Histopathological survey on farmed meagre (Argyrosomus regius) affected by Systemic Granulomatous Syndrome (SGS)

Meagre (Argyrosomus regius) represents a promising fish species for the diversification in European aquaculture, on the basis of its rapid growth performances. Nevertheless, there are still several bottlenecks for optimization of meagre production, among which the Systemic Granulomatous Syndrome (SGS) is considered a widespread emerging threat. In order to evaluate the impact of SGS on the health of farmed meagre in Croatia, a histopathological investigation has been carried out. A total of 98 meagres were monthly sampled during the period July 2015-May 2016 from three cages with different feeding protocols: commercial pellet, hydrated commercial pellet and frozen sardines. From each sampled fish, portions of gills, stomach and intestine, liver, kidney, spleen and heart were fixed in 10% neutral buffered formalin and processed for histology. The SGS has been observed in the meagre from the 3 cages throughout the period under study. Few days after introduction of 6 g fish from a foreign hatchery, the SGS was already evident, with a disseminated granulomatosis in the kidney, mild granulomatosis in the liver and macrovacuolar hepatic steatosis. No significative differences in the fish from the three different cages have been observed, as well as a worsening of granulomatosis throughout the study period. In general, in all the examined fish the kidney and, to a lesser extent, the liver were the organs principally affected by SGS, with mild to severe granulomatosis, usually disseminated, in some case with coalescent granulomas. In the liver, the observation of macrovacuolar steatosis was constant. Based on histological observation and the few data from literature, SGS in meagre seems to be a pathological condition caused by concurrent factors, such as the presence of a Calcium/ Phosphorus imbalance (high CO2), worsened by the co-occurrence of a particularly susceptible life stage (0+ juvenile) and influenced by cold-water temperatures during winter period.When the ongrowing phase starts with fish showing an already established SGS during the nursery/hatchery phase, it greatly hampers health and productivity of meagre during the following production cycle. An improvement of knowledge on the optimization of diet in farmed meagre and related health issues is necessary to overtake one of the main bottlenecks that still limit the productivity of meagre.