LAKE OSSA, Cameroon—A Cameroonian lake choked with invasive weeds is a science project for biologists, but for Charles Elingua, it means starvation. “I have been fishing in this lake for more than 30 years,” Elinga, 56, and leader of fishermen in Lake Ossa, said. “The salvinia weed has disrupted fishing considerably.  I once was able…