Science Advisory Report  2012/067

Limit Reference Points and Minimum Populations of Harp Seals (Pagophilus groenlandicus)

Summary

• The Atlantic Seal Management Strategy provides a framework that identifies precautionary and limit reference levels which define healthy, cautious and critical zones of abundance, along with management actions that are triggered when these levels are crossed in order to reduce serious harm to the resource.
• Currently, the precautionary and critical reference levels are defined as 70% and 30% of the maximum observed or inferred population size.
• As the true size of most wild populations is unknown, our perception of the population size will change as new data become available and/or the methods used to estimate abundance improve. This uncertainty must be acknowledged when setting precautionary reference levels by using proportions rather than fixed numbers, as proportions maintain the relative conservation levels in the face of changes to our perception of the population abundance.
• The lowest population level observed should not be used as limit reference level. Recovery from low levels under past environmental conditions does not guarantee recovery will occur under current conditions.
• The Limit Reference Level (LRL) should be set as a proportion of carrying capacity (K) or some proxy for K. If an estimate of K can be obtained, the LRL can be expressed as a proportion of the number of seals required for a maximum sustainable yield (N_MSY) which would be consistent with DFO guidelines and international approaches.
• Because of uncertainties in our ability to estimate K and the shape of the density dependent relationship, we recommend that if N_MSY is used, the LRL be set at 50% N_MSY.
• If K or N_MSY cannot be estimated, the maximum population observed or inferred can be used as a proxy for K. However, with few exceptions, this will be lower than K and therefore less conservative. Given that 50% N_MSY is estimated to be at least 30% of K, care should be taken when setting a reference level based upon N_max. The current LRL (N₃₀) is at the low end of the possible values.
• The framework proposed here is appropriate for other marine mammal populations which are considered to be data rich.
• Precautionary reference levels should remain the same between periodic reviews. For harp seals, levels should remain constant between major assessments which occur every 4 to 5 years.
• Using the 2011 assessment model that assumed a carrying capacity of 12 million seals, a maximum population of 8.3 million and a N_lim of 2.5 million, the minimum population required to support a sustainable harvest varying from 100,000 to 400,000 was estimated under different levels of risk. Generally, a larger minimum population size was needed to support a larger harvest; a larger population size was also needed to have a higher probability (i.e. lower risk level) of respecting the management objective.
• For example, a population of approximately 5.3 million seals was estimated to be necessary to sustain an annual catch of 100,000 with a 95% likelihood of remaining above the limit reference point for a 15 year period if reproductive rates do not change while a population of 4.7 million is required if we assume that reproductive rates will increase as the population declines.

This Science Advisory Report is from the October 29 to November 2 “Annual meeting of the National Marine Mammal Peer Review Committee (NMMPRC)”. Additional publications from this process will be posted as they become available on the Fisheries and Oceans Canada Science Advisory Schedule.

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