Due to the high demand for lumpfish as cleanerfish in salmonid ocean farming operations, increasing hatchery production and rearing efficiency is of great importance. As juveniles, lumpfish are cannibalistic which is controlled, to some extent, though frequent size grading of the fish, however, cannibalism still occurs. Understanding and mitigating for factors that exacerbate aggressive behaviors in juvenile lumpfish, which can lead to cannibalism, would help achieve the goal of increasing juvenile production in the hatchery. We hypothesize that lumpfish cannibalism is linked to a specific ontogenetic period related to fish size and can be exacerbated by various stressors such as stocking density and photoperiod. To test this hypothesis, we subjected two different size classes of juvenile lumpfish (5g and 10g) to varying densities (40g/L, 65g/L, or 90g/L) under different photoperiod regimes (ambient, constant low light, or constant bright light) for a 10-week duration in winter 2022. Fish growth, survival, and aggression were measured biweekly, and stocking densities adjusted to baseline levels biweekly by removing any necessary fish. Final results, including daily growth rates, overall mean percent growth, survival, and occurrence and severity of fish aggression, as well as recommendations for lumpfish facilities, will be shared with session participants.