| Summary: | The unidentified pathogen, known as 'Frill on Gill’ (FOG), infects the shallow-water Cape hake Merluccius capensis off the south and west coast of South Africa. This pathogen, a parasitic castrator, occurs on the gill filaments of infected fish. This study investigates the effects of this pathogen on the body condition of M. capensis on the south coast of South Africa, through the use of two body condition measures, condition factor and hepatosomatic index (HSI). A total of 24275 M. capensis were sampled, of which 825 (3.4%) were infected with the FOG pathogen. The smallest M. capensis infected with FOG was 10 cm, and the largest was 84 cm. The smallest uninfected M. capensissampled was 6 cm and the largest was 106 cm. The presence of FOG had a significant impact on the condition factor and HSI of M. capensis (three-way analysis of variance (ANOVA): p-value = 0.002, and p-value = 5.13e-09 respectively), with the effect of the pathogen varying with length. For fish with a total length smaller than 26 cm, infected M. capensis were in poorer condition than uninfected fish at the same length, with lower condition factors and HSI values. However, for M. capensis with lengths between 30 and 34 cm, 42 and 46 cm, 54 and 62 cm, and 78 and 82 cm, the condition factors of FOG infected fish were higher than uninfected fish. The HSI values of M. capensis infected with FOG also steadily increase above those of uninfected fish, for lengths greater than 42 cm. The lower body condition measures of uninfected M. capensis could be linked to the large amounts of energy that these fish expend during reproduction and gonad maturation. In contrast, M. capensis infected with FOG do not partake in the energy intensive task of reproduction. Merluccius capensis infected with FOG don’t contribute to reproduction, but still compete with uninfected M. capensis for resources. The effect of FOG as a type of parasitic castrator on the regulation of M. capensis populations could become a concern for fishery managers, as they reduce reproductive output and impact fish body condition.
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