| Summary: | Constantly increasing water temperatures caused by climate change, can lead to major consequences for the welfare of farmed fish. The interactions between the expected changes and the fish physiology are still not understood, in specific how the changing environment may affect the fish's health, and how this may impact aquaculture production in general. The fish's gill health can be used as an indication of the fish's overall condition, and a greater understanding of this organ can help to influence decisions that salmon farmers make for operational procedures, e.g., by targeting particularly vulnerable environmental situations in combination with delousing and transportation. Identifying temperature thresholds and tolerance can lead to modified procedures for monitoring and handling of fish. In this thesis we have looked at how elevated and fluctuating temperatures affected gills in Atlantic salmon and compared it with fish from a control group at lower temperature. The hypothesis was that fish at elevated and fluctuating water temperatures may induce changes in gills that may further impact the fish overall robustness, compared to fish at constant lower temperature. Histological changes and gene transcription in gills related to temperature stress may lead to increased understanding of this topic. The results were consistent with the assumption, showing that elevated and fluctuating temperatures have an effect on gill health in Atlantic salmon, which is previously linked to negative effects for the fish. Gills from fluctuating temperature regime generally had several morphological changes compared to gills from the high-temperature group, including the presence of fusion, clubbing and lifting. Changes in gene responses were generally not observed, however, important markers for oxidative stress as Cu/zn sod were down regulated and Mn SOD were up regulated. Genes were mostly down-regulated in the high and fluctuating groups, indicating a general down-regulation of genes. Overall, results show that fluctuating ...
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