Science Advisory Report 2023/024
Abundance and population trajectory of St. Lawrence beluga
- Abundance for the St. Lawrence Estuary (SLE) beluga population has been estimated using systematic aerial surveys covering their entire summer range since 1990. The current assessment is based on 11 photographic surveys flown over 8 summers between 1990 and 2019, and 52 visual surveys flown over 14 summers between 2001 and 2022.
- Previous assessments of beluga abundance and population trends did not include the visual survey estimates. Updated correction factors for animals missed because they were diving (availability bias), or present at the surface but undetected by observers (perception bias) now allow for the use of both visual and photographic surveys.
- With these updated correction factors, revised abundance estimates from photographic surveys are now 1.3 to 1.5 times higher than previous estimates. Revised visual survey estimates are now 1.2 to 2.8 times higher than previous estimates, and generally more variable than photographic surveys.
- These revised aerial survey estimates along with various other long-term data sources were integrated in a population model to estimate the abundance and trajectory of the SLE beluga population. This model accounts for what is known about SLE beluga, and the level of uncertainty about the dynamics of the population.
- Model-estimated trends were consistent with the previous assessment (2012) from 1980-2007. After 2007, the inferred trends diverge between assessments, with the new model showing an increasing trend instead of a decrease. The observed reduction in the incidence of cancers may have progressively increased survival of older adults, and contributed to the population increase from 2010-2018.
- Although there is always greater uncertainty near the end of a time series, the model results indicate population trends have levelled off between 2018 and 2022, likely reflecting poor survival of calves and pregnant females since 2010, and a possible downturn in survival of other adults over the last few years.
- The model estimated abundance of 1,850 SLE beluga (95% CI: 1,500 – 2,200) in 2022 corresponds to a Potential Biological Removal (PBR) of 3.4 individuals per year.
- The maximum population size that could be sustained in the SLE without anthropogenic sources of mortality is estimated at 6,700 beluga (95% CI: 4,300 – 10,400). This carrying capacity was used to estimate reference levels.
- The mean abundance estimate for 2022 falls below the Precautionary Reference Level (PRL = 3,219) and above the Limit Reference Level (LRL = 1,609), which is the Cautious Zone according to the Precautionary Framework.
- Assuming continuation of recent (2010 – 2022) environmental conditions and natural variation over the next 100 years, there is an 78% probability that the population will remain below the PRL, and a 41% probability it will fall into the Critical Zone, i.e., below the LRL.
- The probability of quasi-extinction (at or below 50 individuals) over 100 years is estimated at 0.06%.
- Proposed recovery targets over the next 28 years (one generation) are to (1) achieve or exceed an average annual growth rate of 1%, (2) reduce calf mortality and pregnancy-associated female mortality by 25%, and (3) reduce annual anthropogenic mortality below PBR. Longer term goals (over 100 years) for the population would be to increase the number of mature individuals to more than 2,500, and eventually exceed the PRL.
- The factors causing elevated mortality of calves and pregnant females are important drivers in our estimates of abundance and population trajectory, and result in large uncertainty in predictions. The proportion of animals not seen during the survey (due to availability or perception bias) has a large impact on the abundance estimate and contributes to uncertainty about current trends.
This Science Advisory Report is from the February 20-24, 2023 national peer review meeting on Population size and trends of St. Lawrence Estuary beluga in 2022. 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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