Science Advisory Report 2011/050
Current Status of Northwest Atlantic Harp Seals, (Pagophilus groenlandicus)
Summary
- Northwest Atlantic harp seals are harvested in Canadian and Greenland waters. After averaging approximately 52,000 seals per year between 1983 and 1995, reported Canadian catches increased significantly to a range of 226,000 to 366,000 between 1996 and 2006. Catches have been significantly reduced since 2007 with a reported catch of 69,101 in 2010. Greenland catches have increased steadily since the mid 1970’s reaching a peak of approximately 100,000 in 2000. Since then they have fluctuated, averaging around 85,000. Catches in the Canadian Arctic remain low (<1,000).
- This assessment relies on pup surveys completed once every four - five years combined with estimates of annual reproductive rates and removals to determine total abundance using a population model.
- Total removals of harp seals were estimated using reported catches, estimates of bycatch, primarily in the Newfoundland lumpfish fishery, and estimates of seals killed but not recovered (referred to as ‘struck and lost’) during harvesting in the different regions. From 1996 to 2004, high catches in Canada and Greenland resulted in average annual removals of 465,500. However total removals have declined to an annual average of 310,300 since 2005, primarily due to the lower catches in the Canadian commercial hunt.
- Annual pregnancy rates have been estimated since the 1950s. Pregnancy rates among 4 year olds are low and without trend while pregnancy rates of 5 and 6 year olds increased during the 1970s to a high of 50% and 90%, respectively and then declined to 30% and 50% respectively by the mid 1980s. Since then they have continued to decline. Pregnancy rates of seals 7 years of age and older remained high until the mid 1980s when they declined to approximately 60%. Since then, rates have fluctuated greatly from a low of 40% in 2004 to a high of 74% in 2008. The overall general decline and marked variability in reproductive rates suggests that density-dependent factors are influencing the dynamics of this population.
- A visual survey of the largest whelping concentration off Newfoundland flown on 10 March 2008 produced an estimate of 589,400 (SE=49,500) pups while a photographic survey of the same concentration on 16 March obtained an estimate of 1,161,600 (SE=112,300). The result from a second photographic survey of this concentration carried out on 12 March (1,026,997, SE=280,445) was similar to that obtained from the 16 March photographic survey suggesting that the 10 March visual survey was an under-estimate.
- Combining the estimates from the two photographic surveys at the Front (1,142,985, SE=104,284) with estimates of pup production in the southern Gulf (287,033, SE=27,561), the northern Gulf (172,482, SE=22,287) and another small group at the Front (23,381, SE = 5,492), resulted in a 2008 total pup production estimate of 1,630,300 (SE=110,400, CV=6.8%).
- Assuming exponential population growth, a population model, was fitted to the survey data and smoothed reproductive rates, resulting in a total population estimate of 8.0 million (95% CI 6.77 to 9.26 million) animals in 2008. However, the model fit to the recent pup survey data was very poor owing to large fluctuations in annual reproductive rates among mature females which likely produced sharp changes in pup production in 2004 and in 2008.
- A model assuming density-dependent population growth, environmental carrying capacity of 12 million and annual reproductive rate data was fitted to the survey data. This approach provided a better fit to the pup survey data resulting in a total population estimate of 8.11 million (95% CI 7.34 to 8.89 million) in 2008. If a higher environmental carrying capacity of 16 million animals is assumed, then the estimated population in 2008 would be 8.73 million (95% CI=7.82 to 9.83 million).
- The future trend in the population is difficult to predict because of uncertainty associated with reproductive rates and the model formulation used to describe the dynamics of this population. Depending on the model approach and reproductive data used, the 2010 population would lie between 8.61- 9.55 million (95% CI 7.80 to 10.80 million) animals.
- Science was requested to examine a variety of harvest scenarios to determine their impacts on the population. Annual harvests of up to 400,000 animals would continue to respect the management plan over the next three years assuming that ice related mortality of young of the year is on average about 30% above normal. Higher levels might be acceptable, but are sensitive to assumptions concerning population growth. Very poor ice conditions are expected for 2011, which may result in higher mortality, but under such conditions catches are expected to remain low.
- The current population is at its highest level seen in the 60 year time series. However, uncertainty associated with how the current population is changing and the variability in reproductive rates as well as uncertainty in harvest levels in Greenland, complicates efforts to model future trends in this resource. It is recommended that the frequency of the pup production surveys be increased.
This Science Advisory Report has resulted from a Fisheries and Oceans Canada, Canadian Science Advisory Secretariat, National Advisory Meeting of November 22-26th, 2010 on National Marine Mammal Peer Review Committee Meeting (NMMPRC). Additional publications from this process will be posted as they become available on the DFO Science Advisory Schedule.
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