Not all infected fish become mushy: phenotypic expression underestimates Unicapsula seriolae infection in yellowtail kingfish (Seriola lalandi).

Yellowtail Kingfish (YTK) (Seriola lalandi) is widely distributed throughout the Indo-Pacific and is a significant recreational and commercial species in Australia. In New South Wales (NSW), a post-cooking mushy flesh condition has been reported, though its cause remains unconfirmed. This study combined morpho-molecular diagnostics with a citizen-science mushy fish reporting platform to identify associated parasites and to map the spatio-temporal distribution of mushy flesh and infections in NSW. Unicapsula seriolae was detected in 38.9% (95% exact CI: 23.1-56.5%) of YTK donated by recreational anglers and charter operators (n = 36) (2024-2025) and confirmed by myxospore morphology and SSU and LSU rDNA sequencing. Most infected fish (71.4%) did not exhibit the phenotype, although mushy individuals were significantly more likely to be infected than non-mushy fish. Infection occurred year-round and across bioregions, whereas the phenotype was restricted to warmer months, yet asymptomatic infections were also present. Collectively, these findings extend the records of U. seriolae-YTK infections on Australia's east coast beyond the initial reports off QLD and indicate that mushy flesh alone is an unreliable indicator of U. seriolae infection. Infection and sea surface temperature alone were insufficient to drive phenotypic expression, with host-environment-parasite interactions likely modulating its manifestation. Citizen-science data (n = 48) revealed widespread, temporally variable phenotypic occurrence and documented socio-economic impacts on anglers and consumers. The parasite's effects on flesh quality may influence angler and consumer perception and marketability; however, the weak concordance between infection and phenotype indicates that visual assessment alone is likely to underestimate infection, allowing infected fish to enter the supply chain, undetected. Although U. seriolae is not considered a significant zoonotic parasite, in Japan, reports of foodborne illness associated with U. seriolae in other Seriola species suggest these findings warrant consideration from a food safety perspective. The results highlight that U. seriolae is present in YTK from the Manning and Hawkesbury shelf bioregions in NSW, but the expression of mushy flesh is conditional rather than a direct, inevitable consequence of infection.