A hidden parasite in New South Wales kingfish
Anglers who regularly catch yellowtail kingfish (Seriola lalandi) may be familiar with the disappointment of filleting a fish only to find its flesh turns mushy and unpalatable once cooked. Some have compared the texture to wet newspaper. The problem is often impossible to spot in raw fish, which can look, smell, and taste normal when served as sashimi, only revealing itself after cooking or preparation as ceviche.
The mushy flesh issue has been associated with the parasite Unicapsula seriolae, though researchers caution it is too soon to confirm the parasite as the definitive cause. What is clear is that the parasite shows up in many kingfish that turn mushy when cooked.
Now, new research suggests infection with Unicapsula seriolae may be far more widespread than previously thought, and that even fish whose flesh does not turn mushy can still be infected.
First detection in New South Wales
The research, led by scientists who have been fascinated by the mushy kingfish problem for over 15 years, worked with New South Wales-based anglers, spearfishers, and charter operators to collect tissue donations in 2024 and 2025. Using microscopic examination, PCR testing, and DNA sequencing, the team detected the parasite in kingfish caught in NSW for the first time.
Previously, scientists had only tested for and detected this parasite in kingfish from Queensland and Western Australia, and in aquaculture settings in Western Australia. Reports of mushy kingfish in NSW date back to the 1990s, but this is the first scientific confirmation of the parasite's presence there.
Most infections show no symptoms
The study found that 38.9% of the sampled fish were infected with Unicapsula seriolae. Notably, 71.4% of infected fish did not develop mushy flesh when cooked—meaning they were asymptomatic. This suggests that relying on the mushy flesh symptom to detect infections will miss most cases.
The parasite is extremely small, similar in size to a human red blood cell, making it impossible to spot with the naked eye. Only some infected fish develop visible signs, in the form of black cysts.
The infection was found in fish caught throughout the year, although mushy flesh symptoms usually appear in fish caught in summer and spring. Researchers suggest there may be a temperature link, but say it is too soon to say for sure.
Broader implications and unanswered questions
While the parasite is associated with mass-mortality events in aquaculture settings outside NSW when other pathogens are present, its impact on wild kingfish health remains unclear. Scientists say they do not yet fully understand how Unicapsula seriolae affects the fish that carry it.
In Japan, the same parasite has been linked to foodborne illnesses in people who ate raw greater amberjack (Seriola dumerili). However, there is no evidence of similar cases in Australia.
The research team is encouraging anglers and consumers to contribute to ongoing research by logging encounters with mushy kingfish on the 'mushy fish app', a citizen science tool designed to help track the problem.
As the research continues, the findings highlight that the 'mushy fish' phenomenon may be just the tip of the iceberg, with a far larger number of infected fish showing no outward signs at all.