Science Advisory Report 2007/056
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 Eastern Shore 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 Eastern Shore, compared to the Gulf.
- Using aerial surveys, total production of Northwest Atlantic grey seals in 2007 was 67,500 (Standard Error (SE)=1,400). This includes 54,500 (SE=1,300) pups born on Sable Island, 3,000 (SE=40) along the eastern shore of Nova Scotia, and 9,900 (SE=600) 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 then surveys indicate that there has been a decline in the rate of increase to about 7% per year. Pup production in the eastern Shore component has also increased from a few hundred in the early 1990s to over 3,000 in 2007. 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 a significant increase in the age at first birth over the last decade. 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 non-Sable Island component 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.
- The reproductive data used in the population model were obtained from animals shot in the Gulf of St. Lawrence. The presence of mid- to late-term fetuses is used to estimate age-specific birth rates. These data will slightly overestimate actual birth rates as some pregnancies will not be carried to term.
- Removals from the population during the last five years include animals taken in the commercial harvest, scientific collections, nuisance seal removals and incidental catches in commercial fisheries. No information is available on incidental catches, and data on number of seals killed as nuisance seals are limited.
- Pup surveys completed approximately every 4 years combined with estimates of age-specific reproductive rates and removals are incorporated into a population model to determine total abundance. The variability associated with model parameters, as well as potential changes in natural mortality rates due to environmental conditions add uncertainty to the non-Sable Island population estimates. Additional uncertainty is associated with the application of reproductive rate data from the Gulf to the Sable Island component of the population.
- With the completion of the 2007 survey, there are now three surveys in the last 15 years. This along with the reproductive data means that the grey seal assessment should now be considered as Data Rich.
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