Fish with cold sores? What the carp?!

Does thousands of diseased fish going belly-up and rotting in our waterways sound like a bad idea? Well, it just might happen.


Carp are voracious feeders!

Carp gather in large numbers in summer to breed.
Image by Shoko Muraguchi via Flikr.

Enter the European carp. This close relative of the humble goldfish was introduced into Australian waters by European colonists in the 1800s. Their numbers have since exploded. It has reached the point that if you took all the fish out of the water and put them on a massive kitchen scale (good luck), carp would be about 90% of the weight of that slimy, writhing mass.


Messy eaters

Carp feed by digging around in the sludgy river bottom, which muddies the water around them. Turning clear water into muck makes life difficult for farmers that use it to water their crops and it makes the water unsuitable to drink. Importantly, this cloudy water makes it difficult for other fish to find food, affecting their health and ability to breed.


Carp feed by sifting through mud, making the water murky. Image by Ingrid Taylor Via Flikr

Carp with cold sores

In 2015, the Minister for Agriculture and Water Resources Barnaby Joyce allocated $15 million to curb carp numbers, appointing a “carpinator” to head the project. Their main idea for controlling these “rabbits of the river” is to infect our rivers with a strain of the herpes virus (the virus that causes cold sores in humans). The disease infects the carp, clogging up their gills and preventing them from breathing, eventually killing them. It’s estimated that up to 90% of the “bottom-dwelling mud-suckers” will die to this virus, clearing up the water and giving the native fish a chance to recover. Studies are still ongoing, but at the moment it doesn’t look like the disease directly affects any other fish, animals or humans.


Blackwater death

While this might all sound great, it doesn’t mean that the grand carp cold sore plan is without complications. The virus works rather quickly, and when nearly all of the living animals in a stream suddenly die, there will be problems. This can lead to what is called a “blackwater event“. The bacteria that decompose the carp carcasses use oxygen. A huge amount of dead carp means a huge amount of bacteria, and a massive loss of oxygen. If there is no oxygen left in the water, anything still alive will suffocate – in this case, the fish that we are trying to save from the carp!

Thousands of dead carp could suck the oxygen out of our rivers. Image by Grondenau via Flikr


Yes, there are plans in place to haul out the dead fish, but it won’t be possible to cover the entire length of every river. If not done properly, the carp herpes virus might do just as much harm as good. So, what are some alternatives?

Judas carp

This devious method gets carp to betray their own kind. Researchers catch a few large individual fish and tag them with tracking devices. They keep a close eye on their movements, and use this information to work out when and where the carp will gather. Then, they sweep in and net them all up. Most importantly, there are no dead fish left in the river. This is also used to control other invasive animals, such as goats and hogs.

“Judas pigs” lead managers to their hard-to-find social groups, making them easier to control
Image by Christine Kaelin via Flikr

Daughterless carp

Another, less obvious, way of suppressing carp numbers is meddling with their genes. To do this, researchers breed specially engineered female carp that are only able to give birth to males. These mutant females are released into the carp-infested areas and left to breed with the natural population. Over time, the relative numbers of male and female fish gets out of whack, with more and more male fish being born. Fewer females mean fewer baby carp, and the number of young carp born in each generation drops, reducing their total numbers.

No room to relax

When we get rid of these alien fish, it is likely to help our freshwater systems. We cannot be complacent, though. If this virus is not managed properly, it may have a disastrous impact on the very rivers we are trying to protect. Before it can be released, we need to ensure that it is safe and we need to have a solid plan in place to clean up the dead carp.

It is also not enough.

Taking too much water from our streams, other alien species, pollution and habitat loss are all issues affecting our rivers. We need to attack these different parts of the problem if we want to reverse the damage that we have done. Killing carp is a piece of the puzzle, but it is not the whole story.

2 Responses to “Fish with cold sores? What the carp?!”

  1. Gabriel Cornell says:

    Thanks Dom.

    Good point. I think the idea is that reducing the numbers will hopefully allow the other fish to recover a bit and suppress the carp. This would prevent them returning to such high numbers if they do reach some level of natural immunity. Some scientists are saying that we need to pair this with other control methods to really hit them very hard.

  2. Dominic Thorn says:

    Love your work Gabriel.
    If the herpes plan goes ahead, how will we avoid another myxomatosis incident like we had with the rabbits? Will the 10% that aren’t wiped out by herpes not just breed and replace the lost carp with new carp immune to herpes?