Abstract
Thermal treatment has become the most used delousing method in salmonid aquaculture. However, concerns have been raised about it being painful for the fish. We studied the behavioural response of Atlantic salmon acclimated to 8 °C when transferred to temperatures in the range 0–38 °C. Exposure time was 5 min or until they reached the endpoint of losing equilibrium and laying on their side, a sign of imminent death. At temperatures below 28 °C, none of the fish reached endpoint within the 5-min maximum. At 28 °C four of five fish reached endpoint, and fish reached endpoint more rapidly as temperature increased further. Fish transferred to temperatures above 28 °C had higher swimming speed immediately after transfer and maintained a high swimming speed until just before loss of equilibrium. Their behaviour was from the start characterised by collisions into tank walls and head shaking. Just before loss of equilibrium they started breaking the surface of the water, swimming in a circle pattern and in some instances displayed a side-wise bending of their body. In other words, salmon transferred to temperatures above 28 °C showed instant behavioural responses indicative of nociception or pain.
Highlights
As the salmon louse (Lepeophtheirus salmonis) has increasingly developed resistance against chemotherapeutants, thermal delousing by exposing fish to heated water has become the most used delousing method in salmonid and especially Atlantic salmon (Salmo salar L.) aquaculture in Norway (Gismervik, Gåsnes, Nielsen, amp; Mejdell, 2018b; Overton et al, 2018)
We studied the behavioural response of Atlantic salmon acclimated to 8 °C when transferred to temperatures in the range 0–38 °C
While none of the fish exposed to 28 °C did so, with time to endpoint decreasing with increasing temperature
Summary
As the salmon louse (Lepeophtheirus salmonis) has increasingly developed resistance against chemotherapeutants, thermal delousing by exposing fish to heated water (appr. 28–34 °C, sometimes higher) has become the most used delousing method in salmonid and especially Atlantic salmon (Salmo salar L.) aquaculture in Norway (Gismervik, Gåsnes, Nielsen, amp; Mejdell, 2018b; Overton et al, 2018). As the salmon louse (Lepeophtheirus salmonis) has increasingly developed resistance against chemotherapeutants, thermal delousing by exposing fish to heated water Thermal delousing has been promoted as an environmentally friendly delousing method, as it only uses heated water. Concerns have recently been raised about thermal delousing being experienced as painful by the fish (Poppe, Dalum, Røislien, Nordgreen & Helgesen, 2018). The Norwegian Food Safety Authorities are concerned that thermal delousing violates EU directive 98/58/EC and the Norwegian animal welfare act stating that keepers of animals, including fish, must ensure that the animals are treated well and are protected from danger of unnecessary stress and strains
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