"Scores of Boko Haram fighters blocked a route linking Nigeria with Chad near the fishing village of Doron Baga on the shores of Lake Chad on Thursday and killed a group of 48 fish traders on their way to Chad to buy fish," Abubakar Gamandi, head of the fish traders association, told AFP.
Gamandi said that after setting up a barricade at Dogon Fili, 15 kilometres (nine miles) from Doron Baga in Borno state, the attackers stopped a convoy of fish vendors around midday, silently slaughtering some and drowning others in the lake.
"The Boko Haram gunmen slit the throats of some of the men and tied the hands and legs of the others before throwing them into the lake to drown," Gamandi told AFP by telephone from Maiduguri, the Borno state capital.
It was unclear if the motive for the gruesome attack was robbery or if there were other reasons for the killings. Boko Haram has at times targeted residents seemingly indiscriminately in its deadly insurgency.
Doron Baga, 180 kilometres from Maiduguri, is the base of the Multi-National Joint Task Force (MNJTF), comprising troops from Nigeria, Chad and Niger fighting the Islamist group.
The MNJTF was formed in 1998 to fight trans-border crime but its mandate has been expanded as part of efforts to tame the Boko Haram insurgency in the region.
Gamandi claimed the assailants killed all of their victims without using their guns.
"The attackers killed their victims silently without the use of the gun to avoid attracting attention from the multi-national troops," he said.
News of the attack was slow to emerge due to the destruction of mobile phone towers in the area by Boko Haram in previous attacks.
A military officer in Maiduguri confirmed it but said details were sketchy.
"We heard of the attack near Doron Baga but we don't have any details because the area falls under the operational jurisdiction of the MNJTF," the military officer said.
In nearby Niger, visiting French Prime Minister Manuel Walls condemned "a new barbaric act" by what he called "a terrorist organisation".
Incessant Boko Haram attacks have disrupted fishing and farming along the shores of Lake Chad. Fishermen from Doron Baga have been forced to abandon fishing and have turned to importing dried fish from neighbouring Chad.
Gamandi said the Dogon Fili route provided the safest passage for traders from Doron Baga to Chad as other routes are infested with Boko Haram gunmen who rob and kill travellers.
Last December at least seven fishermen were killed when Boko Haram Islamists attacked Doron Baga in a nighttime raid that left many homes burnt.
In August, the Islamists raided Dogon Baga and kidnapped 97 people after killing 28 villagers.
The hostages, including women and children, were loaded on speed boats and ferried across the lake into Chad.
Chadian troops rescued 85 of the hostages when they intercepted a convoy of buses transporting them from the shores.
More than 13,000 people have been killed since the insurgency began in 2009 and Boko Haram is now said to be in control of more than two dozen towns in Nigeria's northeast in its quest for a hardline Islamic state.
The Islamists have made major gains over the past 18 months and violence has continued at a relentless pace in three northeastern states that had long been under a state of emergency.
The emergency measures expired this week and President Goodluck Jonathan, who is running for re-election in February, has yet to get a parliamentary approval for an extension.
In Lagos, Nigeria's main opposition party claimed Sunday that security agents had ransacked its office in Lagos, arresting workers and seizing documents, in the latest flareup ahead of February elections.