Science Advisory Report 2016/007
Estimates of abundance and total allowable removals for Atlantic walrus (Odobenus rosmarus rosmarus) in Foxe Basin
- Walrus are a challenging species to enumerate owing to their aggregated distribution and correlated haulout behavior resulting in highly variable proportion of animals hauled out at one time.
- Several methods have been used to estimate walrus abundance. All methods that estimate the number of animals on land or ice require a correction factor to adjust the counts for animals that are in the water (not visible) when the surveys are completed.
- Based on a simulation study, the Simple Count (SC) method, using mean counts provides a reliable and unbiased estimate of the number of animals hauled out.
- Abundance information from surveys completed in 1983, 1988, and 1989 provided useful information on trends in walrus abundance in Foxe Basin.
- Abundance information from surveys flown in 2010 and 2011 were also re-examined using the Simple Count method. This resulted in an estimated population of 10,400 (SE=4,500) in 2010 and 14,100 (SE=6,700) in 2011.
- A surplus production population model that included reported harvest information was fitted to the Foxe Basin times series of abundance estimates, using Bayesian methods. One of the strengths of the Bayesian approach is that it provides a framework to account for uncertainty in model inputs.
- Model results showed that the stock has likely remained stable over the last 60+ years.
- The estimated population from the model in 1954 was 11,900 (95% Bayesian Credible Intervals (BCI) =10,200-17,900) and in 2014 was 12,500 (95% BCI=8,600-18,500). The reported harvests do not appear to be having an impact on the population as it appears to be stable. However, our ability to detect a decline is limited owing to the infrequency of surveys.
- Total allowable removals of walrus in Foxe Basin have been estimated using the Potential Biological Removal (PBR) method. In previous assessments, a maximum rate of increase (Rmax) = 0.07 has been used. This review concluded that an Rmax of 0.08 is more appropriate.
- In previous assessments, a recovery factor (FR) of 0.5 has been used. In this assessment, advice using FR=1 was also provided.
- A PBR estimate based on the estimated 2014 population size from the model was 211 or 422 using recovery factors of 0.5 or 1.0, respectively.
- The Foxe Basin stock was last surveyed in 2011. Model uncertainty increases with increasing time since the last survey, which affects our ability to evaluate the impact of harvesting on the population.
- The PBR calculation includes removals from all sources of human induced mortality. Current average reported harvests from this area are 170 animals per year, implying a total removal of 243, if a Struck and Lost value of 30% is assumed.
- A Precautionary Approach framework with clear management objectives is needed. This would allow for more risk-based advice to be provided. In the meantime, updating the model with new survey data and using model outputs to provide PBR estimates uses all of the scientific information in providing management advice.
This Science Advisory Report is from the October 20-23, 2015 Annual Meeting of the National Marine Mammal Peer Review Committee (NMMPRC). Additional publications from this meeting will be posted on the DFO Science Advisory Schedule as they become available.
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