Infections of Philometra madai (Nematoda: Philometridae) in the Gonads of a High-growth Broodstock Population of Red Seabream Pagrus major

Abstract

Aquaculture of the red seabream Pagrus major predominantly utilizes artificial seedlings of high-growth strains. ​Although effective broodstock management is crucial for sustainable production, internal parasitism in brood fish, particularly by the gonadal nematode Philometra madai, is often neglected. ​Despite its common occurrence in adult red seabream gonads, infection status of broodstocks and the effects on the brood fish health remain unexplored. ​This study conducted a 14-month survey on 3- to 4-year-old brood red seabream. ​Worm carcasses were consistently found in both male and female gonads throughout the year, with overall average prevalence of 86.4%. ​In some cases, these carcasses filled the entire gonad, including ovarian cavity and sperm ducts, extending to the area near the cloaca, potentially impeding ovulation and sperm release. ​However, no significant effects on gonadal development were observed and there was no correlation between worm weight and host weight, condition factor, or gonadosomatic index. ​Live P. madai were detected only during the red seabream spawning season, from March to May, with their maturation synchronized with host reproduction. ​This study represents the first survey of P. madai infections in high-growth strain red seabream broodstock.