Reluctance of farmed Atlantic salmon to feed in cold water revealed during automated hydroacoustic feeding control

Waste feed remains a major issue in open sea-cage Atlantic salmon aquaculture. “Echofeeding” is an appetite-led feeding method that stops meals based on fish biomass detected by an echo sounder. The method reduced waste feed and upheld fish growth in a relatively vertically unstratified coastal farming environment. Here, we tested echofeeding at a commercially relevant scale over an 8-month period in a fjord environment with seasonal vertical temperature and salinity gradients. We compared fish behaviour and growth between echofed fish, fed at high intensity and near surface, and control fish, with feeding regulated by pellet detection without surface feeding restriction (conventional practice). Growth (SGR>1.81) and FCR (<0.87) were excellent and similar for three months after sea-transfer in August. However, a strong halocline in late November (<5°C surface water) led echofed fish to avoid surface feeding, resulting in underfeeding. Following the setting of a deeper depth interval for triggering feeding, the echofed fish fed more, and fed at similar levels to control fish when feeding intensity was reduced. Echofeeding underperformed in early spring as rising surface temperatures attracted salmon, making it difficult for the system to distinguish between feeding and routine behaviours. Both groups contracted salmonid alphavirus during winter, reducing appetite and promoting early harvest. Results highlight the need for echofeeding to take environmental changes into account. Further, as fish grew, a gradual decline in the echo signal measured during feeding suggests a method for refining meal termination threshold to minimize waste feed while maintaining good fish growth. • Atlantic salmon show a trade-off between surface temperature and hunger level. • Simple and consistent proxies for measuring salmon appetite are highly promising. • Individual size variation was not affected by feeding intensity or underfeeding. • Feeding control based on pellet detection is robust to environmental changes. • Biomass threshold levels for hydroacoustic feeding control depends on fish size.