At least 62 people, including children, drown while 83 others swam ashore after the overcrowded boat capsized.
Fears of a serious escalation increased after the arrest of the commander of the presidential guard and his deputy, both loyalists of former President Abdul Aziz.
Experts warn barren pastures in the two West African states threaten livestock herding, a key pillar of food security.
Mkhaitir was initially sentenced to death for comments about Prophet Muhammad and the role of Islam in society.
Government candidate Mohamed Ould Ghazouani wins 51.9 percent with nearly all votes counted, provisional results show.
Some 1.5 million people eligible to vote in elections amid opposition warnings of potential vote fraud.
Prime elezioni dall'indipendenza. Favorito ministro Difesa
Voters set to choose among six candidates vying to succeed President Mohamed Ould Abdel Aziz.
Musicians of local instrument, the ardine, rely on the internet to spread their cultural music.
Second round of voting will be held on September 15 with a record 98 parties taking part in the elections.