The effect of climate related stressors on liver in Atlantic salmon (Salmo salar)

Abstract

Climate change impacts the average temperature in the world’s oceans, as well as other environmental parameters, such as oxygen levels. These changes are threatening and challenging animals living there, including farmed species. In this trial Atlantic salmon (Salmo salar) were exposed to fluctuating temperature (12-17C) as a single stressor, and in combination with lowered level of dissolved oxygen (~70%), to evaluate how combined environmental stressors affect the fish health- and welfare. The two groups exposed to environmental stressors were compared to a control group (12C). In earlier studies, both elevated temperature and lower levels of oxygen have resulted in signs of reduced fish health- and welfare. Combining these stressors, as well as exposing salmon to a fluctuating temperature regime, can provide important knowledge connected to the affect climate related stressors may have on caged salmon, when, for example, a heatwave occurs. In this study the salmon liver was major focus, because this organ can provide important information regarding the overall health- and welfare of the fish. The parameters studied were metabolism, stress, apoptosis, and immune response, as well as growth, scale loss, and gill score. The methods used were histology and gene transcription. In addition, development of a new digital histopathological tool for salmon liver was commenced. Exposing fish to conditions simulating heatwaves, as well as lowered level of dissolved oxygen, resulted in cellular differences and morphological changes in the liver, such as reduction in lipid storage, increased amount of connective tissue, and higher occurrence of melanin spots. Further, these changes may affect the homeostatic balance in the fish, thus causing reduced health and welfare. The fish exposed to environmental stressors combined had the lowest growth-related indicators, as well as the highest score of examined welfare parameters, indicating reduced fish welfare compared to the control group. However, there is a need for further studies in order to generate more knowledge on how climate related stressors affect the Atlantic salmon, and to understand the biological effects of the changes observed in this study. Regarding the development of the digital histopathological tool for salmon liver, results from the final version showed several errors, indicating that the algorithm needs additional training. In this trial, approximately 80 hours of drawing was done in the training of the algorithm, resulting in increased accuracy in detecting the different components, and for some features, such as melanin, there were no errors in detection. Continuing developing tools like this may be useful in the evaluation on how climate change effect the liver of the fish, as changes may be difficult to observe using traditional histology.