Dead fish spotted on Bermuda’s beaches and floating close to shore are probably not a cause for concern, according to an ocean expert.

The rotting carcasses have been seen this week on the sands at Shelly Bay and floating in St George’s and Hamilton harbours, and the Bermuda Government said it is the worst fish die-off around the island in eight years.

Chris Flook, from Bermuda Institute of Ocean Sciences, said the phenomenon happened every year and was caused either by an algae bloom or by fish being unable to breathe because of a change in temperature in the water — known as a thermocline — caused by several calm hot days of weather.

He said: “This usually dissipates in a matter of hours and is a completely natural phenomenon. Without being right there to test the water around the dead fish, it’s impossible to know what exactly was the reason.”

A Department of Environment and Natural Resources spokeswoman said: “We would like to thank those members of the public who have taken the time to contact us.

“We record all reports of this nature, regardless of how minor they may appear, as it helps us to track the timing and distribution of any large scale die-off events, which in turn can help us determine the cause.”

She said warmer temperatures promote the growth and reproduction of naturally occurring bacteria, viruses and other parasites that can affect fish health. These fish pathogens are normally present at low levels, and are generally not pathogenic to humans.

“The current die-off event is affecting a large number of fishes from a wide range of species,” she said.

“The last time there was a fish die-off on this scale was during September of 2009. At that time, the bacterium Vibrio harveyi and a gill parasite, Brooklynella, were found to be the cause. Neither of these organisms is pathogenic to humans. However, at this stage, it is too early to speculate about possible causes behind the current die-off.”

The Department said dead fish on private shorelines should be collected carefully using rubber gloves, placed in a sturdy plastic bag and taken directly to the incinerator at Tynes Bay.

The department urged people to contact them when they spot affected fish, particularly any that are still alive but have visible signs of ill health.

Key signs include sores or lesions on the skin, frayed fins or unusual behaviour. Suspect behaviours include swimming slowly, upside down or near the surface, rapid movement of the gill covers, or appearing to gasp at the surface of the water.

Anyone who is able to provide such a sample should call the marine resources section on 293-5600.