Divers, meanwhile, recovered dozens of additional bodies from the wreckage, raising the death toll to 275.
Just 155 migrants, most if not all from Eritrea, survived Thursday's shipwreck. Survivors said there were some 500 would-be asylum-seekers aboard when the ship capsized.
Tens of thousands of migrants from Africa and the Middle East try to cross the Mediterranean Sea each year, seeking a better life in Europe, but the journey is fraught with danger.
Thursday's sinking has one of the highest verified death tolls of migrant ship disasters in the Mediterranean.
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Prosecutors in Agrigento, Sicily, said they had detained the Tunisian suspect, who was transferred from Lampedusa to Argrigento under police custody aboard a ferry. He faces charges of aiding illegal immigration and multiple counts of homicide.
Coast Guard Commander Filippo Marini said 43 bodies were recovered from within the ship's hold, while one was spotted by a helicopter floating near the wreck, before the operation was suspended for darkness.
A disproportionate number of the dead are women: So far the bodies of 81 women have been recovered, while only six of the survivors were female. Eight of the dead are children.
He said the scene inside the ship's hold is tough for divers, but that "maintaining cold blood is a quality that helps the operation."
The survivors were helping identify the bodies, mostly through photographs. In some cases, divers also have recovered documents.