Stopping the killing of elephants for their tusks could add some $25 million (23 million euros) to Africa's annual tourism income, more than offsetting the anti-poaching spend, a study said Tuesday.

While the figure pales in comparison to the estimated value of the black market ivory trade in China, it represents about a fifth of tourist income for game parks in 14 countries, where half of Africa's elephants are located, the study said.

"We find that the lost economic benefits that elephants could deliver to African countries via tourism are substantial, and that these benefits exceed the costs necessary to halt elephant declines in east, southern and west Africa," the authors wrote in the journal Nature Communications.

The conservation of elephants, they concluded, "is a wise investment decision for countries in the savannah regions of Africa".

The estimated value of the ivory trade is almost $600 million per year, illustrating the "economic difficulties of elephant conservation," the authors conceded.

Africa's elephant population has been reduced by about 30 percent overall in just the period 2007 to 2014, the study said.

The beasts' tusks are used for ivory carvings in parts of Asia where ownership of such sculptures denote wealth and success.

The research team used available data on tourist behaviour and elephant density for the study, which they claimed was the first to quantify "lost economic benefits" from poaching.

They found that tourists are more likely to visit parks with many elephants, and calculated that each extra animal boosted visits by 371 percent.

However, for forested central Africa where elephants are harder to spot and their numbers are less linked to tourist revenue, anti-poaching spending would not offset conservation spending to the same extent.