The culling of killer whales more than two decades ago has had a lasting effect on their population in the Southern Ocean, according to research from Deakin University.
The longitudinal study has revealed the killer whale population in subantarctic waters near the Crozet Islands remains less than half what it was when illegal fishing started in 1996.
In the paper published this week, the research says the loss of family members in the whales' highly-organised social groups had a catastrophic long-term impact on the remaining population.
Deakin marine scientist Paul Tixier said killer whales are animals who bond in highly social family groupings.
"Once numbers were depleted, some of the whales tried to associate with whales in other groups, potentially to maintain the effectiveness of group hunting, which is a key feeding strategy for killer whales when capturing large prey," Dr Tixier said.
"But these new bonds were weak, sporadic and resulted in a lower survival for those whales remaining in the groups most impacted by the culling events in the 1990s."
The research team studied the behaviour of 221 killer whales between 1987 and 2014.