By PHILIP ENGLISH
A survey of 25 harbours in the upper North Island has turned up populations of a newly arrived fish, the bridled goby, a native of southern Australian estuarine waters.
Individual specimens were first found in the Tamaki River upstream of the Waitemata Harbour in 1996, but the 25-harbour survey by the National Institute of Water and Atmospheric Research last summer found bigger numbers of the fish in more places.
Another rare find was tiny snapper 12mm to 70mm long - just out of their planktonic larval stage - settling on seagrass beds in several harbours.
Their association with seagrass is classed as important because seagrass is in decline.
Dr Trevor Willis, an Auckland University marine lab researcher at Leigh, was the first to study the goby, which has since been found in the Upper Waitemata, the Waiwera Estuary, at Leigh and the Whangarei Harbour and in the Matapouri River north of Tutukaka.
He suspects the fish arrived in ballast water discharged by ships arriving in the Hauraki Gulf.
"We can't see them being transported as larvae across the Tasman naturally. I won't rule it out completely, but I think it is highly unlikely."
The bridled goby was too new for its potential effect on native marine life to be assessed.
"It is possible that they might out-compete other species for space. We know they seem to be using mud crab burrows in soft mud," said Dr Willis.
"We don't know whether they are just occupying empty burrows or evicting the crabs or what they are doing.
"That's the beginnings of a study over the next two years."
Dr Mark Morrison, of Niwa in Auckland, said the aim of the 25-harbour survey was to produce classification schemes to rank harbours' importance as habitats or nurseries for fish.
"We are concentrating on fish that are pre-reproductive. We call them juveniles - but anything before they reach the adult population when they can breed.
"We also found things like trevally, gurnard and kahawai - all sorts of species that are commercially important. We found juvenile gurnard down to 2cm long. They're quite cute."
Dr Morrison said the survey was part of Niwa estuarine and coastal habitat research that included tracking snapper acoustically with sonar tags and recording different predators of fish in different parts of the Manukau Harbour.
The work would identify estuaries or fish nursery habitats most vulnerable to damage from human activities.
Northland diver and marine conservationist Wade Doak said his wife Jan had photographed another Australian goby species, the exquisite goby, in the Matapouri recently.
Not only had the two gobies turned up, but a new worm was causing havoc for the Coromandel Peninsula scallop fishery and had spread to the Poor Knights.
Mr Doak said a tiny mussel about the size of a grain of rice was another new invader.
"We see plenty in the news, with good reason, about these sorts of things happening on land. But biosecurity in the sea doesn't get quite as much attention although it has serious implications."
Links
National Institute of Water and Atmospheric Research
Fish invaders spread to new waters
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