Animal domestication is constrained by the capacity of species to live under close confinement and perform well at high densities in captivity, but how fish adapt to crowding is poorly understood. We examined behavioural and transcriptomic responses to crowding in Nile Tilapia (Oreochromis niloticus). A mirror test was used to assess agonistic interactions and hypothalamus gene expression was assessed among the most and least aggressive fish at each density.