Trait values of individuals are affected not only by their genetic makeup, but also by environmental factors and interactions with other individuals. The heritable effect of an individual on the trait values of other individuals it interacts with is known as an indirect genetic effect (IGE). Such IGEs may affect response to selection. Fish selected for high growth rate, for example, have been shown to be more aggressive and competitive, which may reduce the observed response in growth rate. The main objective of this study is to quantify the genetic and non-genetic indirect effects for harvest weight in the GIFT strain of Nile tilapia. A total of 6330 fish with harvest weight information were used to estimate genetic and non-genetic parameters. A bivariate analysis of harvest weight and survival was conducted by fitting different mixed models to investigate the presence of IGEs and other non-genetic effects. The full set of genetic parameters could not be estimated simultaneously with the inclusion of maternal common environmental effects. A confounding between maternal common environmental effects and direct genetic effects resulted from the mating strategy, where one sire was mated to only one or two dams. A 1 male to 2 females mating design is common in aquaculture, but it has limited power to estimate genetic parameters. Models without maternal common environmental effects showed significant IGE on harvest weight, which contributed 48% of total heritable variance. Models with maternal common environmental effects suggested the presence of IGE. The direct-indirect genetic correlation for harvest weight was negative (-. 0.38. ±. 0.19), indicating that traditional selection, if performed in an environment where the fish have to compete with each other for the resources, will increase competition. A strongly negative genetic correlation between direct effects on survival and indirect effects on harvest weight (-. 0.79. ±. 0.30) showed that individuals with better genes for survival suppressed growth rate of their social partners. Our results suggest that heritable competitive interactions affect harvest weight in Nile tilapia. Hence, breeding schemes may need to be adapted to avoid an increase in aggressiveness due to selection for growth rate in a competitive environment. Further studies are required to investigate the relevance of IGE and its implications on different systems of commercial aquaculture production. Statement of relevance: Sociable fish will help to improve aquaculture production.