Center of Expertise in Marine Mammology: Scientific Research Report 2012-2014

St. Lawrence Estuary Beluga

Véronique Lesage

The St. Lawrence Estuary (SLE) beluga are listed as threatened under the Species at Risk Act; a Recovery Strategy is defined, and their summer Critical Habitat has been identified. A review of the SLE beluga status (2007) concluded that the population was stable over the period 1988-2006. However, in recent years there has been an increase in reports of death of young-of-the-year, and an apparent increase in adult female perinatal mortalities, suggesting that the status of SLE beluga may have changed.

In the fall of 2013, a thorough review of the available scientific information was conducted at the annual national marine mammal peer-review committee meeting to provide an update of the status of the SLE beluga population, in particular information on current population size and trends, an analysis of factors that are affecting the population trend, including recruitment levels, sources of mortality, environmental conditions, contaminant loads, and their potential impact on mortality and fecundity. This review, led by DFO, was the result of concerted efforts from over 20 scientists, approximately two-thirds of whom were from DFO.

Beluga adult and calf (photo: DFO).

Beluga adult and calf (photo: DFO).

This review confirmed that the SLE beluga population was stable or increasing at a slow rate until the beginning of the 2000s, and also appeared stable in terms of calf production, pregnancy rate, age structure, and adult mortality. During this period, environmental conditions such as sea ice and water temperature were favourable to an Arctic species, although several of the groundfish stocks, including known prey of beluga such as Atlantic cod, collapsed in the early 1990s. The lower-than-expected population growth observed during the 1990s suggests that food, environmental conditions or other factors were limiting population growth. Other limiting factors may include high contaminant loads, negative effects from marine traffic, or high mortality rate associated with occasional events such as toxic algal blooms. The relative importance of all these factors in limiting population growth is not known.

A model informed by various data sources suggests that the SLE beluga population began to decrease in the early 2000s. This decline was concomitant to an increased, and more variable newborn calf mortality with cascading effects on the reproductive pattern of females, and to an observed and predicted decrease in the number of younger age classes in the population. During this period, females having lost their calf became available for reproduction earlier than the normal 3-year cycle, resulting in years with over half of the mature females being pregnant at the same time. These peaks were usually followed by peaks of high newborn mortality. Based on the analysis of some ecosystem indices in the Gulf of St. Lawrence, this change in population dynamics coincided with a period of warming temperatures, decline in ice cover, negative spring herring and large demersal prey abundance indices, and change in indices of beluga diet. This period also followed one characterized by an exponential increase in some chemical substances such as polybrominated diethyl ethers (PBDEs) in beluga and their environment, and when these substances were at their maxima.

Beluga whale (photo: DFO).

Beluga whale (photo: DFO).

The decrease in population size documented in recent aerial surveys appears to be a lagged response resulting from lower recruitment propagating through the population. Although the events that initially triggered this perturbation are not fully understood, the problem seems to have worsened in the last six years, following a harmful algal bloom of Alexandrium tamarense in the SLE beluga summer range, which probably played a major role in the high mortalities observed in 2008, on its own or in conjunction with other unfavourable environmental and anthropogenic factors. The 2010 and 2012 anomalies in the numbers of newborn calves washing ashore cannot be explained by higher calf production alone. These anomalies resulted from a combination of an increased calf production, and reduced calf survival, likely related partly to unfavorable environmental conditions for the species, although the mechanisms leading to these additional mortalities in newborn calves are not understood.

This review also suggests that SLE beluga live in an environment less favorable to an Arctic species since the late 1990s, also characterized by chronic exposure to potentially aggravating stressors such as marine traffic, persistent organic pollutants, and occasional toxic algal blooms.

Climate variability resulting, among other things, in increases in water temperature and associated declines in ice-cover may further affect this beluga population, for instance through changes in food resources and increases in inter-specific competition as other species expand their range due to loss of ice cover. In the short term, efforts can be directed towards reducing anthropogenic stressors such as disturbance in sensitive areas and critical periods for females and calves, chemical contamination, nutrient enrichment, habitat loss, and competition for food resources from fisheries. This underlines the importance of maintaining a critical population to withstand periodic downturns, in particular by addressing anthropological factors that could delay recovery.

Environmental shifts resulting in poor foraging conditions have been observed in other ecosystems and have resulted in several years of little or no population growth, for example, North Atlantic Right Whales in response to low copepod abundance; northern resident killer whales in the Pacific in response to declines in salmon abundance. However, observations from other systems also show that populations of long-lived mammals do respond to favourable conditions when they occur.

DFO continues to monitor the SLE beluga population dynamics, and trends in habitat quality, and contributes by its innovative research to further our understanding of the ecological needs of beluga, and the mechanisms of action and relative importance of current environmental stressors and to implement actions that are the most appropriate to help the recovery of this population.

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