Prospectors check on river life

A trio of French scientists is currently wading through – and, on occasion, falling into – Rarotonga’s rivers and streams in an attempt to catalogue the critters residing in our fresh water.
Clad in hip waders and armed with assorted nets and plastic buckets, Phillip Keith, Gerard Marquet and Philippe Gerbeaux have, to date, forded rivers in French Polynesia, Samoa, and Wallis and Futuna.
“We prospect different rivers to know what fish and shrimp there are,” said Keith of the work he and cohorts are conducting for the National Museum of Natural History in Paris. “It will be interesting to know if there are some species only found here in the Cook Islands.”
DNA samples are also being taken from prawns and eels.
“We know now that the fresh-water fauna from the Cook Islands is really similar to that of Tahiti, but really different from the fresh-water fauna from Samoa,” said Keith while standing on the bank of a stream in the Takuvaine Valley. “We think the Melanesian fauna stops at Samoa and never goes to the Cook Islands or French Polynesia. So the Cook Islands is really the barrier between two different types of fauna.”
Keith’s job, at least on this day, is to strap a battery pack to his chest and then dangle a metal rod in the water. The machine transmits an electric current that temporarily paralyzes the assorted fauna, making them easier to scoop up in the large nets wielded by Marquet and Gerbeaux. Turn off the current and the fish shimmy away unharmed and none the worse for wear.
It’s a methodical job that, at times, puts the pain in painstaking. While working at the higher end of one river, Marquet took a nasty tumble that resulted in several abrasions on his face and a finger so dislocated it was actually bent at a right angle.
Once they have finished their explorations, the scientists will submit a report to the Cook Islands government and then take their results back to the museum to add to its database.
At the same time, they are also aiding Gerald McCormack with his research.
“This is fantastic, that people like this come here,” said McCormack, the director of the Cook Islands Natural Heritage Trust. “These sorts of experts are just invaluable in terms of allowing our Natural Heritage database to eventually record all the biodiversity here.”
McCormack admitted he feels like an eager school kid tagging along to watch how the experts operate.
“They are able to pick up a live specimen and, very quickly, look at a feature or the way it moves, and they know what species it is,” he said. “That’s what I’m picking up from these guys: the real live biology of identifying things that are living. It’s great. It’s been fantastic.”
Along with the usual suspects, the scientists also made a surprising discovery.
“They found a bathygobius,” said McCormack. “It’s a tiny fish about three centimetres long. They don’t know what species that is and, certainly, it’s not on our database. They need to do some DNA testing on it before they can tell which species it is. It’s very unlikely to be a new species – it’s just an unrecorded species.”
McCormack said the French team has added at least five species to the Natural Heritage Trust’s database during their time on the island.
“Mainly from the lower areas of the streams, which is a place where it is very hard to see these species unless you have the electric shock machine,” he said.
Eels are a particularly interesting creature to study. Rarotonga is home to three varieties of eels, including one that, reportedly, dines on rats. Only adult eels live in the rivers, after spawning in the ocean, usually in the deep sea near Fiji and Samoa. Once they finish producing the next generation, the eels make a beeline for fresh water.
“Some eels return to their native streams, but generally it’s haphazard,” said Keith. “If they sense some fresh water in the sea, they follow the fresh water and go up the river. They go up rivers in Tahiti or Samoa, even if the adult was in the Cook Islands.”
This whole circle of life thing only works, however, if there are actually rivers to live in.
“The main problem in the future for different islands like the Cook Islands will be the fresh-water supply for the population,” said Keith. “Many rivers in the Cook Islands have water intake and as we take all the water for the water intake, there is no more water flowing in the river so the different species cannot climb up.”
McCormack said it’s unfortunate that there has to be a pecking order when it comes to water use.
“I suppose the human need of water is greater than fish need of water,” he said. “The sheer amount of water that we’re needing to take out, and with increasingly strong El Ninos, there is simply going to be less water available. And when the choice comes, people are going to take it.”

By John Ireland

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