Parasitism

Climate Change is Lousy for Salmon

Sandvik, Anne D., et al. “The effect of a warmer climate on the salmon lice infection pressure from Norwegian aquaculture.” ICES Journal of Marine Science (2021). https://doi.org/10.1093/icesjms/fsab069

If you go to the store, you’ve probably seen some Norwegian smoked salmon on the shelves. Norway is one of the major producers of the world’s salmon and is the world’s 2nd largest exporter of seafood. However, salmon farming in Norway has become more difficult recently due to climate change. As temperatures in the Arctic increase, so does the habitat range for harmful marine species, including parasites. One parasite of particular concern is the salmon louse.

The three main life stages of the salmon louse: reproducing adult (top), mature adult (middle) and immature larvae (bottom). Photo by Thomas Bjørkan via Wikimedia.

The salmon louse is a type of marine parasite that attaches itself to salmon and feeds on their skin, blood and mucus. If they feed on an infected salmon, they can also spread the infection or disease to others in the farm. The lice also significantly lower the quality of farmed fish, sometimes making the salmon unfit to sell. To learn more about how lice impact salmon, check out this article from NHM.

In this new paper, a team of Norwegian researchers created several predictive models to see how warmer temperatures might affect the relationship between salmon lice and both wild and farmed salmon. To better understand the connection between increasing temperature and salmon infection, the team looked at three main aspects of lice life cycle across 170 Norwegian salmon farms:

  1. Number of hatched lice eggs
  2. Survival rate of larval lice
  3. Number of salmon infected by lice
The reference map of the study site in Norway (S0), with the three models (S1, S2, S3) based on predicted temperature increase. The red indicates high levels of lice infection and transmission, while the yellow shows medium rates and the green shows low rates. From Sandvik et al. 2021.

 

After doing some fancy math and integrating the current climate models, the researchers came up with several concerning results. First, salmon are more likely to get infected as temperature increases. According to the model, salmon are almost 55 times more likely to become infected after a temperature increase of 2 degrees C. Second, the geographic area of lice infection will likely increase (see map on the right). Warmer temperatures will mean lice have a longer window of time to infect salmon. This will allow them to reproduce and spread faster than before. The researchers stated this estimate is conservative, since salmon lice generally produce more eggs than assumed in the model. They did find a silver lining, however. During the study, the researchers observed that lice tend to stay toward the surface. As ocean temperatures increase, less dissolved oxygen will be available at the surface. This will force fish deeper, decreasing their exposure to the lice, and likely won’t harm them otherwise. The team advised that salmon farmers start to think about how to adapt their farms in order to take advantage of this situation.

While the team was able to better understand how salmon lice infections might respond to warming temperatures, they say more research should be done on how infection rates may respond to other effects of climate change (e.g., changing ocean salinity and currents, more frequent floods or increasing atmospheric carbon dioxide). The habitat range of wild salmon will also likely increase, allowing for the spread of salmon lice to other populations of wild salmon, as well as other fish species such as sea trout and stickleback. In order to lessen this problem, the researchers urge fishery managers to stop the release of infected fish and implement fishery practices to stop interactions between farmed and wild salmon. 

Otherwise, salmon in the future may be having a pretty lousy time.

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