Thirty-seven Iberian killer whales: what the latest demographic study says, and why adult females are a concern

Iberian killer whales are known above all for their interactions with the rudders of sailing boats, and more recently for the shooting of their oldest female. A study published in October 2025 in Marine Mammal Science casts a quite different light, less spectacular and more decisive: the demographic record of this population over thirteen years.

Painstaking work

Ruth Esteban, Pauline Gauffier, Alfredo López and their colleagues have reassessed the parameters of the subpopulation between 2011 and 2023, from two sources: 18,554 identification photographs and 26 strandings.

The method rests on capture-recapture. Each killer whale is recognisable by its dorsal fin, its grey saddle patch and its scars. Photographing the same individuals year after year makes it possible to estimate how many animals there are, including those that have not been seen, and above all to estimate survival rates: an animal regularly photographed and then absent for several years in a row is probably dead.

What capture-recapture makes possible

The method comes from population ecology and can be grasped through an image. If one photographs a hundred animals in one year, then a hundred the following year, and eighty of them were already in the first set, the population is probably close to that hundred. If only ten recur, it is far larger.

In a population of a few dozen individuals monitored over years, this principle becomes very powerful: almost all the animals are known, and the question is no longer how many there are, but which ones are still there.

This is what makes it possible to estimate survival rates by age class and sex, the interval between two births, and the age at first reproduction. These demographic parameters are far more informative than a total headcount, because they indicate the direction the population will take before the number itself moves. Our page on cetacean demographics sets out these indicators in detail.

The 26 strandings in the dataset play a complementary role: they confirm certain deaths, allow causes of death to be determined and provide biological samples that photography does not give.

The figures

Population in 2023	37 individuals
Annual growth	0.46%, population apparently stable
Interval between two births	8.3 years
Survival of young animals	rising compared with the period before 2011
Survival of adults	falling, particularly among females
Birth season	mainly summer and autumn

These killer whales also show a morphological peculiarity: they are smaller than the average for the species worldwide.

Why “stable” is not reassuring

Growth of 0.46% a year, for a population of thirty-seven individuals, amounts to one additional animal roughly every six years. Formally, the population is not declining. In practice, it has no margin.

The point that should hold the attention lies elsewhere, in the composition of that stability. The survival of young animals has improved, which is good news and probably the result of more favourable feeding conditions. But adult survival is falling, and more so among females.

Yet it is the survival of adult females that governs the future of a slow-breeding population. With an interval of 8.3 years between two births, a female that disappears at forty takes several potential births with her. And in killer whales, whose society is matrilineal, she also takes knowledge with her: it is the older females that lead the groups to resources.

A dependence that makes everything worse

The IUCN classified this subpopulation as Critically Endangered in 2019, on two criteria. The first is the population size, well below fifty mature individuals. The second is dietary: these killer whales depend closely on Atlantic bluefin tuna, which they follow into the Strait of Gibraltar in spring.

This specialisation makes them reliant on a resource that is itself exploited. The comparison with the northeast Pacific blue whale, a krill specialist put in difficulty when the krill collapses, is instructive: a specialised diet is a strength as long as the prey is abundant, a vulnerability as soon as it is lacking.

A society in small groups

Thirty-seven individuals do not form a single group. They are distributed among a few stable subgroups, organised around related females, which do not necessarily travel or hunt together.

This structure has a demographic consequence that the total conceals: the loss of an older female does not affect the population uniformly, it can compromise an entire subgroup. In killer whales, hunting techniques and spatial knowledge are transmitted socially, and a group deprived of its matriarch loses knowledge that is not rebuilt within a generation. Our page on social cohesion describes these mechanisms.

This is what makes the shooting of an older female, as in the case of the oldest female found riddled with shot, disproportionate in relation to the unit lost.

The interactions with sailing boats, put back in proportion

Since 2020, Iberian killer whales have been pushing and damaging the rudder blades of sailing boats in the Strait of Gibraltar and along the Atlantic coast. The phenomenon has been widely covered in the media, often in terms of attack or revenge.

What can be said from the available data is more modest. The behaviour is concentrated in a small number of individuals and has spread socially, like other fashions observed in this species. No human has been injured. No motivation can be established: play, curiosity, learning, a reaction to a negative experience, all these readings remain hypotheses.

Above all, this behaviour is not the main threat to the population. The study by Esteban et al. cites fishing activities and these interactions as emerging threats, without making them the driver of the demography. The real risk associated with interactions with boats is indirect: it lies in the hostility they arouse among some users, in a population where the loss of a few females would be enough to change the trajectory.

What the study points to for the years ahead

The authors point to two emerging threats: fishing activities and the recent interactions with recreational boats. The second is today the more widely reported, but the study does not present it as the main one, and nothing indicates that it affects the demography.

Their conclusion is sober and unambiguous: the future of this subpopulation depends on recruitment, on juvenile survival and on the effective reproduction of females. In other words, on what is currently deteriorating. Our pages on cetacean demographics and IUCN statuses place these indicators in context.

Source: Esteban R., Gauffier P., López A., Gil-Vera F. J., Selling J., Haristoy L. et al., “Demographic Parameters of Iberian Killer Whales Between 2011 and 2023”, Marine Mammal Science, 22 October 2025.