Transmission Bottlenecks of Sparicotyle chrysophrii in a Gilthead Seabream Aquaculture: A Next-Generation Matrix...

Sparicotyle chrysophrii is a major ectoparasitic pathogen of gilthead seabream (Sparus aurata) in Mediterranean aquaculture. Despite its high reproductive capacity, the parasite does not consistently cause severe outbreaks on commercial farms. In this study, we applied a next-generation matrix (NGM) framework to quantify intrinsic transmission potential and compare it with observed seasonal infection patterns. Field data of S. chrysophrii collected from a commercial seabream farm in Türkiye across four seasons were used to parameterize the model under marine cage conditions. The NGM analysis indicated that intrinsic transmission potential (R₀) can reach high values under biologically plausible reproductive scenarios and varies only slightly across the observed seasonal temperature range. In contrast, field observations revealed pronounced seasonal variation in infection levels, with moderate prevalence in spring and summer, a marked increase in autumn, and no detectable infection in winter. Sensitivity analysis identified mortality during the free-living larval stage and adult parasite loss as the dominant processes limiting parasite replacement. These findings suggest that the apparent mismatch between high intrinsic transmission potential and moderate field infection levels may reflect environmental and system-level constraints that reduce realized transmission in open-cage aquaculture systems. By decomposing transmission into stage-specific components, the NGM framework clarifies key bottlenecks regulating persistence of S. chrysophrii and provides a mechanistic framework for evaluating potential control strategies in marine aquaculture.