Science Advisory Report 2010/091
Stock assessment of Northwest Atlantic grey seals (Halichoerus Grypus)
- Grey seals form a single genetic population that can be divided into three groups for management purposes based on the location of breeding sites. Most pups (81%) are born on Sable Island, 15% are born in the Gulf and 4% are born along the coast of Nova Scotia. This distribution has changed over time, with a decline in the fraction of the population born on the ice compared to on small islands, and an increase in the proportion of animals born on the coast of Nova Scotia, compared to the Gulf.
- Total pup production of Northwest Atlantic grey seals in 2010 was 76,300 (SE=6,500). This includes 62,000 (SE=600) pups born on Sable Island, 3,000 (SE=100) along the coast of Nova Scotia, and 11,300 (SE=6,400) in the Gulf of St. Lawrence.
- Pup production on Sable Island increased at an exponential rate of 12.8% per year between the 1970s and 1997. However, since 1997 surveys indicate that there has been a decline in the rate of increase to about 4% per year between 2007 and 2010. Pup production along the coast of Nova Scotia has increased from a few hundred in the early 1990s to about 3,000 in 2007 and 2010. However, the number of pups born at small breeding colonies in southwest Nova Scotia doubled between 2007 and 2010 and colonies have expanded to adjacent islands. Pup production in the Gulf has been more variable. It has increased from roughly 7,000 animals in 1984 to 11,000 in 1996, but has fluctuated between 6,100 and 15,600 since then.
- On Sable Island there has been an increase in the age at first birth and a reduction in apparent survival of juveniles over the last decade. For example, the proportion of animals aged 5 years old appearing for the first time with a pup has declined from 30% during the period 1985-1989 to 12% during 1998-2002. In the Gulf herd of the population, the mean age at first birth is 5 years old. This has not changed since the late 1960s, nor have changes been observed over time in age-specific pregnancy rates. There are no data on age-specific pregnancy rates for the coast of Nova Scotia.
- Removals from the population during the last five years include animals taken in the commercial harvest, scientific collections, nuisance seals, and incidental catches in commercial fisheries. Estimates of the number of seals killed as nuisance seals are incomplete. There is no data available on incidental catches, but the numbers are thought to be small.
- Pup surveys completed approximately every 4 years combined with estimates of age-specific reproductive rates and removals are incorporated into a Bayesian population model to determine total abundance. The variability associated with model parameters, as well as potential changes in natural mortality rates due to changes in environmental conditions add uncertainty to the Gulf population estimates. Additional uncertainty is associated with the application of reproductive rate data from the Gulf to the Sable Island herd.
- Total estimated population size at the end of the 2010 breeding season (i.e., including pups) was 348,900 (95% CI 291,300-414,900). This is 4% higher than the equivalent estimate for 2009 of 335,200 (95%CI 292,000-395,100) and an order of magnitude higher than the estimate for 1977 of 35,800 (95%CI 24,700-53,100). Estimates of average annual rates of population increase from the model were 6% in the 1980s, 9% in the 1990s, and 6% in the 2000s.
- Outputs from the population dynamics model were used to investigate the consequences of a range of harvest strategies. The management objective was to find harvest levels that have an 80% probability of maintaining the population at or above 70% of the largest estimate to date, i.e., above 244,200.
- The management objective could be achieved with harvests as high as 70,000 and 45,000 animals per year over a 3-year and 5-year period, respectively, given a harvest that was 50% young-of-the-year and 50% older animals, and assuming that mortality was distributed among ages, sexes and regions in proportion to relative abundance in the population. This objective could also be achieved with harvests of 50% young-of-the-year and 50% older animals as high as 30,000 animals per year over a 20-year period.
- Removals of 95% young-of-the-year and 5% older animals could achieve higher total harvest levels, up to 70,000 animals per year, and still meet the management objective over 20 years. This is possible because adult females have such long reproductive lives. However at the end of the 20-year period such high harvest levels would result in a population collapse. Higher quotas could be sustained over shorter time periods, but for a long-lived species such as grey seal, more work is needed to determine if 20 years is a long enough time window to judge long-term sustainability.
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