He said 47 seals were in the area that day, but the dead one was "isolated out in its own place'".
When he edited the photograph, taken with a telephoto lens, the following morning, he "just kind of jumped", seeing for the first time the seal was "clearly decapitated".
He contacted Doc and the Otago Daily Times hoping someone would come forward with more information, which would lead to a conviction for the person who did it.
"I think a lot of people get very upset when this happens to seals, or any kind of wildlife, but I think it's pretty disgusting myself."
Doc Dunedin operations manager Annie Wallace said the decapitation "looks suspiciously like human intervention" and she was "absolutely'" interested in anyone with information contacting the department.
"We're disgusted by it. These are special marine mammals that we are lucky to have living on our coast, we need to learn to live together with them.'"
The dead seal was found "around a reasonably inaccessible place" about 50m east of the DoC viewing area at Shag Pt, on a rocky outcrop about 7m above the waterline.
"And the seal couldn't have washed there after decapitation. It looks like a place that you could access, but it's fairly well hidden away," she said.
The department was talking to contacts in the Shag Pt and Moeraki areas.
If someone killed the seal, they could be charged under the Marine Mammals Protection Act.
A 53-year-old man faces charges after an 11-month-old female sea lion named Rua was found dead near Quarantine Pt, north of the Portobello Marine Aquarium, on November 6, 2016.
