Security forces believe Eta has killed more than 850 people in its campaign for independence for the Basque Country in northwestern Spain.
But in recent years the movement has been severely weakened by hundreds of arrests.
The increase in the number of arrests is said to be due to improved co-ordination between authorities in Spain and France, which Eta has long used as a hideout.
'Setback' for Eta
Bill Bond, a journalist in Madrid and expert on Eta, said the latest arrest is an obvious blow to Eta.
|Gogeascoechea was captured in a joint operation between France and Spain [AFP]
"But ... there is always someone waiting to step in, and it usually doesn't take long to do that. At the same time, it is very difficult for them to keep having to do that," he told Al Jazeera.
"[Police] have arrested somebody every other day for the last 60 days. The war is not yet over.
"But every time this happens it is a setback and it means the Eta people that are left are concentrating on reforming and hiding again and not on bombing people.
"That is one thing for which the authorities are very grateful."
A leading member of Batasuna, the group's political wing, last week called on Eta to continue its struggle by peaceful means.
But Batasuna has lost some of its influence over Eta since the collapse of peace talks with the Spanish government following a bomb attack on Madrid airport that killed two people in 2006.
Eta's last fatal attack was the killing of two police officers on the island of Majorca in July 2009.