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Salmon can outrun sea lice by swimming fast

Fast swimming salmon catch less sea lice than fish cruising at normal speeds observed in sea cages. These new results can probably be exploited by farmers so that salmon catch fewer lice.

ild salmon already take advantage of that it can swim from lice. Their migrations from rivers and into the ocean are basically so high that it affects the chances of lice to attach.

The new results were recently published by researchers from the University of Melbourne and IMR.

- We got the idea from some evidence that suggested that salmon farms located at exposed locations, where current speeds are higher and fish should swim faster on average, had low lice levels, explained Francisca Samsing, PhD researcher at the University of Melbourne and IMR.

Testing swimming speeds relevant for farmed and wild salmon

In specially designed swimming tunnels, groups of salmon were swum at different speeds and the same numbers of sea lice larvae were released into each tunnel.

- The three current speeds we used reflected the average night-time and daytime swimming speeds of farmed salmon in sea-cages (0.2 to 0.7 body lengths per second), and the high speeds (1.4 body length per second), at which wild salmon often swim at to migrate from rivers to the open sea, said Samsing.

Video of the salmon in specially designed swimming tunnels.

Normal swimming speed gives most lice attachment

Fish which swam at 0.7 body lengths per second caught 2.5 times more lice than fish that swam at 1.4 body lengths per second, and 1.3 times more lice than fish swam slowly  at 0.2 fish lengths per second. However, after lice became attached, swimming speeds did not change how well lice survived.

- Our results are important because they add a new dimension to our understanding of how both farmed and wild salmon interact with sea lice. That salmon in fish farms typically swim at speeds that are best for sea lice to attach is a factor that likely contributes to the industry’s ongoing problem with lice that we have not recognised until now. stated Frode Oppedal, fish welfare researcher from the Institute of Marine Research.

For wild salmon, the story is slightly differently.
- Wild salmon swim fast when they leave their rivers to migrate to the sea. Scientists had previously assumed they did this to outrun predators in coastal waters before they reached the relative safety of the open sea, but now it seems this behaviour might provide some protection from infection by outrunning sea lice that are concentrated in coastal waters, stated Tim Dempster, head of the Sustainable Aquaculture Laboratory at the University of Melbourne.

Reduced sea lice infestation at faster swimming?

The study opens up new opportunities in aquaculture to combat lice. Farming in more exposed locations, where high currents are naturally present and fish must swim faster, or using farm management techniques to encourage fish to swim faster for short periods, could reduce lice levels in salmon farms.


Gone with the flow: current velocities mediate parasitic infestation of an aquatic host

Facts about sea lice

Latin name: Lepeophtheirus salmonis
Distribution: occur naturally in Norwegian waters. Their numbers have risen significantly in parallel with the growth of the aquaculture industry.
Biology: sea lice are parasites with eight life stages, three of which are free-swimming, two of which are stationary and three of which are mobile. They attach themselves to salmon in the third life stage.
Size: adult female: 12 mm (approx. 29 mm including egg strings); adult male: 6 mm.
Diet: the skin and blood of salmonids. The lice only start feeding when they have attached themselves to a host fish (stationary and mobile stages).
Reproduction: all year round, but reproduce increasingly quickly as temperatures rise in spring.
Dispersal: free-swimming stages spread on currents in fjords and coastal waters.
Treatment: biological methods (wrasse) or chemicals (medication).

Sea lice


Frode Oppedal
971 66 781
Tim Dempster
55 23 85 00