Science Advisory Report 2014/010
Stock Assessment of Canadian Grey Seals (Halichoerus Grypus)
- Canadian Grey seals form a single genetic population that is divided into three groups for management purposes based on the location of breeding sites. Most pups (81%) are born on Sable Island (Sable), while 15% are born in the Gulf of St Lawrence (Gulf) and 4% are born along the coast of Nova Scotia (CNS). These proportions have changed over time, with a decline in the fraction of the population born in the Gulf.
- A population model incorporating estimates of reproductive rates up to 2012 was fitted to pup production estimates up to 2010 to describe the dynamics of the grey seal population in Atlantic Canada. Combining all three herds, the model estimated a total 2014 grey seal pup production in Atlantic Canada of 93,000 (95% CI=48,000-137,000) animals, with an associated total population of 505,000 (95% CI=329,000-682,000). The model predicts that population size in all three management areas continues to grow.
- The current estimate of population size is higher than what was presented in the last assessment. The higher estimate of population size results from differences in the reproductive rates applied between the two assessments, as well as relaxing the limits on environmental carrying capacity in the current assessment which allowed the modelled population to increase more rapidly. Although, there is evidence of a reduction in juvenile survival on Sable Island, there is not sufficient information in the time series of pup production to estimate the carrying capacity. This is reflected in higher estimates of total population.
- Removals from the population during the last five years include animals taken in the commercial harvest, for scientific collections and as nuisance seals (seals removed for damaging gear and catches). Estimates of the number of seals killed as nuisance seals are poorly known. There are no data available on incidental catches, but the numbers are thought to be small.
- Projections from the population dynamics model were used to investigate the consequences of a range of harvest strategies. To maintain an 80% probability of staying above N70 and assuming that young of the year (YOY) comprise 95% of the catch, then harvests of 39,200 animals (Sable:33,000; CNS:1,200; Gulf:5,000) could be taken. If YOY comprise 90% of the catch then the maximum total harvest would be 36,600 (Sable=31,000; CNS: 1,100; Gulf: 4,500). If YOY comprise 70% of the catch, then the total harvest would be 28,200 (Sable: 24,000; CNS: 700; Gulf: 3,500). Higher harvest levels would result in lower probabilities of remaining above N70.
- If the management objective is to maintain an 80% probability of staying above N50 and assuming that YOY comprise 95% of the catch, then harvests of 43,400 animals (Sable:36,000; CNS:1,400; Gulf:6,000) could be taken. If YOY comprise 90% of the catches then the maximum total harvest would be 40,700 (Sable=34,000; CNS: 1,200; Gulf: 5,500). If YOY comprise 70% of the catch, then the maximum total harvest would be 30,900 (Sable: 26,000; CNS: 900; Gulf: 4,000).
- Advice was request to evaluate the risk of falling below N70 and N50 for harvests of 50,000, 60,000, 70,000 and 90,000, 100,000 seals per year, assuming age compositions of 95%, 90% and 70% young of the year. Assuming that harvests are proportional to pup production in each herd (0.04 CNS, 0.19 Gulf, 0.77 Sable), the risk of falling below N70 and N50 for a total harvest of 50,000 with 95% YOY, are 0.76 and 0.73 for CNS, 0.56 and 0.52 for the Gulf, and 0.25 and 0.22 for Sable. For a total harvest of 50,000 with 70% YOY, the probabilities of falling below N70 and N50 are 0.86 and 0.86 for CNS, 0.73 and 0.71 for the Gulf, and 0.36 and 0.34 for Sable. The risk of falling below N70 and N50 increases with increased harvest levels.
- A new pup production estimate is needed to reduce uncertainty in the estimate of grey seal population size.
- Several factors should be monitored to determine if a multiyear TAC should be re-evaluated. In general, significant changes in any of the major assumptions used in the projections should trigger a new analysis; the most important being annual reproductive rates. Significant changes in the age structure of the harvest or mortality should also result in a re-analysis.
This Science Advisory Report is from the October 7-11 2013 Annual Meeting of the National Marine Mammal Peer Review Committee (NMMPRC). 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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