LAUSR

Abstract Inadequate stock density in fish larviculture can cause a dominant behavior that harms feeding and results in stress, lots of heterogeneous fish and low productive rates. This study aimed to evaluate the effect of stocking densities on growth performance, cortisol levels and arapaima (Arapaima gigas) larvae production cost. Arapaima larvae (1.32 ± 0.25 g; 5.90 ± 0.33 cm) were used and stocked at densities of 400, 800, 1200, 1600 and 2000 larvae m−3. Fish were housed in 15 tanks (50 L) according to a completely randomized design in a recirculating water system with constant aeration. Larvae were fed an extruded commercial diet (45% crude protein) at the 10% biomass proportion, divided into six meals day−1 for 30 days. At the end of the trial, fish were measured and weighed, and four fish per tank were euthanized to analyze plasma cortisol. The arapaima larvae production cost was calculated for all the stocking densities. The fish reared at the 2000 larvae m−3 density presented feeding voracity and the best zootechnical performance: final weight (12.16 ± 0.89 g), weight gain (10.84 ± 0.89 g), relative growth rate (7.69 ± 0.26%) and lower stress levels, which resulted in a 93.33% survival rate. This group of fish presented a low production cost and, consequently, the highest income per larva (2.34 US$). For arapaima larviculture, using 2000 larvae m−3 is recommended because it results in higher growth, more homogeneous fish, better survival rates which, in turn, result in the lowest arapaima larvae production cost.