Responses of Atlantic cod to high water temperature and bacterial infection

Aquaculture is one of the fastest growing food production sectors in the world and further expansion is expected throughout the 21st century. However, climate change is threatening the development of the sector and action is needed to prepare the industry for the coming challenges. Using downscaled temperature projections based on the Intergovernmental Panel on Climate Change (IPCC) climate projection (Shared Socioeconomic Pathway, SSP2-4.5), we analysed potential future temperatures at a selected Atlantic cod (Gadus morhua L.) farm site in Northern Norway. Results showed that the farming area may experience increased temperatures the next 10–15 years, including more days with temperatures above 17°C. Based on the predicted future conditions, we designed a study with Atlantic cod (Gadus morhua L.) to evaluate effects from high temperature alone and in combination with Fransicella noatunensis infection. Fish were kept at 12°C and 17°C for eight weeks and samples of skin and spleen collected at different timepoints were analysed with transcriptomics, histology, scanning electron microscopy and immunohistochemistry. Atlantic cod, produced at the Center of Marine Aquaculture (Nofima, Norway), were transferred to The Aquaculture Research Station (Tromsø, Norway). Fish were kept for acclimatization for 4 weeks in flow-through tanks at 8 oC, and fed Amber Neptun (Skretting, Norway). Temperature increase was performed in two stages. First the fish were acclimated to 12oC for one week. Second, 120 fish were distributed in 4 tanks, 30 fish per tank, and temperature in two tanks increased to 17 oC. Temperature in the two remaining tanks were kept at 12oC. After acclimatization, fish were moved to the challenge facility at the station where they were kept in 200 L tanks with similar conditions. First day after transfer (T0), three fish from each tank were killed by an overdose of anaesthetics and sampled. At week 8, fish were challenged by cohabitation with Francisella noatunensis (type strain NCIMB 14265).