In France, rainbow trout farming in conventional flow-through systems raises environmental concerns. To address this, there is a growing interest in organic aquaculture. In this study, we employed an attributional life cycle assessment (LCA) to analyze the environmental impacts of rainbow trout production, comparing conventional and organic practices in a model fish farm. Our life cycle impact assessment revealed that organic farming significantly reduced environmental impacts per tonne of trout in seven of the nine selected impact categories. Notably, freshwater ecotoxicity exhibited the most significant difference, with organic systems showing a 35% decrease. The only exceptions were freshwater eutrophication and water dependence, where organic production led to higher impacts per tonne of trout. In conventional farming, emissions amounted to 14 kg of P eq./tonne, whereas in organic farming, the emissions were slightly higher (15 kg of P eq./tonne). For water dependence, one tonne of trout production in the conventional system mobilized 128 103 m3 vs. 185 103 m3 in the organic system. The environmental benefits of organic production were even more marked when using a surface-based functional unit (m2y). We demonstrated the benefits of organic trout production from an environmental perspective. However, our findings highlight the caution needed when interpreting LCA comparisons of such production systems that can be highly influenced by methodological choices such as the functional unit used.