St Lawrence belugas: twice as numerous as was thought, and no healthier

It happens that a figure doubles without anything changing in the reality it describes. The population of belugas of the St Lawrence estuary is now estimated at about 1,850 individuals, close to double the figure previously accepted. This is not good demographic news, and the explanation of the paradox is more instructive than the number itself.

Why the estimate changed

The belugas of the estuary have been surveyed from the air for decades. Photographs are taken along flight lines, and the visible animals are then counted.

The difficulty lies in what is not seen. A beluga spends part of its time diving, and appears on the photograph only if it is at the surface as the aircraft passes. A correction is therefore needed for the proportion of animals available at the moment of the overflight, a correction that rests on assumptions about diving behaviour.

It is this correction that has been revised, drawing on finer data on time spent at the surface. The same surveys, reanalysed, give a higher figure. The 2022 population was not 900 animals that became 1,850: it was already 1,850, and we were underestimating it.

Revision of this kind is frequent and healthy, but it imposes a rule: an old figure cannot be compared with a new one if the method has changed in between. Our pages on cetacean demographics and on distribution in French waters return often to this trap.

The figures that did not change

Revised estimate, 2022	about 1,850 individuals
Population in the early 20th century	7,800 to 10,100 individuals
Hunting	banned since 1979
Trend	stable or in slight decline
Warning sign	high newborn mortality over the past ten years or so

The contrast between the first two lines is the real subject. Even at 1,850, the population amounts to about a fifth of what it was a century ago, and it has not rebuilt itself since hunting stopped, forty-five years on.

A population isolated since the last glaciation

These belugas are not stray visitors. They form a resident and isolated population, and that history explains part of its fragility.

The beluga is an Arctic species. The St Lawrence animals are its southernmost population in the world, separated by several thousand kilometres from the nearest Arctic groups. It is a remnant of the end of the last glaciation, when the Champlain Sea filled the valley of the river and cold waters reached much further south.

When the climate warmed and the sea withdrew, some of the belugas stayed in the estuary instead of following the retreating ice. Genetic analyses confirm this isolation: the population is distinct from the Arctic groups and exchanges almost no individuals with them.

Two consequences follow. First, what is lost here is not replaced: no immigration will offset excess mortality, unlike what happens in a connected population. Second, genetic diversity is reduced, which limits the capacity to adapt.

It is the same mechanism of isolation as in the Mediterranean sperm whale or the killer whales of the Strait of Gibraltar: an enclosed population, with no outside input, in which every loss is final. Our page on conservation status explains why assessment is carried out by population and not by species.

The signal that gives concern

It is not the population size, it is newborn mortality, high and persistent for about ten years. Carcasses of very young animals, and of females that died giving birth, are found every year.

This signal means the same thing as in the grey whale or the right whale: a population can decline without any visible die-off, simply because renewal has stopped. And it acts with a delay, because the young animals missing today are the breeders that will be missing in fifteen years.

Projections indicate that the population could decline over the coming decade, despite an estimate revised upwards. The two findings do not contradict each other: one bears on the level, the other on the slope.

Three cumulative pressures

The St Lawrence belugas live in an industrialised and busy estuary, and three pressures are documented.

Contaminants. This population is one of the most studied in the world from this point of view, and analyses have revealed high concentrations of persistent organic pollutants and of metals, as well as an unusual frequency of tumours. A female passes a large part of her burden to her first calf through milk, as our pages on chemical pollution and on lactation explain.

Noise. The estuary is a major shipping lane, and the beluga is an extremely vocal animal, whose nickname of sea canary is well earned. Mothers and their calves communicate constantly, and the masking of those exchanges in a noisy environment is a serious hypothesis for part of the juvenile mortality. Our page on ocean noise sets out this mechanism.

Prey. The availability of certain fish species has changed in the estuary, and a female in poor condition feeds her calf badly.

The limits

Three reservations are in order.

1. The revision remains an estimate. The figure of 1,850 depends on assumptions about diving behaviour. A further methodological improvement could shift it again.

2. The time series is weakened. Changing method makes it necessary to reanalyse the whole historical record in order to keep a comparable trend, heavy work and never entirely satisfactory.

3. No single cause is demonstrated. Newborn mortality is observed, its causes remain multiple and entangled, and no analysis allows a precise share to be attributed to each of them.

What this case teaches

It illustrates a common confusion: a figure that rises is not necessarily a population that rises. Here, the revision reflects an advance in measurement and nothing else.

It is also a reminder that stopping one pressure, hunting in this instance, is not enough when others have taken over. Our introductory video on the beluga and our page on conservation status place this population within the species as a whole, whose situation is far more favourable in the Arctic.

Sources: reanalysis of the aerial surveys of the beluga population of the St Lawrence estuary, with the estimate raised to about 1,850 individuals for 2022; Canadian work on newborn mortality and contaminants.