Parasite Behind Mushy Fish May Be More Widespread

If you're a keen angler and regularly catch yellowtail kingfish (Seriola lalandi), you may have encountered instances of the fish's flesh turning mushy and unpalatable once cooked. Some have said it's a bit like trying to eat wet newspaper.

This can be disappointing for the angler and for the hungry guests you've invited to dinner. It is also a waste of food - and of the fish's life.

It's often hard to pinpoint which fish will turn mushy when cooked as they can look, smell and taste the same when raw, such as when served as sashimi. The issue only appears once the fish is cooked or prepared as ceviche. In other words, you need heat or a pH change for the problem to reveal itself.

The mushy flesh problem is associated with the parasite Unicapsula seriolae but it's too soon to say it's definitely the cause - that research hasn't been done yet.

What we do know is that this parasite shows up in a lot of kingfish that turn mushy when cooked.

Now, our research suggests infection with Unicapsula seriolae may be more widespread than first thought.

In fact, we found even flesh that doesn't go mushy after cooking can still be infected.

Mushy kingfish and parasite infection in NSW

I've been fascinated by the mushy kingfish problem for over 15 years now, following my own encounters in the waters off Sydney, the mid north coast of New South Wales, and at the dinner table.

There are reports dating back to the 1990s of people in NSW catching kingfish that later turns mushy when cooked.

But our research wanted to go beyond anecdotal evidence, so we scientifically tested for and detected this parasite in kingfish caught in NSW. For the first time, we show it is present in kingfish from NSW.

Scientists had previously only tested for and detected this parasite in kingfish caught in Queensland and Western Australia, and in aquaculture settings in Western Australia.

Our recent research involved working closely with NSW-based anglers, spearfishers and charter operators to obtain donations of kingfish tissue in 2024 and 2025.

We tested for the presence of microscopic parasites in kingfish by:

• looking at samples under the microscope

• doing polymerase chain reaction (PCR) testing and

• DNA sequencing.

In the samples we tested, we found 38.9% of fish were infected with the myxosporean parasite, Unicapsula seriolae. This falls within the range of infections in wild kingfish in previous Australian reports , which put the figure at between at 29.3% and 80%.

But crucially, we also showed that a larger proportion of infected fish (71.4%) did not develop mushy flesh when cooked, or were asymptomatic.

This indicates that relying on fish turning mushy once cooked will mean most infections get missed.

This microscopic marine parasite is similar in size to the human red blood cell, making it impossible to spot with the naked eye. Only some fish will develop signs of infection in the form of black cysts that can be seen with the naked eye.

We also found the parasite in fish caught throughout the year, even though the issue of flesh turning mushy when cooked only usually shows up in fish caught in summer and spring.

This might suggest there's a temperature link that causes this issue to become more prevalent in warmer months, but it's too soon to say for sure.

How does this parasite affect fish health and human health?

Scientists don't yet fully understand the impacts this parasite can have on the health of kingfish. But we do know that in aquaculture settings outside of NSW this parasite has been associated with mass-mortality events (when other pathogens are also present).

Despite this, we still do not know the exact role the parasite plays in the health and physiology of wild kingfish.

Scientists are also unsure what the consumption of raw Unicapsula seriolae-infected kingfish means for human health.

Currently, there is no evidence to suggest it causes food-borne illnesses here in Australia. In Japan , the same parasite has been linked to cases of food-borne illnesses in people who ate raw greater amberjack (Seriola dumerili) - a species closely related to the yellowtail kingfish.

We do not have any accounts of similar cases here in Australia. More research is required to investigate the impacts to human health, if there are any.

All in all, we found most infected kingfish from NSW waters do not become mushy when cooked. And we still do not exactly know why the flesh turns mushy when cooked for some infected kingfish but not others.

Further research is required to understand the significance and consequence of infection, and to develop improved rapid detection methods.

Although sample sizes were small, we are continuing our work to better understand infection.

If you're an angler or consumer and would like to contribute to the ongoing research you can become a citizen scientist and log your encounters with mushy kingfish on the mushy fish app .

The research upon which this story is based was supported by the Charles Sturt University Faculty Early Career Researcher scheme, the Gulbali Institute and the SUNY Center for Biotechnology and Biodiversity.