Hindcasting Farmed Salmon Mortality to Improve Future Health and Production Outcomes

Area of Science:

• Aquatic animal health
• Fish disease epidemiology
• Mathematical modeling in biology

Background:

• Bacterial pathogens, including Piscirickettsia spp. and Tenacibaculum species, are linked to summer mortality in farmed Chinook salmon (Oncorhynchus tshawytscha).
• A 2022 mortality event in Pelorus Sound, New Zealand, coincided with high water temperatures (>18°C) and bacterial skin disease.

Purpose of the Study:

• To understand the progression of bacterial infections and identify drivers of mortality during the 2022 event.
• To simulate the disease outbreak using a networked susceptible, infectious, and recovered (SIR) model.

Main Methods:

• A networked susceptible, infectious, and recovered (SIR) model was employed to simulate the 2022 mortality event.
• Parameter exploration was conducted to reconstruct observed mortality rates across three affected farm sites.
• Disease parameters were compared with established ranges for Piscirickettsia salmonis.

Main Results:

• The SIR model successfully reconstructed mortality rates, identifying plausible drivers of the disease outbreak.
• Bacterial pathogens likely spread within healthy fish populations over a 10-week incubation period before mortality was observed.
• Vaccine use at one site was estimated to have prevented 10% higher mortality, with potential for greater reduction through alternative site selection.

Conclusions:

• Modeling provides insights into the progression and drivers of bacterial disease outbreaks in farmed salmon.
• Vaccine efficacy in aquaculture can be enhanced through strategic site selection, highlighting the importance of future vaccine development.
• Understanding disease dynamics is crucial for mitigating losses in aquaculture operations.