Optimal Salmon Lice Treatment and Tragedy of the Commons

Wednesday 28 Aug 19


Andre William Visser
DTU Aqua
+45 35 88 34 25


Ken Haste Andersen
Professor, Head of Section
DTU Aqua
+45 35 88 33 99

Sea lice on salmon farms are controlled via treatments. Authorities mandate treatments when sea lice infections reach a given threshold. Using a bio-economic model, we show that isolated farms profit most from a high treatment threshold, whereas a low threshold is optimal for farm networks. Operating with a low treatment threshold hence places farms in a tragedy-of-the-commons environment.

Salmon lice has for decades plagued salmon aquaculture by decreasing profits and impacting wild salmon stocks. To protect migrating wild salmon stocks and avoid excessive cross-farm infections, authorities require treatments when sea lice level reach a given threshold. The treatment threshold is set to protect wild salmonid stocks but also to avoid costly lice infections on neighboring farms.

Intuitively one might think that decreasing the treatment threshold will increase number of treatments. While this is true for isolated individual farms, which maximize profits by operating with a high threshold, the maximum profit (least number of treatments) for an entire network of farms occurs at a low threshold (about 0.1 gravid female lice/salmon). These findings substantiate the Norwegian policy of lowering the lice treatment threshold below 0.5 gravid lice/salmon.The results also demonstrate that too low a treatment threshold results in high treatment rates.

The difference between the optimal treatment strategy of individual farmers and that for the total system demonstrates that management of salmon lice infections operates in a tragedy-of-the-commons environment, where individual farmers may have an incentive to disregard legislation at the expense of the others in the network.

The paper argues that strong enforcement is needed to achieve optimal management of salmon lice infections.


The paper can be read here:


Kragesteen TJ, Simonsen K, Visser AW and KH Andersen 2019 Optimal salmon lice treatment threshold and tragedy of the commons in salmon farm networks Aquaculture 512: 734329
13 AUGUST 2022