Recovery potential assessment for the St. Lawrence Estuary Beluga ‪(Delphinapterus leucas)‬ population

Abstract

The St. Lawrence Estuary (SLE) beluga (Delphinapterus leucas) population occurs at the southernmost limit of the species’ distribution, and is a relict from the Wisconsin glaciation. While the historical abundance of this population is highly uncertain, it was most likely above 10,000 individuals. However, the effective carrying capacity for SLE beluga has fundamentally changed from historical levels, and is estimated under current conditions at 6,700 beluga (95% credible interval, CI95 = 4,300 – 10,400). Abundance in 2022 was estimated to be most likely (95% CI) between 1,500 and 2,200 individuals, with a point estimate of 1850 beluga, putting the population in the Cautious Zone according to the DFO Precautionary Framework. Average temperatures in the Gulf of St. Lawrence over the period 2010 – 2022 have increased by three quarters of a degree Celsius relative to the average for 1970 – 2009. Assuming an additional warming of Gulf temperatures by 0.5 Celsius over the next 100 years, and continuation of natural variation without changes (positive or negative) in other mortality factors, projections indicate a low (0.3%) risk of quasi-extinction (i.e., of being reduced to ≤ 50 individuals), a 13% probability for the population reaching the Healthy Zone (i.e., at least 3,219 beluga) over the next 100 years, and a 61% probability of falling into the Critical Zone (i.e., below 1,609 beluga). These results suggest that recovery is theoretically feasible, and is even more so if some of the current threats to the population, such as factors leading to increased pregnancy-related and calf mortality, can be mitigated. Allowable harm from human-related stressors is estimated at 3.4 beluga per year using the Potential Biological Removal approach. Both historical and contemporary data indicate that SLE beluga distribution is the most constrained during summer, and the most extensive in the spring, when a portion of the population is still present in the northwestern Gulf of St. Lawrence, but that the core of the distribution remains within the SLE year-round. Data was sufficient to identify important habitat for both summer (June-October) and winter (Jan-March) for the population; data was insufficient for spring and autumn. Functions could not be attributed to specific locations within the seasonal distribution range, and for most features, there are also insufficient data to support quantitative attributes.