Effects of Body Size and Stocking Density on Routine Oxygen Consumption in the Chilean Silverside (Basilichthys microlepidotus) Reared in a Recirculating Aquaculture System

The Chilean silverside (Basilichthys microlepidotus) is an endemic freshwater species considered Vulnerable, and the species-specific bioengineering parameters needed to design efficient recirculating aquaculture systems (RAS) for this species, particularly oxygen consumption, remain unknown. A factorial experiment (2 × 2) was conducted to evaluate how body size and stocking density affect the routine oxygen consumption of B. microlepidotus. The study considered two body-size groups and two stocking-density treatments, each represented by three replicate tanks. Oxygen uptake rates were determined from mass-balance calculations in a static closed respirometry system during sequential 24 h periods. Body size significantly influenced mass-specific oxygen consumption, with smaller fish exhibiting higher rates than larger fish, consistent with allometric metabolic scaling, whereas no significant independent effect of stocking density was detected; however, with only three tanks per treatment and two similar densities, the test had low power and this result is inconclusive. A pronounced diurnal pattern was evident in all treatments, with the main oxygen consumption peak near 14:00 h, approximately three hours after the single daily feeding (11:00 h), which is consistent with, but does not demonstrate, a specific dynamic action (SDA) response because no unfed control was included; a smaller peak near 08:00 h preceded feeding and may reflect routine laboratory activity at the start of the working day. These results provide the first species-specific reference values of routine oxygen consumption for B. microlepidotus, offering a preliminary basis, within the size and density ranges tested, for estimating the oxygen demand of fish in RAS designed for this species, with direct relevance for both conservation-oriented restocking and its development as a candidate for freshwater aquaculture diversification.