Two-headed fish may remain a mystery
News analysis Scientists may never be able to work out whether pesticides caused a mass spawning of two-headed fish larvae in a popular Australian holiday region.
A Queensland government taskforce was last week established to investigate recent reports of deformities found on a fish farm in Noosa, near Brisbane, and claims that sprays from nearby farms were to blame.
Queensland's chief veterinary officer, Dr Ron Glanville, says the deformities are "highly unlikely" to be due to natural genetic variation, but getting to the bottom of what caused them is no simple matter.
"As far as we know that was a one-off event and it may or may not happen again. You can't actually diagnose something in retrospect," he says.
"If it doesn't happen again we may never get an answer to what happened."
In October last year fish health expert, Dr Matt Landos, reported to authorities, on behalf of a fish hatchery in Noosa, problems that had occurred in three different species of fish over the past two and half years.
In one incident, four separate matings of seven adult bass collected from the Noosa River spawned hundreds of thousands of larvae, 90% of which had two heads and died within 48 hours, says Landos, who is affiliated with the University of Sydney.
More than two heads
Apart from the headline-grabbing two-headed fish larvae, other incidents documented by Landos include mass fish kills and fish suffering neurological convulsions.
Landos says a range of chemicals, including organochlorines, organophosphates and synthetic pyrethroids and carbendazim - have historically been used on nearby macadamia farms.
He says the best evidence to date that chemicals were involved comes from a report by the owner that she successfully treated convulsing fish in a 2007 with atropine - an antidote for organophosphate poisoning.
Glanville agrees this suggests organophosphates were involved in that particular event, but says the reports have not been validated by government tests.
"We need to keep an open mind on this," he says. "If you jump to a conclusion early, you can get caught ... it might be something completely different."
Glanville says government scientists have only been able to directly test water and fish tissue from a mass fish kill that occurred in December 2008, and these tests failed to detect any chemical residues.
"If they are there they would be extremely low levels," says Glanville.
Low level exposure
Landos says tests to date have been flawed, and the problems could have been caused by chronic exposure of fish to chemicals at levels below that which equipment is capable of detecting.
He says this possibility needs to be tested by laboratory experiments that deliberately expose fish at different life cycle stages to varying levels of chemicals.
Landos adds that low levels of chemicals can interrupt genetic, endocrine or physiological systems leading to a host of different symptoms, depending on the stage of development a fish is exposed.
He says the most recent DPI tests, yet to be officially released, have found the fish killed in December 2008 had a fungus in their brain that was a likely cause of death.
But, he says, the tests also found that fish had also likely been exposed to pesticides, which could have depressed their immune system, making them susceptible to fungi.
Glanville says there are plans to monitor fish, water and sediment on both the fish farm and in the Noosa River.
He says scientists will also expose normal fish to suspect water and if the problems arise again they will then attempt to detect any chemical residues at the time.
Dr Diane Webb of the Department of Environmental and Aquatic Sciences at Curtin University of Technology in Perth agrees chemical exposure could damage fish development leading to the range of problems seen in the fish.
But, she says, there is a lack of data on the impact of chronic low level exposure of pesticides on Australian species.
Webb says the relevant Australian authority, does not appear to require tests that investigate the effects of pesticides on the early development of eggs and sperm prior to fertilisation.
She says the proposed studies detailed by Glanville are "in the right direction", but laboratory studies of the kind Landos is calling for are also required.
Glanville agrees with Landos' call for studies, but says the task force, which includes Landos, is yet to decide what tests to carry out.
"That is extremely complex and difficult work to do. That's the sort of work that chemical companies do when they register a new chemical and it generally costs significant amounts of money."
In addition, he says judgements will have to be made about what chemicals are worth investigating.
"There's a whole range of chemicals used on farms in the area," he says. "You can't test every chemical."
But Webb says there are further challenges in testing the pesticide hypothesis.
She says there are many different variables to consider including the interaction of different chemicals - not only of active ingredients, but of surfactants and other chemicals in pesticide formulations.
Factors, such as the chemistry and temperature of the environment can also affect the toxicity of chemicals, says Webb.
So even if all the tests being recommended by experts are carried out they may still not reproduce a similar combination of circumstances that led to the problems in the first place, she says.