The salmon is a good marathon swimmer

New experiments show that salmon can swim for up to three days without becoming exhausted, as long as it does not use more than 80 percent of its maximum swimming capacity.

Published: 07.10.2021

Author: Veronica Nagelsen

In exposed fish farms at sea, salmon can be exposed to worse weather, more current and stronger waves than they do in traditional farming along the coast.

Now, researchers at the Institute of Marine Research have tested how long the salmon can swim without being exhausted.

– We ended the experiment after 72 hours without the fish being exhausted. We were surprised at how long it could actually swim without it collapsing, says researcher Malthe Hvas.

Run a marathon or sprint

– As long as the salmon stayed within its aerobic capacity and did not have to produce lactic acid, it showed an impressive ability to swim for a long time, says Hvas.

Aerobic means with oxygen, and means activities that you can do for a long time. Anaerobic means without oxygen. It requires a lot of strength in a short time so that the heart is not able to pump enough oxygenated blood to the muscles, and you start producing lactic acid.

Or to put it more simply; the difference between running a marathon or sprinting all the way up a steep hill. In other words, the salmon is a good marathon swimmer.

Useful knowledge to assess animal welfare

The maximum swimming speed of fish is most often measured as the critical swimming speed (Ucrit). Here the fish is tested in short time intervals with increasing speeds until it becomes exhausted.

In the study, the researchers tested different constant swimming speeds on the fish based on a percentage of Ucrit in a swimming tunnel at the Institute of Marine Research’s station in Matre.

– As long as the fish stayed at a speed of 80 percent Ucrit, it managed to swim for a long time. If the fish swam faster than this, it became exhausted within the first hours. This shows that the limit between aerobic and anaerobic swimming was about 80 percent of the maximum swimming speed for salmon, says Hvas.

– This is very useful knowledge when we are to assess animal welfare and current conditions at exposed facilities at sea. The speed and duration of strong water currents can not exceed the fish’s swimming abilities, says Hvas.

figure exhaustion
The figure shows how long the salmon is able to swim at constant speeds based on one percent of the fish’s critical swimming speed (Ucrit). At higher speeds, the fish are exhausted more quickly, but at 80 percent Ucrit it does not become exhausted.

Can not ride the Tour de France and at the same time relax

But even if the salmon has good swimming capacity, it is still not optimal for the fish to swim that long. At the same time, it must have the capacity to both grow and eat in order to thrive in the cages.

– It can be compared to cycling the Tour de France. You can not cycle that long, and at the same time have time to eat and relax. In the same way, the salmon will not be allowed to grow or eat if it has to swim continuously and close to its maximum capacity over long periods, says Hvas.

Researchers currently know little about how conditions actually are in offshore fish farms, or how salmon react to, for example, waves, as offshore aquaculture is still a new strategy for the aquaculture industry.

– This is something we need to find out more about, but this study indicates that the salmon has good sustained swimming capacity and can therefore withstand strong water flow for many hours, says Hvas.

– This means that it can probably do well in farming at sea.

Measured how fast the fish swam

The researchers also looked at the frequency of tail strokes at different swimming speeds. The tail stroke frequency of fish is easy to measure, it only requires that you have a stopwatch and count how many times the fish moves its tail back and forth.

The breeders can observe this via camera in the cage, and can thus provide useful information about the fish’s swimming capacity.

When the fish was at 3.2 tail strokes per second, it swam at its maximum capacity, that is, 100 percent Ucrit. When the fish kept this speed in the experiments, it became exhausted within the first two hours.

When the fish had 2.5 tail strokes per second, it maintained a speed equivalent to aerobic swimming (80% Ucrit), and which the fish could maintain over several days.

Tail strokes of less than 2 per second (less than 60% Ucrit) were considered a speed that did not require much effort by the fish. At this speed, the fish will also have the capacity to eat and grow.

swimming speed
Salmon neck rate at increasing swimming speeds. The frequency of tail strokes is easy to observe on fish, and gives an indication of how fast they swim.


Hvas, M., Folkedal, O., & Oppedal, F. (2021). What is the limit of sustained swimming in Atlantic salmon post smolts ?. Aquaculture Environment Interactions, 13189-198.

Lenke: What is the limit of sustained swimming in Atlantic salmon post smolts? (

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