OCN said last week that thousands of dead seal pups had been found at Pelican Point peninsula, a tourist destination known for its colony of seals and school of dolphins.
Naude Dreyer, a marine biologist at OCN, said investigations in the past week had revealed a spike in the number of dead female adults.
He said investigations had mainly centred on the Pelican Point Beach, so the overall number of dead seals could be higher.
The Ministry of Fisheries and Marine Resources said last week it would send samples to South Africa to determine the cause of death. Dreyer said this had not yet been done because of the need to observe strict guidelines on shipping dangerous animal products.
The ministry was not immediately available for comment.
OCN said last week starvation was the most probable cause as fish, the seals’ main source of food, have moved from Pelican Point, but other reasons could be toxins or disease and nothing could be ruled out without a full study.