Colour changes in fish are often stress-induced. The main pigmentation controlling hormones α-Melanocyte Stimulating Hormone (αMSH) and Melanin-Concentrating Hormone (MCH) are pleiotropic and not only control skin pigmentation but also regulate the response to stressors. In vertebrates in which the pigmentation of the skin can be changed by hormonal stimulation, the colour of the background and the illumination are determining factors for the intensity and / or the pattern of skin pigmentation.

The aim of the present task was to identify the major environmental and husbandry factors implicated in skin colour regulation and to evaluate the stress response of red porgy in relation to changes in the chromaticity parameters. The study was to evaluate the relationship between pigmentation and stress and to identify the appropriate culture conditions for the development of natural hue.

The objectives of the experiments were:

• to determine the effects of environmental and culture related factors (light, background colour, handling, and stocking density) on colour change,
• to evaluate the relationship between pigmentation and stress,
• to identify the appropriate culture conditions for the development of natural hue.

The experimental approach involved:

1. the investigation of the effect of illumination and background colour on skin colour,
2. the study of the changes in colour after acute capture and chronic crowding stress,
3. the study of the mortem

Results

Results clearly showed that background, lighting spectrum, light intensity and stocking density have an important effect on skin chromaticity parameters. White background increases skin brightness in levels similar to that of wild fish but reduces skin chroma. Blue lighting spectrum increases skin brightness in dark background (black and red) held fish but did not show any significant effect on hue or chroma. Low light intensity resulted also in higher L in fish held under dark backgrounds.

Stocking density affected significantly L, Hue and ECI. In particular, fish held under dark backgrounds and high stocking density showed lower L and ECI values than that held at low stocking density. There was no effect of background, lighting spectra, light intensity and stocking density on the number of melanophores per mm2 of skin. Higher skin melanin content was observed in red porgies held under dark backgrounds and high light intensity and stocking density compared to that in fish held under low light intensity and stocking density.

In all experiments, there was no significant effect of background colour, lighting spectrum; intensity and stocking density on the circulating stress indicators determined (cortisol,  αMSH, glucose, lactate, osmolality, electrolytes). In addition, there was no correlation between chromaticity parameters and the used stress indexes. However, significant differences were observed in circulating catecholamines and in the complement and lysozyme activities. In all experiments skin paling was corealted with increased norepinephrine plasma concentrations. High light intensity induced a significant decrease in lysozyme activity. Full lighting spectrum seems to induce higher levels of lysozyme than the blue spectrum and white background induces earlier changes than the black and red background, but later on (Day 32) fish in all groups showed the same type of recovery dynamics.

The results altogether indicate that the observed differences in skin lightness are related to differences in the motility of skin chromatophores; melanophores in pale fish are aggregated while in control fish dispersed. Concerning the neuroendocrine regulation of this motility, melanophore aggregation is not mediated through circulating cortisol or MSH, but through changes in plasma catecholamines.