Science Advisory Report  2014/011

Status of Northwest Atlantic harp seals, Pagophilus groenlandicus

Summary

• Northwest Atlantic harp seals are harvested in Canadian and Greenland waters. Canadian commercial catches have declined from a peak reported catch of 355,000 in 2006 to 91,000 in 2013. 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 annual removals have been less than 250,000 animals 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 particularly since the early 2000s.
• Harp seals require stable ice as a platform for resting, pupping and rearing their young. The annual extent of ice cover in Atlantic Canada has varied considerably over the last 44 years, particularly in the Gulf of St Lawrence (Gulf), where overall total ice cover has declined. An estimate of mortality of young of the year caused by poor ice conditions has been incorporated into the assessment. If the frequency of winters with poor ice cover observed over the past decade continues or increases, continued breeding in the southern Gulf may be markedly reduced or cease.
• Photographic and visual aerial surveys flown in the Gulf, Strait of Belle Isle, and off Newfoundland (Front)  during 27 February – 16 March, 2012 resulted in pup production  estimates of 115,500 (SE=15,000), 74,100 (SE=12,400) and 601,400 (SE=66,900), respectively. Combining these estimates resulted in an estimated total pup production of 790,000 (SE=69,700, CV=8.8%).This estimate is significantly lower than that estimated in 2008, in part due to lower reproductive rates.
• 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 estimated 2012 pup production to be 929,000 (SE=148,000) and a total population of 7,445,000 (SE=698,000). Projecting the model to 2014 results in an estimated pup production of 853,000 (SE=202,000) and a total population size of 7,411,000 (SE=656,000). The population appears to be relatively stable, showing little change in abundance since 2004.
• 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 level called N₇₀ that is set at 70 % of N_max. A secondary precautionary reference level, known as N₅₀, is set at 50 % of N_max, and a critical limit reference level, known as N₃₀, is set at 30 % of N_max. Based upon the current model, N_max was estimated to be 7.8 million, N₇₀=5.5 million, N₅₀=3.9 million and N₃₀=2.3 million animals.
• Assuming that the Greenland harvest, reproductive rates (average=0.34), and ice-related mortality remain within the range of values observed over the last 5 years, harvests of 125,000, 100,000, and 75,000 animals consisting of 95 %, 90 % and 70 % Young-of-the-Year (YOY), respectively, would result in an 80% likelihood of remaining above N₇₀. Harvests of 250,000, 225,000 and 150,000 seals consisting of 95 %, 90 % and 70 % YOY, respectively is estimated to have an 80 % likelihood of remaining above N₅₀.
• Assuming a harvest of 95 % young of the year and reproductive rates similar to those seen over the past 5 years, the probabilities that the population would decline below N₇₀, with catches of 300,000, 400,000, 500,000 and 600,000 would be 0.68, 0.91, 0.98 and 1, respectively. The probability for similar catches to result in the population declining below N₅₀ would be 0.29, 0.6, 0.85, and 0.96 respectively. The probability of the population declining below these thresholds would be greater if the proportion of young in the catch declines to 90 % or 70 %.
• Assuming that the Greenland harvest, reproductive rates (average=0.48) and ice-related mortality remained within the range of values observed over the last 10 years, then a harvest of 325,000, 275,000, and 175,000 animals would have an 80 % probability of the population remaining above N₇₀ for an age composition of the harvest of 95 %, 90% and 70 % YOY respectively. Harvests of 450,000, 375,000 and 225,000 animals would have an 80 % probability of respecting N₅₀ assuming age compositions of the harvest of 95 %, 90 % and 70% YOY.
• Given reproductive rates observed over the past 10 years and a catch of 95 % YOY, the probabilities that catches of 300,000, 400,000, 500,000 and 600,000 would result in the population declining below N₇₀ were 0.16, 0.35, 0.59 and 0.80 respectively. The probabilities that the population would decline below N₅₀ were 0.04, 0.11, 0.28, and 0.49. Harvests consisting of lower proportion of YOY would have lower probabilities of maintaining a population above the respective reference levels.
• Assuming a similar approach to incorporating uncertainty in the population estimates, harvest levels set based on N₅₀ increased the risk of falling into the critical zone when compared to using the current approach of using N₇₀. Overall, it is likely that the risk of falling below the precautionary thresholds (N₇₀ and N₅₀) and, subsequently, the critical limit (N₃₀) is underestimated. For example, if future reproductive rates are more similar to rates that have been observed over the last 5 years, a harvest level based upon an assumption of 10 year average rates would result in an overharvest by 40-50 %.
• The status of the harp seal population is assessed every 4-5 years coincident with a new estimate of pup production. Since the harvest targets YOY, and females are not fully recruited until they are 8-10 years old, a decline in pup production, resulting from very high exploitation rates or unusual mortality will not be detected for a minimum of 10-15 years. Setting a harvest level based upon N₅₀ will result in a higher risk that the population will fall into the critical zone before such declines can be detected.
• Reproductive rates of females have been declining since the 1980s in response to increased population size and an increase in the occurrence of late term abortions. The abortion rate appears to be influenced by prey availability and ice conditions, either as a direct influence on pupping habitat or as a proxy for other ecosystem changes. These ecosystem changes are predicted to continue. Therefore, it is likely that reproductive rates will remain low.
• 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, particularly Greenland harvest or pup mortality associated with poor ice conditions, should also result in a re-analysis.

This Science Advisory Report is from the 7-11 October, 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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