26 DECEMBER • WORLD AQUACULTURE • WWW.WAS.ORG Phosphorus falls into the category of essential nutrients. This means that our bodies can’t make it on their own. It is a highly reactive element, binding to sugar molecules and rings of carbon with nitrogen atoms to form DNA in all organisms. To create energy phosphorus connects with sugar and a bunch of hydrogen and nitrogen atoms. It also tends to connect with alcohol and fatty acids to form phospholipids, an essential part of cell membranes which select what goes in and out of a cell. Crucially, phosphorus binds to calcium, mineralising bones of vertebrates. It is also a staple of industrialisation and capitalism. Phosphorus based preservatives can be found in a variety of products from meat, to cola and ice cream. It is also used as a fertilizer in agriculture and can be found in the diets of terrestrial and aquatic farmed animals. Norway’s Atlantic salmon (Salmo salar) is no exception. Aquaculture in Norway Norway is situated at the northwestern edge of the European continent. Its rugged coastline stretches across 100,000 km. It is carved by hundreds of deep fjords creating an optimal place for aquaculture. This fjordic coastline is home to more than 900 seacage sites, making Norway the world’s leader in salmon farming. An Unwanted Product of Aquaculture: Phosphorus Waste Sea-cages are floating nets that facilitate the continuous uncontrolled exchange of water, and anything smaller than the mesh size, between the farm and the environment. Open water farming has many advantages. Mainly, it is energetically efficient as the dynamic currents naturally draw in clean, oxygen rich water while allowing oxygen depleted water to move out, all without the need for artificial pumps. But this free flow of water can equally be a drawback. The uncontrolled water movement allows phosphorus-rich faecal waste and uneaten feed particles to enter the water column. Such phosphorus injections can lead to unwanted growth of harmful algae that eventually start to decay with the help of decomposing microorganisms, leading to oxygen depletion. With Norway’s salmon industry projected to almost double its annual production from 2.8 million tonnes to a staggering 5 million tonnes by 2050 (Pandit et al. 2023, Norwegian Seafood Council 2025), effective phosphorus management has become a critical issue. A good start would be looking at what we feed our salmon — and whether we can feed them smarter. Where Can We Find Phosphorus and Why Do We Tend to Use Too Much? Being naturally carnivorous, salmon were historically fed 70% fishmeal-based diets. This is a product made from wild caught fish, and typically low trophic species such as anchovies, sardines, herring or whiting. Fishmeal is high in phosphorus, but the bioavailability, the amount of mineral that is turned into useful molecules inside the body, varies with the species and parts of the fish that are used. Phosphorus trapped in bone material is completely unavailable. Even during times of feeding farmed salmon with phosphorus rich fishmeal diets, salmon couldn’t use the trapped phosphorus in the bones and developed signs of phosphorus deficiency (Albrektsen et al. 2009). The major signs of phosphorus deficiency in salmon include low bone mineral content, soft curly spines, and bent inner vertebral structures (Figure 1). This is because the fish continues to grow even without enough phosphorus and forms bone matrix that doesn’t get mineralised, which makes it soft and rubbery. Aquaculture is now moving away from fishmeal towards Are We Wasting It? The Story of Phosphorus and Salmon Lucia Drábiková, Saskia Kröckel, P. Eckhard Witten, Guido Riesen, Paul Morris, Agnés Ostertag, Martine Cohen-Solal, Thomas W.K. Fraser and Per Gunnar Fjelldal Lucia Drábiková and Guido Riesen preparing for sampling at the sea-cages.
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