The darker side of exotic ornamental fish

Millions of beautiful ornamental fish are imported each year and, despite strict quarantine measures, some might be also importing diseases that threaten domestic fish species and industries

;" Imported Dwarf Gouramis have been identified as carrying disease despite appearing to be healthy
Photos: 123rf.com

By Bianca Nogrady

Exotic ornamental fish are a popular pet and desk accessory, but there is a darker environmental side to these often brilliantly coloured creatures.

In recent years, exotic ornamental fish diseases have been implicated in several mortality events among native species such as the Murray Cod (Maccullochella peelii), and have also caused serious problems for domestic producers of ornamental fish such as Goldfish (Carassius auratus).

While quarantine procedures exist for imported ornamental fish, a recent research project led by Joy Becker, from the University of Sydney, suggests that many diseases are slipping through the net and escaping into Australian waterways.

Australia imports about 18 million ornamental fish each year, says Joy Becker, senior lecturer in aquatic animal health and production.

And every fish must comply with Australia’s import conditions, including a health certification from the point of origin.

The fish are also subject to quarantine for varying lengths of time depending on the species, with Goldfish quarantined for 21 days and Dwarf Gouramis (Trichogaster lalius) for 14 days.

The aim of quarantine is to allow for the detection of any pathogens that might be latent in the fish; however, Joy Becker’s research suggests that some significant exotic diseases are able to remain at a subclinical level for much longer.

“It can be difficult to detect them during the quarantine period; either the incubation period is much longer, or there has to be some sort of stress event that occurs for the fish to go from being a subclinical carrier – basically just harbouring the pathogen – to becoming overtly sick and being able to release the pathogen.

“However, sick fish are identified during quarantine and not allowed in the country, so it does work,” Joy Becker says.

Using highly sensitive molecular diagnostic techniques, Joy Becker and colleagues recently examined the presence and prevalence of two notifiable diseases – the dwarf gourami iridovirus and cyprinid herpesvirus 2 – at all levels of the ornamental fish industry.

“We looked at fish coming in from overseas and then we looked at different populations of fish that are already in Australia at pet shops, ones that had just come out of quarantine at wholesalers, and then ones that were domestically produced in aquaculture farms and then finally in the wild,” she says.

The research identified both viruses in a range of species, in different parts of the industry.

In the case of the dwarf gourami iridovirus, it was detected in about 20 per cent of otherwise healthy Dwarf Gouramis imported from overseas and under quarantine and up to 15 per cent of fish released from quarantine to wholesalers.

The virus was found in almost 30 per cent of sick Dwarf Gouramis at pet shops and it was also found at one domestic fish farm.

Cyprinid herpesvirus 2 was also found on wholesaler premises and fish farms, and was even detected in several populations of wild Goldfish.

“It’s a really clear demonstration of how we can get an ornamental fish virus that’s exotic into the country and then from pet stores and pet owners, it gets into the wild,” Joy Becker says.

Cyprinid herpesvirus 2 affects Goldfish, causing high levels of mortality, and is an issue for the domestic Goldfish aquaculture industry, but it does not affect any other species or people.

The dwarf gourami iridovirus is of much greater concern because it not only affects the popular Dwarf Gouramis but has also been responsible for mortality events among farmed Murray Cod.

“The dwarf gourami iridovirus infects several species. Right now it’s considered a freshwater pathogen but there’s evidence that it can infect species that are estuarine, as well as marine species, so that just increases its scope for the number of hosts it can impact on,” Joy Becker says.

Vulnerabilities to exposure

New South Wales Goldfish producer Steve Wood says cyprinid herpesvirus 2 is a major issue for him. He is a director of the Pine Creek Fish Hatchery at Bonville, on the NSW north coast, a business that has been breeding ornamental fish, and Goldfish in particular, for more than 80 years.

It is a point of pride for the hatchery that the operations and fish are free of pests and diseases.

However, he says the lack of exposure to exotic diseases also makes them highly vulnerable when they are exposed; this is in contrast to imported fish, which seem to have built up some level of immunity through exposure to pathogens.

“If we put our fish in an aquarium that’s got cyprinid herpesvirus 2 coming in, they will die pretty quickly.” He says his business has lost customers because his fish have died when mixed with others in fish tanks at pet shops.

There is little hard data on exactly how these viruses are spreading and escaping into the wild, but Joy Becker says the source is unlikely to be individual pet owners.

“Your home aquarist putting tank water down the drain is not going to be the issue; it’s going to be places where you have a lot of fish and you have that potential for pathogen amplification, so that could be a pet shop or a wholesaler or even a domestic farm,” she says.

One scenario for release is simply a large amount of water from these venues entering the wild. Another, rarer scenario might be a tank break that releases fish into the wild.

The suggestion has also been made that feeding ornamental fish to farmed fish may be another route of transmission.

“It does raise the other issue of people releasing pet fish into the wild, which is a big ‘no-no’,” Joy Becker says.

“The water is probably a lower risk from a home aquarium but a human putting a fish back into the wild, that would create a higher risk.”

The other issue is how to prevent these diseases from entering Australia in the first place. Since the release of these findings, quarantine authorities have accepted that cyprinid herpesvirus 2 is present in Australia and have revoked the requirement for imported Goldfish to be certified free of the disease.

However, the challenge is prevent or limit the entry of other diseases, including the dwarf gourami iridovirus.

“From the research that we did, we made some recommendations that have included diagnostic testing or veterinary health certification for the dwarf gourami iridovirus to be considered offshore, so then we know the fish arriving at the border are are free of dwarf gourami iridovirus,” Joy Becker says.

Other options include increasing the quarantine period for certain species, with the aim of detecting subclinical disease that might emerge after longer periods of time, or increasing surveillance and mandatory testing of consignments offshore to ensure they are free of disease before they arrive in Australia.

Steve Wood says he believes ornamental fish should be subject to the same entry procedures as any other imported animals.

“If you brought 100 sheep in and 10 died, the other 90 wouldn’t get released,” he says.

Joy Becker is now planning a large study examining all of the currently notifiable diseases associated with ornamental fish and using molecular diagnostic methods to look for the presence of any of these diseases in ornamental fish entering the country.

“It’s all protecting Australia’s industries, whether it’s aquaculture industries, fisheries industries or ecological industries,” she says.

FRDC Research Code: 2009-044

More information

Joy Becker, 02 9351 1787
joy.becker@sydney.edu.au