Salmon on the Lam

Image Credit: Isabell Schulz, CC BY-SA 2.0

Image Credit: Isabell Schulz, CC BY-SA 2.0, Image Cropped

Salmon on the lam: Drivers of escaped farmed fish abundance in rivers (2020) Mahlum et al., Journal of Applied Ecology,

The Crux

In a world with a growing human population and overfished seas, farming fish (aquaculture) could be a viable solution to our food security problems. Salmon aquaculture is already a massive industry worldwide, having grown substantially over the last half-century.

Yet the industry carries its own issues, one of which being its effect on wild salmon, which are of huge cultural importance to most lands that they’re found in. Wild salmon lifestyles see them migrate up rivers from the ocean to breed, with most salmon returning to the same rivers they were born in. Yet salmon escaping from fish farms have no spawning grounds to which to return, and can end up anywhere. This can result in deteriorating wild populations, with the farmed fish spreading disease and competing with the wild fish, as well as reducing wild fish health through interbreeding.

Because of this, figuring out where escaped salmon end up could be a major step forward for fish farms and local rivers alike. This week’s paper looks at what sort of variables lead to a river full of farmed salmon, and whether or not we can predict when and where they are likely to show up.

What They Did

The researchers used a technique called drift diving (more on that below) to count escaped salmon and wild salmon abundance at 486 sites over 14 years along Norwegian rivers. They then compared the numbers of farmed salmon to environmental values, including the number of wild salmon present, the total amount of water flowing through the river (discharge) and the intensity of nearby aquaculture.

That last value presented a challenge, as there are many different ways to quantify nearby aquaculture intensity. The researchers tried using the number of escaped salmon reported by nearby farms, the total number and mass of fish produced, and the number and allowed total production of each farm. They did this at a number of different scales, the smallest of which only counted fish farms within 10 kilometres of the site, the largest of which counted farms within 100 kilometres.

Did You Know: Drift Diving

Electrofishing has long been used to measure fish abundance. It involves luring fish it with an electrode and stunning them, allowing them to be caught, sampled, and usually released. However it’s very invasive, and drift-diving is one less harmful alternative. It essentially involves swimming the length of a stream and having a diver count the number of each species of fish. I have to admit, when I first read about the technique I was surprised that someone could not only get a reliable count of salmon from drift diving, but also differentiate between farmed and escaped salmon. However having been lucky enough to have worked in the same building as the Norwegian Nature Institute’s salmon division this year, I’ve been reassured it is highly accurate.

Farmed salmon cages like these can be found along coastal areas in many parts of the world, including Norway (Image Credit: Sam Beebe/EcoTrust, CC BY-SA 3.0)

What They Found

Total allowed production within 75 kilometres turned out to be the best indicator of aquaculture intensity. It had a positive effect on the number of escaped salmon in a river, as did wild salmon abundance and the river’s total discharge. The model had a reasonably good ability to predict where escaped salmon would turn up, but it worked better when it grouped rivers into broad risk categories.


I was actually a reviewer on this paper, so if I have problems with it I don’t really have anyone to blame. Potentially I should have noted the difficulty in using a broad value like “number of farms within 100km” for fish, which obviously don’t travel as the crow flies.

So What?

The fact that farmed salmon turn up in rivers filled with wild salmon could indicate that they are wont to follow the herd wherever possible, which may make it tricky to produce effective management strategies. The fact that the number of reported escaped salmon didn’t match up that well with the observed escaped salmon could indicate that there need to be more stringent measures in place regarding monitoring escapees from fish farms.

This is a good indication of progress for a controversial industry. While I’m not a big fan of fish farms, it’s nice to see work done that could help mitigate their impact of wild freshwater systems.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who hates the amount of money you get for studying salmon when there are perfectly good sticklebacks lying in wait in any river. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

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