The interaction between immune competence and stress responses in relation to fish health problems in aquaculture. Deel I: "Effects of genotype and genotype-environment interactions on the stress responses in common carp exposed to chronic stress"

Projectleader: Prof.dr. C.J.J. Richter
Researcher: Dr.N.M. Ruane
Project period: maart 1998 mei 2002

Summary
Stress and stress disorders in a teleost fish, the common carp Cyprinus carpio L. This 4-year project focussed on the measurement of stress levels in common carp under aquaculture conditions and how this response to stress could be influenced by genetic and environmental factors.
There is a clear need for basic information on the mechanisms behind the interaction between health of farmed fish and how they respond to their husbandry environment (stressors). Genetically inherited stress disorders in model fish species can help increase our understanding of the interaction between the immune system and the stress response in fish, and help in developing guidelines for welfare regulations in fish farming in Europe.

Objectives: The general objectives of the project were to investigate to what extent environmental factors (e.g. increased rearing densities) and genetic factors (different carp strains), influenced the response of the fish to a standardised acute stressor.

Methodology: As a model, we used all-male isogenic carp. This strain is a cross between two homozygous inbred strains, produced by androgenesis. Using isogenic animals enhances the power and reproducibility of the experiments, and allows for comparisons of rearing conditions on the same genetic background. We used a standardised 3-h net-confinement stressor, which produces a reproducible acute stress response in carp. Netting is a common disturbance on fish farms, and easy to set up in the laboratory.

Results: We first determined the effects of feeding history and rearing densities (100 kg/m3) on the acute stress response. Fish reared at high densities initially exhibited a slight increase in plasma cortisollevels, but values had returned to control levels after 3 days. No effect on growth was observed over a 28-day period, but the response to additional net-confinement was modified, suggesting that disturbance of fish at high densities should be avoided until the animals become adapted. Periods of low feeding preceding a stressor also compromised the stress response. Chronic stress levels in fish are difficult to establish through the measurement of blood indicators. A technique to measure cortisol in water was developed and a correlation was found between fish numbers in a tank and cortisollevel in the water.

Various researchers have shown that genetically different fish strains can exhibit different cortisol stress responses. One of our inbred strains (ES) consistently showed a highly reduced cortisol response during net-confinement. The physiological basis is a heritable defect in the synthesis of the stress hormone cortisol. These fish are valuable models to investigate the interactions between the stress response and other functions, such as growth, reproduction and the immune response.