Researchers fed rainbow trout different diets made partly from mushroom stem waste and found that these ingredients beneficially changed the bacteria living in the fish’s gut. Different parts of the fish’s digestive system showed different changes in bacterial communities, suggesting these mushroom ingredients work in region-specific ways. The mushroom-based diets reduced harmful bacteria and increased beneficial ones, potentially improving fish health. This research shows how mushroom waste can be recycled into nutritious feed for farmed fish while promoting their health.