Researchers tested whether mushroom stem waste products could be used as sustainable fish feed ingredients. When juvenile rainbow trout were fed diets containing 30% mushroom stems from three different species, their gut bacteria changed in beneficial ways without harming the intestinal lining. Different parts of the gut showed different bacterial responses, with the most beneficial effects being increased growth of helpful bacteria like Mycoplasma and Legionella. This research suggests mushroom processing byproducts could improve aquaculture sustainability while supporting fish health.