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Salmon Are Weirder Than You Might Think

The title is located in the centre of the image, reading “Salmon Are WEIRDER Than You Think.” A dark blue body of water containing a darker blue silhouette of an adult salmon occupies the top half of the image. In comparison, a lighter blue body of water containing a darker blue silhouette of a spawning adult salmon occupies the bottom half.

Written by Kristen Derbyshire
Illustrated by Cynthia Pham

While most fish spend their whole lives in either saltwater or freshwater, salmon have an unconventional habit of migrating in and out of freshwater and saltwater habitats at different life stages. Salmon are physiologically built to live in both habitats, and frankly, that is weird. Let’s dive in.

Salmon are what is termed as anadromous, migrating from the freshwater in which they were born, to the seawater in which they live most of their lives until it’s time to return to breed in the same freshwater they came from.¹ In order to survive the drastic change in water conditions, salmon have developed some convenient adaptations.

The word “ANADROMOUS” is located at the centre of the image, surrounded by a white border. A grey-blue salmon in its ocean adult phase occupies the top half of the image, emerging from the left side. It is depicted wearing a green backward-facing baseball cap and a blue backpack. A salmon with a grey-blue head and a red-orange body in its spawning adult phase occupies the bottom half of the page, emerging from the right side. It is depicted wearing a brown-brimmed hat and “holding” a brown briefcase with its fin. Aside from the colour difference, the spawning adult salmon differs from the ocean adult salmon in that its jaw is elongated and hooked and its body possesses a hump. Text reading “ocean adult” and “spawning adult (to river)” with accompanying arrows are located the illustration of each respective fish

Like us humans, salmon need salt to survive, but a balance of salinity within the fluids of our bodies is crucial. We intake salt and water through our diet, and the excess salt our bodies don’t need is transferred to our kidneys and eliminated.² This internal balancing act between water and salt is called osmoregulation, and it is the key to survival for salmon when migrating between habitats.

Freshwater has a lower concentration of salt than the body of a salmon, so when a salmon is swimming in freshwater, the salt in its body wants to leave the fish and escape into the water where it has more space for itself. On the flip side, when a salmon is swimming in saltwater, which has a higher concentration of salt than the salmon’s body, the salt from the water is compelled to permeate the salmon’s body. Too little salt, the fish can become overhydrated. Too much salt, and the fish can become dehydrated.

So how do they mitigate these risks? Salmon have evolved behavioural and physiological adaptations which allow them to cope with both habitats. When in saltwater, the salmon will drink several litres of water a day to prevent dehydration. The salmon’s kidneys will produce urine sparingly, in a highly concentrated form with lots of excess salt. In freshwater, where overhydration is the threat, the only water the salmon intakes is what happens to be consumed when it eats. To process the excess water in its body, its kidneys produce a larger volume of watery urine.³

It doesn’t stop there. Salmon also have a unique molecular pump in their gills which moves sodium, and other molecules which form salt, in and out of the fish’s cell walls. In freshwater, the pumps will take sodium in, while in saltwater, the pumps will shoot excess sodium out of its body.⁴ This unusual adaptation is a key player in affording salmon the ability to survive in these contrasting conditions.

Depending on the salinity of the water a salmon is in at any given moment, its body is able to adjust through behavioural as well as physiological adaptations, allowing it to get along “swimmingly” with the immediate conditions. Salmon are evolutionary marvels within the context of having the ability to osmoregulate so effectively in relation to other fish species, and that’s what makes them so cool and weird.

Sources:

  1. National Park Service. Anadromous Fish, National Park Service, U.S. Department of the Interior. Last Updated 12 September 2022. https://www.nps.gov/olym/learn/nature/anadromous-fish.htm
  2. Boundless. 41.1: Osmoregulation and Osmotic Balance – Introduction, Libre Texts Biology. Accessed 21 November 2024. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/41%3A_Osmotic_Regulation_and_the_Excretory_System/41.01%3A_Osmoregulation_and_Osmotic_Balance_-_Introduction
  3. The University of New Mexico. Acclimation of Osmoregulatory Function in Salmon, The University of New Mexico. Accessed 21 November 2024. https://www.unm.edu/~toolson/salmon_osmoregulation.html
  4. Cook Inlet Aquaculture Association. Osmoregulation: How salmon survive in freshwater and saltwater, Cook Inlet Aquaculture Association. 29 September 2022. https://ciaanet.org/osmoregulation-how-salmon-survive-in-freshwater-and-saltwater/

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