Fifty thousand humpback whales off the east coast of Australia

Every austral winter, humpback whales travel up the east coast of Australia towards their breeding grounds. For 2026, the estimates put forward place the passage at more than 50,000 animals, a figure presented as higher than the population that existed before industrial whaling. The success is real. The comparison deserves a closer look.

The starting point

The east Australian humpback whale population was reduced to a few hundred animals by the middle of the twentieth century. Commercial whaling on this population ceased in the early 1960s, after taking tens of thousands of animals in a few decades. Our historical whaling statistics give the scale of those catches.

The rebound that followed is one of the best documented recoveries in a large marine mammal, with sustained growth over several decades. It is a solid result, and it comes down to a simple cause: the killing stopped.

How a migration is counted

The figure announced is not obtained by adding up animals seen. It rests on land-based counts, made from fixed stations on headlands that the migration passes, during defined time slots and over part of the season.

These counts are then corrected in three ways. First for temporal coverage, since the animals also pass at night and outside observation hours. Then for spatial coverage, because part of the flow passes too far offshore to be seen from land. Finally for detection probability, which depends on sea state and visibility.

Each of these corrections is a multiplicative factor, and the uncertainty propagates. A total announced at 50,000 should be read as an order of magnitude carrying a margin of several thousand, not as a head count.

Why comparing with the pre-whaling level is delicate

The difficulty is that nobody counted the whales before whaling. Historical population sizes are reconstructions, obtained by two main routes.

The first works back in time from the catch records, modelling the dynamics of the population to recover the number of animals that accounts for the recorded catches. It depends on the quality of the archives, which are often incomplete, and sometimes distorted, in particular by the falsified Soviet returns of the twentieth century.

The second rests on present-day genetic diversity, which retains the trace of past population sizes. The two methods regularly give diverging results, sometimes by a large factor.

Saying that a population today exceeds its pre-whaling level therefore amounts to comparing a modern count with a reconstructed estimate. The claim may well be correct; it does not have the same standing as a measurement.

What is certain, and why that is enough

Whether the exact figure is 40,000 or 50,000, and whether or not it exceeds an uncertain historical level, the main fact does not change: this population has gone from a few hundred animals to several tens of thousands in sixty years. It is one of the rare cases where a conservation decision produces an effect visible within a human lifetime.

That is worth recalling at a time when other populations, such as the Iberian killer whales or the North Atlantic right whale, are still counted in tens or in hundreds.

Growth that cannot continue indefinitely

A population growing by several per cent a year doubles in about twenty years. That regime does not hold: as numbers approach what the environment can feed, growth slows, then stops. Ecologists speak of density dependence.

The mechanism holds no mystery. When more whales use the same Antarctic feeding grounds, each finds slightly less krill, puts on slightly less fat, and breeds slightly less often. The population does not collapse, it levels off.

This levelling off is expected and normal. It does raise a formidable problem of interpretation for the years ahead: if growth slows, how is a natural ceiling to be told apart from the beginning of a deterioration. The two produce the same signal at first, and only observation of the animals’ body condition separates them. Our pages on demographics and diets set out these indicators.

The question is not theoretical. Antarctic krill, the basis of the diet of these populations, depends on sea ice, which is shrinking. A large population dependent on a single resource is vulnerable to variation in that resource, however successful its recovery has been. That is the point made by our page on climate change.

Three things that do not come back with the numbers

Recovering a number is not the same as recovering an earlier state, and three things do not return with the animals.

Genetic diversity first. A population that has passed through a bottleneck of a few hundred animals loses variants that reappear only through mutation, over very long timescales. A large population grown from a small core remains genetically poorer than the original population was, with the usual consequences for adaptive capacity.

Transmitted knowledge next. Humpback whales learn their routes and their feeding grounds socially. An area deserted for several generations can stay that way for a long time, not because it has become inhospitable, but because nobody knows any longer that it exists. Some areas used before whaling are still not reoccupied.

The ecological role last. Large cetaceans transport nutrients, at depth as at the surface, and between latitudes, which fertilises surface waters and supports plankton production. That function, reduced by a considerable factor by industrial whaling, recovers only as numbers rise again, and never completely while other populations remain remnants.

This is why the Australian recovery, real as it is, does not generalise. Other populations of the same species, isolated and small in number, show no comparable rebound. Our pages on conservation status and marine protected areas give that contrast.

A success that raises its own questions

A population this large brings new problems: more entanglements in coastal fishing gear, the risk of collision with dense shipping traffic, and tourist pressure on the watching areas. Our pages on watching hotspots and demographics deal with these effects of success.

Sources: estimates of the numbers passing the east Australian coast for the 2026 season, relayed by the Australian press; published work on the recovery of the east Australian humpback whale population. The pre-whaling population sizes cited in these comparisons are reconstructions and not counts.