A new study reveals the remarkable power of seaweed to clean—and in some cases entirely eradicate—fish waste from aquaculture.
In the new work based out of the University of Miami, Florida, researchers tested out the abilities of several seaweed species to clean water polluted with fish effluent. Starting with multiple tanks, each containing one of four seaweed species, including a type of red seaweed, and a sea lettuce, the researchers evenly pumped through effluent-filled water into each one from a tank stocked with yellowtail snapper, a common aquaculture species in the region.
The fish were kept in tanks at commercial production densities, to mimic the polluting effects of a fish farm. All the seaweed species received the same levels of effluent flow to compare their responses. These were also tested against control tanks that contained the seaweeds but weren’t exposed to waste from the fish tank. Researchers took regular readings of water oxygen, CO2, phosphate and ammonia levels in the water, and also tested the seaweeds for levels of protein, fat, minerals, and metals
This steady analysis revealed that the seaweed had varying—but often striking—abilities to cleanse the water of fish waste. The most significant finding is that one species called Agardh’s red weed, a dense, frilly seaweed with a burgundy hue, reduced levels of polluting ammonia from the fish-contaminated water to below detectable levels, seemingly absorbing all this waste and using it to power its own growth.
Additionally, tests of the water before and after it had run through the seaweed tanks showed that sea lettuce in particular was very good at absorbing carbon from the water, taking up more carbon than any other seaweed species and leaving the water significantly cleared of dissolved CO2. It also generated more oxygen than any other species. Two other seaweed species also significantly decreased the dissolved CO2 content of the effluent-infused water.
Meanwhile, a species known as sea grapes contained the highest amounts of protein when researchers did sample testing on the effluent-fed algae, pointing to its potential nutritional value, and suggesting it could make sense to produce seaweed commercially alongside fish aquaculture. In nearly all the seaweed species, the researchers also found that samples were enriched with the analyzed nutrients after exposure to the fish-fed water.
Ultimately, the study shows how mimicking natural seascapes, by bringing together fish and seaweed, could produce double-benefits: significantly curbing pollution in the world’s fastest-growing food production sector, while providing an additional source of human nutrient and farmer revenues.
In the short-term, they hope their investigation will convince farmers to at least give seaweeds a go, and to make smarter choices about which species they select when they do.
Lasco et. al. “Evaluation of native macroalgae species of the Southeast U.S. and Caribbean for use in integrated multi-trophic aquaculture (IMTA).” Aquaculture International. 2026.
Image: ©Anthropocene Magazine. AI-generated
