Fishing communities along Africa's coastline are at a greater risk of extinction as countries eye oceans for tourism, industrial fishing and exploration revenue to jumpstart their "blue economies," UN experts and activists said on Monday.

The continent's 38 coastal and island states have in recent years moved to tap ocean resources through commercial fishing, marine tourism and sea-bed mining, according to the United Nations Economic Commission for Africa (Uneca).

"There is a great risk and a great danger that those communities will be marginalized," said Joseph Zelasney, a fishery officer at UN's Food and Agriculture Organization (FAO).

"The resources that they depend on will be decimated," he added at a side event at the Blue Economy Conference organised by Kenya, Canada and Japan in Nairobi.

The world's poorest continent hosts a blue economy estimated at $1-trillion but loses $42-billion a year to illegal fishing and logging of mangroves along the coast, according to Uneca estimates.

Seismic waves generated by prospectors in the search for minerals, oil and gases along the ocean floor have scared away fish stocks, said Dawda Saine of the Confederation of African Artisanal Fishing in Gambia.

"Noise and vibration drives fishes away, which means they (fishermen) have to go further to fish," Saine said.

Pollution from a vibrant tourism sector and foreign trawlers have reduced stocks along the Indian Ocean, Salim Mohamed, a fisherman from Malindi in Kenya, said.

"We suffer as artisanal fishers but all local regulation just look at us as the polluter and doesn't go beyond that," he said.

The continent's fish stocks are also being depleted by industrial trawlers which comb the oceans to feed European and Asian markets, experts say, posing a threat to livelihoods and food security for communities living along the coast.

Growth of blue economies in Africa could also take away common rights to land and water along the coastline and transfer them to corporations and a few individuals, said Andre Standing, advisor with the Coalition for Fair Fisheries Arrangements.

Most of the land and beaches along Africa's thousands of miles of coastline is untitled, making it a good target for illegal acquisition, activists said.

"There is a great worry that we could see privatisation of areas that were previously open to these communities," Standing told the Thomson Reuters Foundation.

"We need to have a radical vision that values communities and livelihoods or they will become extinct."