Science1 publisher3 min readPublished
Most kingfish carrying the mushy-flesh parasite cook up firm, so the cook test misses them
A New South Wales survey detected Unicapsula seriolae in 38.9% of yellowtail kingfish, and 71.4% of the infected fish never turned mushy after cooking.
The Scientist · Science desk
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What happened
- Yellowtail kingfish (Seriola lalandi) flesh sometimes turns mushy and unpalatable once cooked; some describe it as being like eating wet newspaper.
- Affected fish can look, smell and taste the same when raw, such as when served as sashimi; the problem only appears once the fish is cooked or prepared as ceviche, meaning heat or a pH change is needed for it to reveal itself.
- The mushy flesh problem is associated with the parasite Unicapsula seriolae, but it is too soon to say the parasite is definitely the cause because that research has not been done; the parasite shows up in many kingfish that turn mushy when cooked.
- The researchers scientifically tested for and detected Unicapsula seriolae in kingfish caught in New South Wales for the first time; scientists had previously only tested for and detected the parasite in kingfish caught in Queensland and Western Australia, and in aquaculture settings in Western Australia.
- The research involved working with NSW-based anglers, spearfishers and charter operators to obtain donations of kingfish tissue in 2024 and 2025.
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Why it matters
Researchers who tested donated yellowtail kingfish (Seriola lalandi) tissue from New South Wales found the myxosporean parasite Unicapsula seriolae in 38.9% of fish, and reported that 71.4% of the infected fish did not turn mushy when cooked [7][9]. That undercuts the only screening method anglers and cooks currently have: waiting to see whether the fillet collapses in the pan catches at most about 28.6% of infections [1][10].
The mushy flesh problem is old news to anyone who fishes the NSW coast, with reports going back to the 1990s, and it has an unpleasant signature that some describe as eating wet newspaper [16][1]. The awkward part is timing. Infected fish look, smell and taste normal raw, including as sashimi, and the texture failure only appears once heat or a pH change is applied, so cooking or ceviche preparation is the reveal [2].
According to the authors, writing at Phys.org, this is the first time the parasite has been scientifically tested for and detected in kingfish caught in NSW; previous detections were confined to Queensland and Western Australia, including WA aquaculture [4]. The samples came from NSW anglers, spearfishers and charter operators in 2024 and 2025, and were screened by microscopy, PCR and DNA sequencing [5][6]. The 38.9% prevalence sits inside, and near the low end of, the 29.3% to 80% range reported in earlier Australian work on wild kingfish [7][8][4].
Two details explain why visual and culinary screening both fail. The parasite is roughly the size of a human red blood cell, so it cannot be seen with the naked eye, and only some infected fish develop the black cysts that are visible without a microscope [11]. And the team found infected fish throughout the year, even though mushy flesh is usually reported in summer and spring catches; they raise a possible temperature link but say it is too early to confirm [12].
Causation is still not established. The parasite shows up in many kingfish that go mushy, but the work proving it is the cause has not been done [3], and the mechanism behind the texture change in infected fish remains unexplained [14]. Applied to the reported prevalence, roughly 27.8% of all fish tested were infected and firm, against roughly 11.1% infected and mushy [2][3].
One caveat on the arithmetic: the published figures are consistent with small denominators. 71.4% is 5/7 and 38.9% is 7/18 to one decimal place, and the source does not state how many fish were tested [5].
On consequence, the authors say there is currently no evidence the parasite causes foodborne illness in Australia, while in Japan the same parasite has been linked to illness in people who ate raw greater amberjack (Seriola dumerili), a close relative [15]. In aquaculture outside NSW it has been associated with mass-mortality events, though only alongside other pathogens [13].
Watch for a study that establishes whether Unicapsula seriolae actually causes the texture failure, a larger NSW survey with a stated sample size, and any seasonal prevalence data that would test the temperature hypothesis [3][12][5].