Cannibalism in early fish developmental stages is a biological phenomenon affecting aquaculture, so seeding density and feeding schemes play a fundamental role in its optimal control. The objective of this research was to evaluate two initial stocking densities, 0.7 larvae/L (D7) and 1.4 larvae/L (D14), which were fed (F) and not fed (NF) in tropical gar (Atractosteus tropicus) larvae on growth, attacks, survival, and digestive enzyme activity, digestive morphology, and oxidative enzymes and aggressiveness gene expression. A total of 420 larvae (0.022 ± 0.003 g; 1.52 ± 0.10 cm) were placed in 70 L tanks connected to a recirculation system. Treatment F larvae were fed a formulated food. D14F larvae had the highest final weight, total length, and specific growth rate (SGR) (p < 0.05). The highest survival percentage was in D7F larvae (46.19 ± 2.74) (p < 0.05). The highest frequency of attacks occurred in D14F larvae (26.66 ± 0.82) (p < 0.05). The coefficient of variation (CV) and size heterogeneity (SH) were higher in D7F larvae (13.56 ± 1.80; 0.81 ± 0.10) (p < 0.05). The F larvae started attacks at 8 days after hatching (DAH), while the NF larvae showed it at 10 DAH. The difference in weight between the attacker and the attacked was 16.44 ± 9.75% and in total length 12.28 ± 4.57% (p < 0.05). Acid and alkaline protease, trypsin, and lipase activities presented differences between both factors, as well as the interaction between them. D14F larvae presented the highest Glutathione peroxidase (GPX) activity (p < 0.05), while D7-NF larvae registered the highest catalase (CAT) activity (p < 0.05). Superoxide dismutase (SOD) activities only showed differences between F and NF larvae (p < 0.05). The liver presented the highest percentage of area with melanomacrophage centers (MMC) in D7-NF larvae, and hepatocytes with the highest area were recorded in D14F larvae (p < 0.05). In gut, the highest enterocytes were recorded in D14F larvae (p < 0.05). cat, cck, and gpx expression did not show difference between feeding regimes. The expression of sod1 and sod2 in D7F and D14F treatments presented the highest expressions (p < 0.05). D7F larvae presented higher expression of avp1, crh, htr1, sstr1, th and tph1 genes (p < 0.05). Our results show that A. tropicus larvae D14F presented the highest growth and morphometric parameters and decreased CV and SH, which shows a more homogeneous growth. Regarding cannibalism, fed larvae (D7F, D14F) presented cannibalism in a greater proportion than unfed larvae regardless of density, however, it is necessary to analyze other physiological and social factors that could be related to the aggressiveness and cannibalism of the species.