Ammonia excretion dynamics of the silverside (Basilichthys microlepidotus) during the juvenile-to-adult grow-out phase, under different body sizes and stocking densities in a recirculating aquaculture system (RAS)

The Chilean silverside (Basilichthys microlepidotus) is an endemic freshwater species with emerging potential for aquaculture and conservation-oriented production. However, key bioengineering parameters required for system design and biomass management remain poorly defined. This study evaluated how body size and stocking density affect total ammonia nitrogen (TAN) excretion rates in a recirculating aquaculture system (RAS) under controlled conditions. Two stocking densities (1.1 and 1.8 kg m-3) and two size ranges (7.8-77.8 g and 80-297 g), as well as their interaction, were tested using a factorial design with triplicate tanks per treatment. Mean TAN excretion rates ranged from 24.5 to 30.6 mg TAN kg-1 fish h-1. A clear diurnal excretion pattern was observed, with peak TAN release occurring 2-4 h after feeding, followed by a gradual decline. Neither body size nor stocking density significantly affected TAN excretion within the evaluated ranges, indicating that these results provide bioengineering parameters for RAS design and scaling.