Silje Kittilsen defended her PhD thesis on December 4th 2009

Ane Gro Siri Skjelfjord

Stress responsiveness in Salmonid fish: The cortisol response and associated traits

Intensive husbandry production systems such as aquaculture may be stressful to the animals. In teleost fish, like in other vertebrates, individual differences in neuroendocrine and behavioural responses confer different susceptibility to stress and disease. Consistent trait correlations often referred to as coping styles, are partly heritable and may be of importance to both the welfare of individuals and production parameters. Selection for divergent cortisol responses in rainbow trout (Oncorhynchus mykiss) has yielded two strains also showing consistent differences in behaviour over three generations. Low cortisol-responsive fish (LR) are typically bolder and start to feed more rapidly after stressful experiences, and usually become socially dominant over high cortisol-responsive (HR) individuals.

The present work explores variation in the cortisol response and some central associated traits in farmed salmonid fish, including behaviour, morphology and susceptibility to disease. Increased knowledge on variation in stress responsiveness and how to identify such variation are interesting from an applied perspective, as selection for reduced stress responsiveness might be one important tool to alleviate some of the problems and concerns related to the health and welfare of farmed fish. Furthermore, phenotypic correlates of stress responsiveness are of fundamental biological interest and can add to current knowledge on the ecological and evolutionary implications of physiological-behavioural trait correlations.

The results show that variation in the cortisol response is associated with both behavioural and morphological characteristics. Locomotor behaviour during acute confinement stress correlated to stress-induced cortisol levels and was significantly different between 10 families of Atlantic salmon (Salmo salar) from a commercial Norwegian breeding programme. Locomotor behaviour during confinement also showed a positive correlation with mortalities in an infectious pancreas necrosis virus (IPN) challenge carried out on the same sibling groups in the breeding programme.

Furthermore, behavioural characteristics associated with cortisol responsiveness appear to be more flexible than previously reported. LR and HR rainbow trout of the 3rd generation shifted behavioural profiles after a short period of food deprivation and transport to a new rearing site. In contrast to previous findings, HR fish now regained feeding sooner in new environments and became socially dominant over LR individuals.

During the work with these lines it was discovered that selection for divergent cortisol responsiveness not only led to differences in behaviour, but also in their visual appearance, more specifically the degree of dermal melanin-pigmentation. Low cortisol-responsive fish were found to be consistently more spotted when compared to HR individuals. A negative relationship between post-stress cortisol levels and degree of melanin-pigmentation was confirmed in another salmonid species, Atlantic salmon. Furthermore, individual salmon with a higher number of melanin-based skin spots harboured fewer mature female salmon lice (Lepeophtheirus salmonis) in two naturally infected broodstock populations. These results fit with established knowledge on glucocorticoid suppression of immune function.

The results support existing knowledge on trait correlations in behaviour and physiology in salmonid fish, but also reveal novel associations between such correlations, morphology and disease susceptibility. Furthermore, behavioural components associated with heritable differences in cortisol responsiveness are more flexible than previously reported. This illustrates the complexity of the factors that cause and sustain individual variation in behaviour and physiology. Thus, further scientific effort is necessary to unveil both proximate and ultimate mechanisms in the relationship between behaviour, physiology and fitnesss related traits such as immunocompetence.

Updated: 08.12.09
Printerfriendly version

Del med en venn:

Department of Animal and Aquacultural Sciences

P.O. Box 5003
N-1432 Ås

Phone: +47 64 96 51 00
Fax: +47 64 96 51 01

E-mail: iha@umb.no

Webmaster: Janne Karin Brodin