Historical Data
Sea otters once ranged from Northern Japan to central Baja California, and had an estimated global population of between 150,000 and 300,000. However hunting which began the downfall of the sea otter in the Northern Pacific Ocean. Initially, it was only the Native Americans who hunted the sea otter, however when Europeans came to North America and began the maritime fur trade, this amplified the problem (Nichol, 2002).
By the mid 1800’s, the sea otter was considered to be on the brink of extinction. In 1911, the global sea otter population dropped as low as 2,000 individuals, and the International Fur Seal Treaty was signed by Japan, Russia, the United States and the United Kingdom (for Canada). It included an article which prohibited non-natives and anyone hunting for commercial purposes from hunting sea otters. However, this did not prevent the sea otter population from continuing to decline, and in 1929, the last verified sea otter in Canada was shot and killed (Nichol, 2002).
For many years, there were no sea otters in Canadian waters and most of the sea otters in the world were located in the Aleutian Islands, off of the coast of Alaska. However in 1969, Canada undertook a reintroduction program which relocated 89 sea otters from Alaska to the coastal waters of British Columbia. This relocation project was initially unsuccessful, as the population dropped to 28 individuals when the otters were first introduced. In the following years, the otter population began to grow and prosper. From 1977-1995, the population showed steady growth and averaged 19.1% growth over this time period (Nichol, 2002).
Although the sea otter was extirpated from the Canadian West Coast, it was reintroduced and once again become a part of the ecosystem, as shown in figure 1. Over recent years, the status of the sea otter was upgraded from endangered to threatened, and as of 2004, the otter was once again upgraded to a species of special concern. In the most recent study in 2008, the sea otter population in British Columbia had increased to about 4,700, up from 3,200 in 2004. In general, the current otter population is stable, however its troubled past and vulnerability to oil spills result in the need to protect the species (Fisheries and Oceans Canada, 2013).
By the mid 1800’s, the sea otter was considered to be on the brink of extinction. In 1911, the global sea otter population dropped as low as 2,000 individuals, and the International Fur Seal Treaty was signed by Japan, Russia, the United States and the United Kingdom (for Canada). It included an article which prohibited non-natives and anyone hunting for commercial purposes from hunting sea otters. However, this did not prevent the sea otter population from continuing to decline, and in 1929, the last verified sea otter in Canada was shot and killed (Nichol, 2002).
For many years, there were no sea otters in Canadian waters and most of the sea otters in the world were located in the Aleutian Islands, off of the coast of Alaska. However in 1969, Canada undertook a reintroduction program which relocated 89 sea otters from Alaska to the coastal waters of British Columbia. This relocation project was initially unsuccessful, as the population dropped to 28 individuals when the otters were first introduced. In the following years, the otter population began to grow and prosper. From 1977-1995, the population showed steady growth and averaged 19.1% growth over this time period (Nichol, 2002).
Although the sea otter was extirpated from the Canadian West Coast, it was reintroduced and once again become a part of the ecosystem, as shown in figure 1. Over recent years, the status of the sea otter was upgraded from endangered to threatened, and as of 2004, the otter was once again upgraded to a species of special concern. In the most recent study in 2008, the sea otter population in British Columbia had increased to about 4,700, up from 3,200 in 2004. In general, the current otter population is stable, however its troubled past and vulnerability to oil spills result in the need to protect the species (Fisheries and Oceans Canada, 2013).
Population Growth Pattern
In 1969, 89 otters were re-introduced to the British Columbian Coast Line (Province of British Columbia, 2003), which marked the beginning of a growth curve. In general, the growth pattern exhibited by the sea otter population is logistic growth (Carter-Edwards, 2011). Obviously in practise, there are many factors that contribute to population growth, and no population will directly follow the logarithmic curve. Despite this, there are still many similarities between the logistic growth curve, and the growth patterns of the British Columbian sea otters.
Firstly, when initially introduced to the new environment, the sea otters experienced repressed growth. On the logistic curve, this lack of growth (the lag phase) is attributed to the lack of individuals, and therefore the lesser ability to create a great number of offspring. In fact, when the sea otter population was initially introduced, it experienced negative growth. This was not only due to a small initial population size, but also a rapid change in environment for the relocated creatures (Province of British Columbia, 2003).
Next, the population began to increase rapidly, in a section of the log curve known as the log phase. In this phase, a rapid increase in population can be attributed to an adequate population size, and an abundance of resources. This too happened in the sea otter population, as they experienced an average rate of change of over 19% per year from 1977 to 2004. In this phase, the sea otter was reclaiming the territory it once occupied, and had adequate resources to do so.
Currently, according to the log curve, and given the carrying capacity of the sea otter population, they should still be in the log phase of the curve. However the actual rate of growth has slowed to around 8% each year from 1995 to 2004 (Environment Canada, 2014). Although this does not make sense in theory, as the current population of around 4,000 is far less than the carrying capacity (14,000), there is an explanation. As the population expanded, they mostly increased in numbers in their current habitat rather than expanding to reclaim lost territory. As a result, the areas currently occupied by the sea otter have reached equilibrium densities, and as a result the growth has diminished. For the population to continue expanding at a high rate, they must expand to other parts of the coastline, and continue to grow in numbers.
Firstly, when initially introduced to the new environment, the sea otters experienced repressed growth. On the logistic curve, this lack of growth (the lag phase) is attributed to the lack of individuals, and therefore the lesser ability to create a great number of offspring. In fact, when the sea otter population was initially introduced, it experienced negative growth. This was not only due to a small initial population size, but also a rapid change in environment for the relocated creatures (Province of British Columbia, 2003).
Next, the population began to increase rapidly, in a section of the log curve known as the log phase. In this phase, a rapid increase in population can be attributed to an adequate population size, and an abundance of resources. This too happened in the sea otter population, as they experienced an average rate of change of over 19% per year from 1977 to 2004. In this phase, the sea otter was reclaiming the territory it once occupied, and had adequate resources to do so.
Currently, according to the log curve, and given the carrying capacity of the sea otter population, they should still be in the log phase of the curve. However the actual rate of growth has slowed to around 8% each year from 1995 to 2004 (Environment Canada, 2014). Although this does not make sense in theory, as the current population of around 4,000 is far less than the carrying capacity (14,000), there is an explanation. As the population expanded, they mostly increased in numbers in their current habitat rather than expanding to reclaim lost territory. As a result, the areas currently occupied by the sea otter have reached equilibrium densities, and as a result the growth has diminished. For the population to continue expanding at a high rate, they must expand to other parts of the coastline, and continue to grow in numbers.
Five Year Population Projection
Based on the current population, and most recent growth pattern found, one can extrapolate the following growth over the next five years for sea otters off the coast of British Columbia:
Therefore, based on current trends, the most recent population and the logarithmic growth equation, one can estimate the sea otter population will be about 6906 in five years. However there are sources of error within this estimation. For example, the population used in this calculation is taken from 2008, which means that any prediction made, starts in 2008. This means that the predicted population 5 years in the future is really only for 2 years in the future (2013). Also, the growth pattern used was based on observed populations from 1995 to 2005, which may have changed more recently. For these reasons, this is only an estimation, and should be used solely as a rough estimate of future populations in the British Columbia area.
Therefore, based on current trends, the most recent population and the logarithmic growth equation, one can estimate the sea otter population will be about 6906 in five years. However there are sources of error within this estimation. For example, the population used in this calculation is taken from 2008, which means that any prediction made, starts in 2008. This means that the predicted population 5 years in the future is really only for 2 years in the future (2013). Also, the growth pattern used was based on observed populations from 1995 to 2005, which may have changed more recently. For these reasons, this is only an estimation, and should be used solely as a rough estimate of future populations in the British Columbia area.
Population Limiting Factors
The main limiting factor of the sea otter population is the result of interactions with other species, including humans. This includes predators, prey, and competition from other species, parasitic infection and changes in environment due to human interaction (Environment Canada, 2011). Human interaction includes such disturbances such as over hunting, oil spills and accidental death such as fishing nets which can trap and drown sea otters. These interactions will be discussed later, but include relationships with the great white shark, bald eagles, kelp, sea urchins, parasites and of course humans.
Other than special interactions, there are other factors that limit the sea otters population. For example the sea otter has a very low biotic potential. Because the sea otter only lives for around 10-20 years (Environment Canada, 2011), and is only able to produce one offspring every year, the population does not have the ability to grow quickly. To compound this problem, many of the pups who are born will not reach the age of sexual maturity for a variety of reasons including predation and habitat contamination.
Also, the sea otter population is relatively sedentary, meaning that they do not migrate and tend to live in the same general area for long periods of time. For this reason, it is more difficult for the sea otter population to expand to other geographical locations. This limits the population because rather than having the space to exponentially expand the population, they are confined to an area which will likely reach its carrying capacity, and stunt the population growth (Environment Canada, 2011).
Other than special interactions, there are other factors that limit the sea otters population. For example the sea otter has a very low biotic potential. Because the sea otter only lives for around 10-20 years (Environment Canada, 2011), and is only able to produce one offspring every year, the population does not have the ability to grow quickly. To compound this problem, many of the pups who are born will not reach the age of sexual maturity for a variety of reasons including predation and habitat contamination.
Also, the sea otter population is relatively sedentary, meaning that they do not migrate and tend to live in the same general area for long periods of time. For this reason, it is more difficult for the sea otter population to expand to other geographical locations. This limits the population because rather than having the space to exponentially expand the population, they are confined to an area which will likely reach its carrying capacity, and stunt the population growth (Environment Canada, 2011).