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."

IS militants launched at least four suicide attacks, with reports saying at least 25 people were killed.
The first of the attacks was near the Turkish border crossing. It is thought to be the first fighting in that area.
The battle for Kobane has left hundreds of people dead and forced more than 200,000 to flee into Turkey.
The US-led coalition is supporting the town's defenders with air strikes.
Turkey has allowed some Kurdish fighters from Iraq to travel through its territory to assist in defending Kobane.
However, Turkey views Kurdish forces with suspicion, following a decade-long battle for autonomy by its own Kurdish minority.
IS controls large swathes of both Syria and neighbouring Iraq.
The militants have been trying to overrun Kobane for the past two months.
In the early stages, it looked as though the town's fall was only a matter of time. But the Kurdish fighters have defended tenaciously and kept hold of roughly half the town.
They have been supported by a small number of well-armed Iraqi Kurdish fighters, some other Syrian opposition units, and of course by the coalition air strikes.
The first suicide vehicle bomb targeted a Kurdish position close to the Turkish border at dawn on Saturday.
Kurdish sources said the bomber had crossed from Turkey, but Turkish officials denied this as "lies".
Clashes followed at the site of the attack and to the south-west of the town.
The UK-based activist group, the Syrian Observatory for Human Rights, said there were three more suicide bombings - one an individual with a suicide vest and the other two vehicle bombs.
As the clashes intensified, there were reports that IS had brought up tanks to join the fighting.
Meanwhile two US-led air strikes were carried out against IS positions in the east of Kobane.
In all, 25 people were killed in the clashes, most of them IS fighters, reports said.
IS has captured parts of the town and dozens of villages in the area around it in an offensive lasting more than two months.
But it has met stiff resistance from local Kurdish fighters, who have held about half the town assisted by small numbers of Iraqi Kurds and Syrian Arabs, and backed by US-led air strikes.
A whale and dolphin-watcher has photographed an attack by a bottlenose dolphin on a porpoise off Aberdeen.
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Fighting has intensified in the besieged Syrian border town of Kobane, where Kurdish forces have been holding Islamic State at bay since September.