Rare footage showing an attack on a porpoise by a pod of dolphins has been filmed in the Moray Firth.

The incident occurred a short distance off Chanonry Point, near Inverness, on May 8.

John Logan waited three hours to capture the moment when the dolphins attacked, swarming around the porpoise and tossing it into the air.

At least four such incidents have been reported around Chanonry Point in recent weeks, although the reason for the attacks remains a mystery.

"The dolphins don't eat the porpoise, they just bash it about," Mr Logan said.

"They don't like each other.

"Dolphins are often seen in the area but I've never seen anything like that."

Marine expert Mariel ten Doeschate said around 40 porpoises wash up on Scotland's coastline each year and around half show evidence of dolphin attacks.

"We don't see the attacks that often but we find porpoise remains with evidence of them," she said.

"We see about 40 porpoises a year and about half show evidence of attacks.

"When we first started seeing animals showing up with these injuries everybody thought they were man-made."

Scientists eventually realised the patterns of the wounds suffered by the porpoises matched the teeth of bottlenose dolphins. However, the reasons behind the attacks are less clear.

"Some people think it's because of competition for resources," Ms ten Doeschate, from the Scottish Marine Animal Stranding Scheme (SMASS), explained.

"But they don't feed on the exact same fish so others think it's practice because dolphins kill each other's calves.

"It could well just be rough play because when the porpoises die or stop responding the dolphins tend to lose interest."

"Dolphins don't eat porpoises so it's not predation," she added.

SMASS scientists plan to carry out an autopsy on the corpse of a porpoise found near Chanonry Point shortly after last week's attack was filmed.