The otter has been spotted several times in Cardiff Bay

Nicknamed Ollie by staff at Cardiff Harbour Authority, the otter is believed to come from the River Ely.

But it has been attracted to the bay by the growing course fish population which live in the waters there.

Ollie has been seen close to the International Sports Village site in Ferry Road several times this summer.

A spokesman for the harbour authority said: "They like cleaner water and an abundant supply of easy-to-catch coarse fish all year round, so the bay seems ideal."

Otters are known to be the biggest indicator of improving water conditions.

Dyfrig Jones, a conservation officer with the Wildlife Trust of South and West Wales, is an expert on the mammal.

"It does not surprise me at all that an otter has been spotted in the bay," he said.

The waters in Cardiff Bay are highly populated with fish

"There is plenty of fish there for it to feed on including salmon and sewin.

"We have known for a couple of years that otters have been making more use of town centres - they are regularly seen in Haverfordwest in west Wales and in Port Talbot.

"The population of otters is growing remarkably and they are in water courses all over Wales.

"It is all because of an improvement in water quality, the habitat and the fish population," he said.

Mr Jones said that Ollie could become a bit of a tourist attraction but visitors would not be guaranteed a glimpse of the creature.

"Otters have very large territories, often 20 to 30 miles and they move around quite a bit," he said.

Cardiff Bay has become a popular attraction for visitors to the city

"This otter is likely to be passing through the bay quite regularly because it is very productive for fish," he added.

But Ollie is not the only water mammal to have enjoyed the catch at Cardiff Bay.

In 2002, a seal named Sammy made several appearances in the waters gorging on the fish there.