How delousing affects the short-term growth of Atlantic salmon (Salmo salar)

Infestations with salmon lice and subsequent salmon lice management is one of the most challenging and costly aspects of marine salmonid aquaculture. Both the handling and treatment, specifically non-medicinal treatment, against salmon lice cause stress and physical injuries to the host, the Atlantic salmon (Salmo salar). This in turn leads to reduced appetite and increased mortality. In this study, we have estimated the short-term growth loss of Atlantic salmon related to treatments (thermal, mechanical, hydrogen peroxide bath, freshwater bath and combination medicinal baths) for removal of salmon lice. To achieve this, we have obtained daily production data at cage-level from 2014 to 2019 from three large Norwegian aquaculture companies. We have used the registered feed-amount, number of fish and seawater temperature at cage level to calculate the thermal growth coefficient (TGC) of 635 fish-groups the week before a pre-treatment starvation period and the week after 2530 different treatments to estimate the reduction in TGC. We modelled this outcome using a mixed effect linear regression model, with treatment method as the main fixed effect of interest and fish weight, seawater tem-perature, smolt-age and year-class included as fixed effects. Results showed a period of suboptimal feeding and growth after all treatment methods, where non-medicinal treatment methods had a significantly larger negative effect on growth compared to medicinal treatments. The results also showed that timing of treatment played a role in the outcome of a treatment. The short-term biomass-loss in one cage following one non-medicinal treatment was estimated to 31,200 kg (average cage containing 150,000 fish weighing 3 kg, and seawater temperature of 10 C). Thus, there could exist a potential for increased production in the Atlantic salmon aquaculture industry by reducing the number of delousing operations.

Automated summary

Infestations with salmon lice and the management of these lice pose significant challenges and costs in marine salmonid aquaculture. Treatment against salmon lice, particularly non-medicinal methods, can lead to stress and physical injuries in Atlantic salmon, resulting in reduced appetite and increased mortality. This study analyzed production data from Norwegian aquaculture companies to estimate the short-term growth loss of Atlantic salmon following different treatments for salmon lice removal. The results showed that non-medicinal treatments had a greater negative impact on growth compared to medicinal treatments, and the timing of treatment played a role in the treatment outcome.