Walter Innes and fellow Shorewatch volunteer Monika Carrie were about to head home after an eventful day "staring at an empty sea" on Tuesday.
They then spotted what looked like a dolphin feeding frenzy and Mr Innes took a picture, believing it to be of a dolphin taking a large salmon.
Expert Charlie Phillips said such attacks were not completely understood.
Mr Innes took his picture at 18:04 on Tuesday just outside the south breakwater at Aberdeen Harbour.
He said: "Monika Carrie and I had been staring at an empty sea for a while and were just about to leave.
"We saw what we thought at first was a feeding frenzy, and went down on the rocks to try and get pictures. I thought it was a very large salmon, until I got home and saw it on the computer screen.
"You hear and read about this behaviour, but to see it on the doorstep is stunning."
Mr Phillips, a Highlands-based Whale and Dolphin Conservation field officer who monitors the Moray Firth's resident population of bottlenose dolphins, said: "It's not completely understood why some bottlenose dolphins attack harbour porpoise like this.
"The example in Walter's great pictures show that it's normally small porpoise, often calves, that seem to be targeted.
"One day we might have a better understanding of this behaviour but until then we can only make guesses or speculation."
Mr Phillips added: "It does happen frequently, as the pathologists at the whale and dolphin strandings department of Scotland's Rural College have carcasses of porpoise showing clear signs of bottlenose attack to examine, and they are massive injuries that have been sustained."
A whale and dolphin-watcher has photographed an attack by a bottlenose dolphin on a porpoise off Aberdeen.