You ever heard the saying, “Water, water everywhere, but not a drop to drink?” Well, that’s true for most of the world’s water supply. Salty oceans make up nearly 97% of water on Earth while freshwater sources make less than 3%. There are two main reasons for this.

When rain falls on land, it erodes the rock and releases ions. These ions are carried into streams and rivers and, then, to the ocean. Meanwhile, hydrothermal vents release dissolved ions from the Earth’s crust that mix with the seawater. 85% of dissolved ions in the ocean are sodium and chloride, which combine to form sodium chloride (aka salt).

We cannot drink saltwater because our kidneys cannot handle such high levels of sodium chloride. Our kidneys can only make urine less salty than seawater, so to get rid of excess salt, we have to urinate more water than we drink. If keep drinking saltwater, we’ll eventually die of dehydration. The saltiest water we can safely drink is about 0.5 parts per thousand. That’s 500 milligrams of salt per liter of water. Meanwhile, the average salinity of seawater is about 35 parts per thousand, which is 35 grams of salt per liter. That’s an insane difference.

Yet, in a crazy twist, fish that literally live and breathe in the ocean ALSO struggle with salt. Because the ocean is so salty, saltwater fish are constantly losing water out of their gills and skin, so they have to keep drinking water to stay hydrated. But, because they are constantly drinking seawater, they have to keep releasing the salt in their urine.

However, unlike us, they have specialized kidneys and cells in their gills that allow them to get rid of the excess salt. That’s why, despite living in the ocean, saltwater fish aren’t any saltier than freshwater fish. So, if you’re trying to reduce salt in your diet, you don’t have to choose catfish over salmon. You just have to watch your seasonings.

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