Environmental authorities have been baffled by a sharp rise in the number of dead turtles washing up on Rottnest Island and believe the increase may be linked to spiralling ocean temperatures off WA.

The Department of Environment and Conservation confirmed an increase in the number of turtles usually associated with tropical waters being found dead at the popular holiday spot.

Although the Rottnest Island Authority suggested about five carcasses - believed to have been green turtles - had been noted in the past 12 months, residents on the island have put the figure upwards of 20.

A DEC spokeswoman said that the agency was unsure what had been causing the deaths but there would not be an investigation because it was considered a natural phenomenon.

However, the spokeswoman said several factors could have played a part including warmer water temperatures and changing ocean currents.

She said that it was known for green turtles to occasionally migrate as far south as Perth but Rottnest was outside their usual habitat, suggesting they may have struggled to find food.

The possible role of warmer ocean temperatures could be the latest symptom of broader changes that have played havoc with some fisheries and may be linked to an increase in shark attacks.

In December, the Department of Fisheries said it was investigating whether a marine heatwave had led to an increase in great white shark activity by pushing cooler water, which they preferred, towards the coast.

"There has been an increase in dead turtles being found on the shore at Rottnest Island in the past five months," the DEC spokes-woman said.

"While the reason for the increase is unclear, there could be a number of factors, including predation, water temperature and ocean currents."

One frequent user of Rottnest's beaches, who asked not to be named, said the number of dead turtles was an increasing topic of discussion among island residents.

She said that locals had known the phenomenon to happen before but never to the same extent.