In cultured fish, stress is regarded as an important cause of impaired health, and particularly resistance to infectious disease, since it can directly or indirectly affect the immune system. Methods to reduce stress responses associated with such immunosuppressive effects are thus actively searched out. It is assumed that domestication may be associated with a reduction in stress responsiveness because stress-resistant animals may be positively selected along captive rearing as a result of an improved fitness. Fish domestication is very recent and the adaptive potential to farming conditions may still be high for numerous species including Eurasian perch for which adaptability to captivity conditions is not yet clarified. The general objective of the present thesis was thus to evaluate the physiological and immune responses of Eurasian perch juveniles to some potentially stressful husbandry conditions and to examine whether these responses can be modulated along domestication. In this respect, we exposed Eurasian perch juveniles from distinct generational levels (F1 vs F4/F5) to chronic confinement, single or repeated hypoxia, and repeated water emersion. We measured usual stress indicators and several immune parameters. Proteomic analyses were also performed to resolve serum proteins. Finally, fish were genetically characterized using a microsatellite approach. Our results showed that Eurasian perch was affected by chronic confinement and environmental hypoxia. Under chronic confinement, only F1 juveniles displayed an increased sensitivity of the HPI axis (higher serum cortisol rise) to an acute handling stressor and a cellular stress response (higher HSP70 level in hepatocytes) in comparison to the respective control groups. Proteomic analyses further evidenced down-regulation of some immune serum proteins in F1 individuals while up-regulation was observed only in the F4 fish. During hypoxia, both generations displayed serum and hematological readjustments (spleen contraction leading to increased red blood cell count, hyperglycemia, up-regulation of C3 components). Under repeated exposure to this stressor, the results suggested the development of acclimation mechanisms (lower hyperglycemia, increased serum transferrin abundance) but also immune depression (lower lysozyme activity and abundance of C3 components). These responses appeared to be similar in both generations. Therefore, chronic confinement and repeated hypoxia negatively affected some immune parameters and domestication reduced the stress responsiveness of fish confronted to chronic confinement but not to hypoxia, probably owing to the different capacity of these stressors to severely affect internal homeostasis. Repeated water emersion did not result in an important chronic stress response since only alpha-2-macroglobulins (a2Ms) abundance was up-regulated in the serum of stressed fish. With the exception of the acute cortisol response (20 min post stressor) observed following netting and anesthesia, we did not detect any cortisol increase throughout the experimental periods when fish were confronted to chronic confinement, single or repeated hypoxia or repeated water emersion. This result suggested that cortisol is not an ideal stress indicator in perch because its increase might be absent or short-lasting under the tested conditions. Using cortisol, it might be difficult to detect such stressing situations since appropriate sampling events should be performed in a relatively short-time window. Domestication was also accompanied by complex changes in the abundance of some immune and acute phase protein isoforms/fragments (i.e. C3 complement components, Wap65, LAAO, fibrinogen, transferrin, Ig heavy and light chains and a2Ms). At present, it is not possible to conclude on the overall disease resistance of domesticated fish since we mainly investigated immune changes in serum and since bacterial challenges were not conducted in optimal conditions. More information should also be obtained on the differentially expressed protein isoforms and fragments as they can greatly differ in their activities. Finally, we observed that the fish from higher domestication level (F4, F5) were globally able to develop similar physiological and immune responses to stressors than the F1 generation despite a low genetic diversity. Actually, the microsatellite analysis highlighted a reduced allelic number, heterozygosity and higher genetic homogeneity along generations. Inbreeding was however not detected (low occurrence of linkage disequilibrium and negative Fis values). Taken together, these results confirm that inbreeding depression did not occur and suggest that loss of genetic diversity at neutral loci and possibly at coding regions may, to some extent, not be considered deleterious under stable and nearly optimal farming conditions. Further investigations such as QTL or associated genetic marker studies would be needed to further improve our knowledge on the connection between the observed phenotypic and genotypic data. All in all, it can be hypothesized that a reduction in stress sensitivity may be part of domestication, depending on the stressor characteristics, and that this change is rather independent of the genetic diversity in farmed stocks.
Fish domestication in aquaculture: modulations of stress physiology and immune system in a percid fish, the Eurasian perch (Perca fluviatilis L.)
Douxfils, J. (Author). 1 Mar 2012
Student thesis: Doc types › Doctor of Sciences