Optimal salmon lice treatment threshold and tragedy of the commons in salmon farm networks

Tróndur J Kragesteen,Knud Simonsen,André W Visser,Ken H Andersen

doi:10.1016/j.aquaculture.2019.734329

Tróndur J Kragesteen, Knud Simonsen + Show 2 more

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https://doi.org/10.1016/j.aquaculture.2019.734329

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Abstract

The ectoparasite Lepeophtheirus salmonis 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. Here we make a bio-economic estimation of optimal treatment thresholds. We are particularly interested in identifying conflicts between the optimal threshold of the entire system of farms and for the individual farmer. We show that isolated individual farms can maximize profit by operating with a high threshold, while the maximum profit for an entire network of farms occurs with a 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. This means that strong enforcement is needed to achieve optimal management of salmon lice infections.

Highlights

Salmon aquaculture industry has grown over the last 3 decades from an annual production of 0.23 mio metric tonnes in 1990 to 2.36 mio metric tonnes salmon in 2017 and is a major industry with an annual production representing a value of €15 billion (FAO, 2006–2019)
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
Salmon aquaculture has nearly always been afflicted by sea lice which currently represents the main bottleneck for further expansion of the industry

Summary

Introduction

Salmon aquaculture industry has grown over the last 3 decades from an annual production of 0.23 mio metric tonnes in 1990 to 2.36 mio metric tonnes salmon in 2017 and is a major industry with an annual production representing a value of €15 billion (FAO, 2006–2019). Salmon aquaculture has nearly always been afflicted by sea lice which currently represents the main bottleneck for further expansion of the industry. There are three main issues with elevated levels of sea lice in salmon farms. One issue is impaired salmon growth where in the worst case, salmon can die of infection either directly or indirectly by secondary infections (Pike and Wadsworth, 1999). Another issue is the treatment itself, which is both costly and can have a negative effect on the local environment. The third and perhaps the most studied issue is the artificially increased infection pressure on wild salmonids (Krkošek et al., 2007; Kristoffersen et al, 2018) which has been shown to pose a serious risk to wild salmonid stocks (Krkošek et al, 2013)

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