Distribution of Cardicola forsteri eggs in the gills of southern bluefin tuna (Thunnus maccoyii) (Castelnau, 1872)

The distribution of Cardicola forsteri eggs in the gills of southern bluefin tuna (Thunnus maccoyii) was analysed. Eggs were confirmed to be C. forsteri using laser-capture microdissection and quantitative polymerase chain reaction (qPCR) analyses. Prevalence of infection was significantly higher in the second gill arch (χ2 = 6.49, P < 0.05) than in the fourth gill arch, and significantly lower in the basal region of the second gill arch (χ2 = 6.29, d.f. = 2, P < 0.05). The intensity of infection was not significantly different between the gill arches (F = 0.03, d.f. = 1, P > 0.05). Similarly, there was no significant difference in the intensity of C. forsteri eggs between the gill arches (F = 3.43, d.f. = 2, P > 0.05), or at different depth of sectioning (F = 0.08, d.f. = 1.12, P > 0.05). Results suggest that the presence of C. forsteri eggs in the gills of tuna is more likely to be detected by sampling the second gill arch. Furthermore, targeting the middle region and increasing the sectioning depth may reduce the proportion of false negatives.

► Prevalence of C. forsteri eggs was higher in the second gill arch.
► Prevalence of C. forsteri eggs was lower in the basal region of the gill filaments.
► Intensity was not different between or within the gill arches.