I learned this while researching the phosphorus circulation of seabirds, but salmon also circulate phosphorus at an incredible rate. Phosphorus is a raw material for genes and enzymes, so it is called the “element of life” that is essential for living things, but because it dissolves easily in water and has a high specific gravity, it tends to accumulate at the bottom. Therefore, in order for phosphorus to circulate, a movement against gravity is required to bring the phosphorus that has accumulated at the bottom to the top.
https://www.shiminkagaku.org/post_146
That’s where salmon’s presence seems to be important. They are born in rivers, and when they grow to a certain size, they go out to the ocean, travelling over 6000km, and then take several years to return to their original river. Phosphorus that flows into the ocean is absorbed by phytoplankton, which are then eaten by zooplankton, which are then eaten by salmon. Perhaps the image is that salmon are migrating across the vast ocean, gathering up phosphorus. Salmon are amazing.
https://www.spf.org/opri/newsletter/477_2.html
To digress from the main topic, it is thought that freshwater fish can only live in fresh water such as rivers, and saltwater fish can only live in the sea, but salmon go down from rivers to the sea and back again, so they can adapt to both. Fish that can adapt to both freshwater and saltwater environments are called euryhaline fish, and apparently they regulate this with a hormone called prolactin that adapts to freshwater. It’s an amazing biological system, isn’t it?
https://suizokuseiri-utokyo.jp/research/research-03
After migrating across the vast ocean, salmon store up phosphorus in their bodies, and then they go against gravity and swim upstream in rivers to spawn. Salmon are also known as “marine-derived nutrients” that store up the bounty of the sea and carry it to rivers. When we think of salmon, we tend to imagine them being caught by bears, but they seem to provide valuable nutrients to many living things even after spawning. Salmon are responsible for the nutrient cycle between the sea, rivers, and forests, enriching the forests and rivers.
https://www.hro.or.jp/fisheries/h3mfcd0000000gsj/o7u1kr0000006sk0/o7u1kr0000006pjk.html
There are still many mysteries surrounding salmon, such as which ocean route they swim and how they can pinpoint the exact location of their origin rivers. When you think that birds defy gravity by flying and salmon defy gravity by swimming up rivers, and that this results in the circulation of important nutrients, it’s amazing how amazing birds and salmon are. Of course, our ecosystem exists only because of gravity, but the Earth is amazing because it has both the mechanism of gravity and the mechanism that can defy it.
