Research Document - 2007/034
Recovery Potential Assessment for Sea Otters (Enhydra lutris) in Canada
By Nichol, L.M.
Abstract
Sea otters were extirpated from British Columbia by 1929. In an effort to re-establish the species in Canada, 89 sea otters from Alaska were re-introduced to Checleset Bay, Vancouver Island between 1969 and 1972. The most recent population surveys indicate that, based on direct counts the B.C. sea otter population includes a minimum of 3,180 sea otters (surveys 2001 to 2004) (Nichol et al. 2005). The population occurs along the west coast of Vancouver Island and on a small section of the central B.C. coast although most of the population (~ 85%) occurs along the west coast of Vancouver Island. Population growth has been positive, at 19.1% per year on Vancouver Island between 1977 and 1995 but slowed to 8.0% per year (1995 to 2004) for an overall average annual rate of 15.6% per year between 1977 and 2004. Growth may have slowed along Vancouver Island as parts of the range have reach equilibrium densities. On the central B.C. coast growth has average 12.4% per year (1990 to 2004). Based on historical information from the maritime fur trade and on modeling of available habitat the population in B.C. likely occupies at most 25 to 33% of its historic range. Coast –wide sea otter habitat carrying capacity estimates of 14,844 (9,798-20,769, 95% CI) for optimal habitat and 52,199 (48,672-59,018, 95% CI) based on otters per kilometer of shoreline suggest that at its current size the Canadian sea otter population is far below carrying capacity.
Population growth and range expansion are inter-related in this species. Sea otters are non-migratory and occupy small overlapping home ranges. Range expansion occurs as areas of occupation near carrying capacity. Male sea otters then move from the periphery of the occupied range into adjacent habitat. Females subsequently occupy the new habitat once the males have moved on. Therefore given this inter-relationship it follows that increasing the geographic range of the species is expected to result in both a larger population and one in which the risk from human activities is reduced. An interim recovery target is therefore to continue the range expansion of the species in British Columbia.
Estimates of allowable harm that would not jeopardize recovery were presented based on the effect of mortality on the time to achieve recovery in two geographic areas that encompass the current range on west coast Vancouver Island and on the central B.C. coast. The resulting estimates, 40 to 90 animals per year on Vancouver Island and 20 to 25 animals per year on the central B.C. coast were predicted to delay recovery by ≤ 10% to a target level in each area of 80 to 95% of the habitat-based carrying capacity estimate. The target levels ≥ 80% of carrying capacity were chosen to reflect the inter-relationship between population growth and range expansion. The assumptions made to calculate these estimates and the uncertainties of some of the model parameters, in particular the habitat-based carrying capacity estimates for the central coast section are discussed. Should a directed take be considered for the B.C. population, harvest should be deferred on the central B.C. coast because of the uncertainties and assumption and because that portion of the population is very small. For west coast Vancouver Island, the lower estimate would be precautionary should a directed take be considered.
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