Climate change is no longer a distant threat to aquaculture production . In recent years, its ramifications have become more evident and prevalent, especially with reports of several episodes of long periods of elevated or erratic temperatures , and low dissolved oxygen levels. In addition, acidified aquatic environments due to carbon dioxide (CO2 ) have been identified and included in future climate projection models . These climate stressors have resulted in mass mortality events in several fish farms. Besides these widely known climate stressors , there are several secondary stressors that are less studied but pose significant compound concerns. For example, jellyfish blooms have affected marine farm s and their frequent occurrence in recent years has been implicated to prolonged period of elevated temperature. The severity of some diseases has also been altered by the changing environments, especially where the virulence and pathogenicity of the causative agent is temperature dependent .
The olfactory organ of fish serves a crucial role in sensing stimuli , odorants and chemical signals in the environment. Its anatomical position makes it a portal of entry for a number of pathogens. Hence, b esides its function in olfaction, the presence of mucosa-associated lymphoid tissue in the olfactory rosette highlights its important role in immunity. Earlier studies have shown that the olfaction of fish is highly vulnerable to climate change, particularly with CO2 exposure. We have previously documented that elevated temperature affected the olfactory organ of Atlantic cod ( Gadus morhua ), which carried associated implications on the nasal responses to Francisella noatunensis , an intracellular bacterium which is the causative agent of francisellosis. It remains to be studied how repeated cycles of prolonged exposure to elevated temperature affect the nasal responses and what are the implications associated when fish are exposed to secondary stressors.
A trial was designed where a group of Atlantic cod were exposed to three temperature scenarios: 1) constant temperature at 12oC, the common production temperature, 2) constant elevated temperature at 16oC, the upper thermal limit, and 3) fluctuating temperatures, 12oC→16oC→12o C. Three weeks after, a group of fish were exposed to secondary stressors, i.e., jellyfish bloom and F. noatunensis. Preliminary results indicate that nasal morphometries changed following exposure to a jellyfish bloom. Increased number of mucus cells and prevalence of olfactory epithelial hyperplasia were observed. In addition, lamina propria degeneration was identified in several individuals . The occurrence of these changes was higher in Gr oups 2 and 3. In addition, inter-treatment differences in the regulation of olfactory receptors, immune and stress markers were documented among the groups. In the infection trial, the pathogen was detected in the olfactory organ, though differentiating bacterial load among the groups could not be conclusively established. Interestingly, pathologies related to francisellosis were more severe in Groups 2 and 3. Genome-wide transcriptomic analysis is currently on-going and will be presented at the meeting.
The work was supported by the Norwegian Troms and Finnmark county (TFFK2021-179, SecureCod) and the Norwegian Research Council (No. 194050, Insight; No. 335990, Frantic ).