Science Advisory Report  2016/018

Population Reduction Scenarios for Northwest Atlantic Harp Seals (Pagophilus groenlandicus)

Summary

• Northwest Atlantic harp seals are harvested in Canadian and Greenland waters. Reported Canadian catches have declined from a peak of 355,000 in 2006 to 35,000 animals in 2015. Greenland catches have fluctuated, between 66,100 and 92,200 animals since 2003. Catches in the Canadian Arctic remain low (<1,000). Additional removals include bycatch, as well as estimates of animals killed, but not recovered (struck and loss). Total removals have been less than 250,000 animals each year since 2009.
• Females age 8 years and older (8+) account for approximately 70% of the pup production. Pregnancy rates for these ages were high throughout the 1960s, but have declined since the mid‑1990s. Annual pregnancy rates have varied considerably, particularly since the early 2000s.
• The population assessment model used as input data, the time series of pup production estimates, including the 2012 estimate, as well as reproductive rates, estimates of ice-related mortality, and harvest information to 2013. The model provides an estimated 2012 pup production of 929,000 (Standard Error [SE]=148,000) and a total population of 7,445,000 (SE=698,000).
• The Atlantic Seal Management Strategy identifies three reference levels based upon the maximum observed population, which is referred to as N_max. The first reference level is a Precautionary Reference Level called N₇₀ that is set at 70% of N_max. The Limit Reference Point, known as N₃₀, is set at 30% of N_max. Based upon the current model, the maximum population is estimated to be 7.8 million, the precautionary reference level is 5.5 million, and the Limit Reference Point is 2.3 million animals.
• The impacts of the different Canadian catch options on the projected population were tested under two scenarios. The first scenario (Model A) assumed that reproductive rates and Greenland catches were similar to that seen over the past 10 years. The second scenario, referred to as Model B, assumed that both reproductive rates increase and Greenland catches decrease when the population declines (i.e., density-dependent compensation).
• Under both scenarios the number of animals that need to be removed annually is strongly affected by the age composition of the harvest, and the speed in which the reduction was achieved. Generally, higher numbers of seals must be removed annually if the reduction is to occur quicker, or if more young-of-the-year (YOY) are included.
• The annual catches required to reduce the population to 6.8 million were similar under the two scenarios. However, the annual numbers required to reduce the population to 5.4 million were much higher when density dependence was assumed (Model B). A reduction to 5.4 million assuming annual catches of 90% YOY over 5 years is not possible under either scenario.
• Under most scenarios, after the population had been reduced, annual harvests had to be reduced considerably to permit the population to remain above the Limit Reference Point.
• These simulation results are very sensitive to model assumptions and should be considered for illustration only. The two scenarios represent two unlikely situations, one assuming reproductive rates and catches do not respond to changes in total population while the other assumes full compensation in reproductive rates and catches as the population declines. Based upon historical changes in reproductive rates, we expect that some density dependent compensation will occur, but recent environmental changes suggest that full compensation may not result. The estimated carrying capacity is based upon historical conditions and may no longer be the same. Therefore, the results presented here are only valid within the context of the modelling scenarios examined in this study.

This Science Advisory Report is from the October 20‑23, 2015 Annual Meeting of the National Marine Mammal Peer Review Committee (NMMPRC) held in Halifax, Nova Scotia. 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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