Characterisation and aetiology of jelly-like gonad condition (JGC) in the common carp, Cyprinus carpio (L)

Occurrence of gonad abnormalities in wild caught fish although rare have been reported in multiple species. Presented here is a jelly-like testis condition routinely encountered from wild caught carp in Tasmania. To delineate the cause/s of the condition, the prevalence, onset and progression was examined and correlated with select biotic and abiotic factors in a single cohort of wild carp over four breeding seasons. Observations this far indicate that the prevalence increased (0 to 17.3% of the male cohort) over four consecutive breeding seasons with progressive severity which could be sub-categorised as low, mild, severe and complete. Statistical analysis revealed a significant positive relationship of the condition to fork-length and body weight, with no significant influence from the abiotic factors tested. Growth of the affected fish was on average 9.22, 12.39, 14.24 and 19.78 % higher than that of female, pooled, male and immature sub-populations. Histopathological examination indicates absence of bacteria, fungal hyphae or viral inclusion bodies. Instead the testes were characterised by enhanced proliferation of sertoli cells in the initial stages followed by substantial apoptosis in the later stages leading to formation of fluid filled lumen that impart the jelly-like appearance. Parallel examinations in the golden galaxiid and brown trout that inhabit the same lacustrine system indicate that the condition is absent in both species. Elucidating the causes and mechanisms underpinning such naturally occurring abnormalities could assist in developing powerful pest fish control methods that disrupt reproductive capability of the target species in a species specific manner.