Science Advisory Report 2017/045

Stock assessment of Canadian Northwest Atlantic Grey Seals (Halichoerus grypus)

Summary

• Grey seals form a single genetic population that is divided into three groups for management purposes based on the location of breeding sites.
• To estimate the number of pups born in 2016, aerial photographic (fixed-winged, helicopter, drone) and visual surveys were completed for all grey seal breeding colonies in Canada. The previous pup production survey was conducted in 2010.
• Pup production on Sable Island was estimated to be 83,600 with 95% confidence interval (CI) of 63,600 to 103,500, an increase of 4% over the 2010 level. Coastal Nova Scotia comprises two main areas of breeding grey seals. Hay Island pup production at 2,500 (95% CI 1,700 to 3,200) was similar to the 2010 level, while the southwest Nova Scotia colonies (Round, Mud, Noddy and Flat) increased to 2,100 pups (95% CI 1,800 to 2,400) compared to an estimate of 417 pups in 2010. A new colony on Red Island in the Bras D'Or lakes had 41 pups in 2016. Pup production in the Gulf of St. Lawrence (GSL) was 10,500 (95% CI=9,000 to 12,200) not significantly different from the 2010 estimate of 11,300 (95% CI=4,000 to 31,800). Total pup production was estimated to be 101,500 (95% CI=80,600 to 121,600)
• The distribution of pup production has changed over time, with a decline in the fraction of the population born in the GSL and continued increases on Sable Island and in southwest Nova Scotia. In 2016, most pups (85%) are born on Sable Island, 11% are born in the GSL and 4% are born along the coast of Nova Scotia.
• In the GSL, the proportion of pups born on the ice has declined from 100% in 2004, to 1% in 2016 due to a decline in winter ice cover. Seals have responded by pupping on nearby islands.
• A mark-resight analysis of individually marked grey seals sighted on Sable Island between 1978 and 2016 was used to estimate age- and sex-specific adult survival. Average adult survival was high (male=0.943, 95% CI 0.937 to 0.948; female=0.976, 95% CI 0.974 to 0.978). Male grey seals had lower survival rates at all ages than females. There is no evidence of a change in survival of adult males or females over time.
• Juvenile survival rate has declined from 76% in early 1990s to 33% in the mid-2000s.
• In previous assessments, it has been assumed that the sex ratio between males and females was 1:1. However, this assumption is not correct due to the different mortality rates observed for males and females. Assuming a stable age distribution, the male to female sex ratio is 0.69:1.
• Information on reproductive rates has been obtained from biological samples collected in the GSL and the mark-resighting program on Sable Island. Both sources of data suggest a decline in reproductive rates at the younger age classes, but also indicate high and stable reproductive rates for older females.
• Population models, incorporating estimates of age-specific reproductive rates, and removals was fitted to pup production estimates to describe the dynamics of the grey seal population in Canada. All three herds continue to increase. The estimated 2016 total population for Sable and Coastal Nova Scotia herd, taking into account the sex ratio, was 380,300 (95% CI 234,000 to 517,200), and 44,100 (95% CL=29,600 to 61,100) for Gulf of St Lawrence herd. The total estimated Canadian grey seal population is 424,300 (95% CI=263,600 to 578,300, rounded to the nearest 100). This estimate is significantly lower that estimated in 2014, due to changes in the population model.
• The population model was updated to incorporate the new estimates of adult female survival, and input data including removals and pup production estimates. Based on the new sex-specific estimates of adult survival, the estimated total abundance also used the estimated male:female sex ratio of 0.69:1 instead of the 1:1 ratio assumed in previous assessments.
• Removals from the population during the last five years include animals taken in the commercial harvest, for scientific collections, nuisance seals (seals removed for damaging gear and catches), and incidental catches in commercial fisheries. Estimates of the number of seals killed as nuisance seals are incomplete and poorly known. There is no data available on incidental catches, but the numbers are thought to be small.
• Projections from the model were used to investigate the consequences of a range of harvest strategies. In the GSL, harvests of 4,500 and 2,400 animals comprising 95% young of the year (YOY) and 70% YOY, respectively, would respect the current management objective of remaining above N70. For the combined Sable and Coastal Nova Scotia (Scotian Shelf) herds, harvests of 30,000 and 17,000 animals comprising 95% YOY and 70% YOY, respectively, would have an 80% probability of remaining above N70.
• In the GSL, harvests of 6,800 and 4,000 animals comprising 95% YOY and 70% YOY, respectively, would have a 50% probability of remaining above N70. For the Scotian Shelf herd, harvests of 51,000 and 31,000 animals comprising 95% YOY and 70% YOY, respectively, would have a 50% probability of remaining above N70.
• The probability of exceeding N70 and N50 for total allowable catches of 60,000, 70,000 and 90,000, 100,000, 120,000 150,000 and 200,000, assuming catches taken in proportion to pup production, and age composition of 95% YOY and 70% YOY were estimated.
• For a total catch of 60,000, the probabilities of exceeding N70 for Scotian Shelf is 53% (95% YOY) and 83% (70% YOY) and exceeding N50 is 48% (95% YOY) and 80% (70% YOY). The probabilities of exceeding N70 for the Gulf is 46% (95% YOY) and 94% (70% YOY) and exceeding N50 is 30% (95% YOY) and 96% (70% YOY). Higher harvest levels reduce the probabilities of remaining above N70 and N50.
• The number of grey seals foraging throughout Atlantic Canada varies seasonally. Model estimates of the number of seals in the population and estimated locations from satellite telemetry were used to calculate the seasonal abundance of grey seals in the GSL. These estimates were very uncertain, and should only be used for illustration. The results suggest that the population of grey seals foraging in the GSL can more than double between January to March and July to September.

This Science Advisory Report is from the October 17-21, 2016 Results of 2016 Northwest Atlantic grey seal pup production survey and sustainable harvest advice. 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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