Giant fish among tropical marine visitors to Bay of Islands wreck

The Queensland grouper at Deep Water Cove was about a metre long. Photo/ Ben Brodie

A fish rarely seen in New Zealand has taken temporary command on the bridge of the sunken former navy ship, Canterbury, in the Bay of Islands.

Still a juvenile, the visitor - from a species usually found in tropical Pacific rim and Indian Ocean waters - is almost a metre long. When it grows up, this Queensland grouper could be 3m or more long and weigh 400kg.

Paihia Dive skipper Ben Brodie took some great photos when he came across the new arrival to the Deep Water Cove wreck-reef site at the weekend.

His boss, Paihia Dive owner Craig Johnson, headed out to try his luck on Monday but the fish proved elusive.

''They like caves and dark places, so we're hoping it hangs around,'' Johnson said.

Also known as the bumble bee grouper and giant grouper [spelled and pronounced groper in New Zealand], these are one of the world's largest deep reef fish species.

Although Johnson has seen them in tropical seas, he has never seen one in 20 years diving in Northland and knows of only a handful of recorded sightings here.

The sergeant major damsel fish was less than 20cm. Photo / Craig Johnson

While there were often isolated instances of rare visitors, this year the popular dive site and nearby environs have been quite the drawcard for tropical species, Johnson said.

The grouper joins a line-up that includes a cruisey green turtle, a potentially deadly frogfish (closely related to stonefish), a gold, black and white sergeant major damsel fish and a devilishly sharp-toothed Lord Howe moray eel.

Divers and fish watchers have been excited and delighted with this array of fish life.

It has also led to much talk about warmer than usual water temperatures, climate change and, not least, the inviting artificial reef and no-take zone at Deep Water Cove.

Marine expert Wade Doak said he has been following the arrival of new species to New Zealand for decades. Some stay for a while after arriving on the warm, rich currents flowing down to New Zealand, some don't survive for long.

"Quite a few of them are one-offs, they're strays. Others come in pulses, short bursts of new arrivals. Queensland grouper have been arriving here sporadically for a number of years."

Doak said he had never heard of a juvenile Queensland grouper in local waters before, but some years ago a huge, older specimen hung around the Poor Knights Islands area known as South Harbour.

Local divers watched the giant fish with interest, hoping it would not move away from the strictly protected marine reserve. But it did, and soon after the watchers heard a spear fisher had shot it.

Due to that incident, marine biologist Dr Roger Grace successfully worked to get the species protected in New Zealand waters, Doak said.

"It's not as if they're likely to become established. I just hope that people realise this one is a special visitor and leave it alone," he said of the Deep Water Cove fish.

He and his late diving mate Kelly Tarlton once swam among a group of them at the Three Kings Islands, 55km northwest of Cape Reinga.

Doak described it as like "swimming in the middle of a herd of cows".

They were calm and at least one was the size of and shape of a Volkswagen car - so big there was no reason for them to be scared of humans, Doak said.