distributed in only a portion of its historic range in BC.

This Science Advisory Report is from the Annual Meeting of the National Marine Mammal Peer Review Committee (NMMPRC) held in Ottawa October 20-24, 2014. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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