Factors affecting pigmentation quality in Atlantic salmon (Salmo salar L.) at elevated temperature

Pigmentation quality is one of the most important criteria in Atlantic salmon (Salmo salar) products. The red colour is a result of pigment carotenoids such as astaxanthin (A\\(_x\\)) and canthaxanthin (C\\(_x\\)) deposited in myofibrillar proteins of white muscle tissue and the concentration of these pigment carotenoids correlate well with the red colour intensity. Sea water temperatures in Tasmanian commonly exceed 20¬¨‚àûC for prolonged periods in summer. This is associated with reduced pigmentation quality, which is characterised by reduced red colour intensity and increased heterogeneity in pigmentation on the fillet surface of individual salmon, leading to substantial product downgrade. This phenomenon is typically concurrent with reduced feed intake rates or even the cessation of feed intake. The aim of the thesis was to improve the fundamental understanding of factors and mechanisms that reduce pigmentation quality in salmon exposed to elevated temperature in order to improve pigmentation management in salmon farming. Post-smolts that doubled initial weights at an elevated temperature (19.5¬¨‚àûC) showed increased concentrations of A\\(_x\\) in white muscle compared to fish held at a control temperature (15¬¨‚àûC). However, in contrast to 15¬¨‚àûC, the concentration of A\\(_x\\) was lower in the anterior/dorsal fillet cut (ADC), compared to the dorsal Norwegian quality cut (dNQC) at 19.5¬¨‚àûC. Differences in the A\\(_x\\) concentration per unit of crude protein in white muscle between the fillet cuts at 19.5¬¨‚àûC may have indicated differences in the affinity of myofibrillar proteins in muscle to bind A\\(_x\\). A novel experimental in vivo carotenoid depletion model (four weeks starvation-challenge at 19.5¬¨‚àûC) was developed. After starvation-challenge, fish showed a marked drop of redness chromaticity (a*) by image analysis on the ADC. A follow-up study confirmed the image analysis results by a marked reduction of A\\(_x\\) concentration in the ADC, and there was also a reduction of A\\(_x\\) in the dNQC when fish were starved at 20.8¬¨‚àûC for four weeks. Fish starved for the same period of time at 15¬¨‚àûC also showed a loss in A\\(_x\\) concentration in the ADC. Thus, starvation is the main factor that causes reduced pigmentation quality and elevated temperature (20.8¬¨‚àûC) exacerbated the effect. Pigment carotenoids are potent antioxidants, but the concentration of A\\(_x\\) was not associated with oxidative stress (OS) in muscle of either growing or starving fish. Further, an increase in the concentration of the antioxidant ˜í¬±-tocopherol in muscle did not prevent the reduction of a*-values at starvation-challenge. There was no idoxanthin (Ix), the first metabolic product of A\\(_x\\), in the muscle of either growing or starving fish, which showed that metabolic conversion did not explain the differences in the concentration of A\\(_x\\). Further, despite the lipophilic nature of carotenoids and interactions with dietary lipids, the dietary fatty acid (FA) composition did not affect A\\(_x\\) deposition at 15¬¨‚àûC and 19.5¬¨‚àûC in feeding salmon, and lipid fluxes in a subsequent starvation phase at 15¬¨‚àûC and 20.8¬¨‚àûC showed no association with the reduction in the concentration of A\\(_x\\). However, a change in FAs used as energy substrate in white muscle occurred during starvation and was related to the reduction of the concentration of A\\(_x\\). These changes may have indicated that a switch in lipid metabolism may have been associated with the reduction of A\\(_x\\) concentration. Energy homeostasis in animals cells is complex and a change in lipid metabolism is likely linked to other changes which may also affect muscle proteins. Proteolytic processes are elevated in immature salmonids at starvation and in maturing salmonids during spawning migration. During spawning migration, anadromous salmonids starve, use vast amounts of muscle protein as energy substrate and reallocate carotenoids from muscle into other tissues. Globally rising sea water temperatures require more knowledge on the effects of elevated temperature on the biology and product quality of aquaculture species produced in the marine environment. The current research indicated that reduced pigmentation quality in salmon at elevated temperature was not associated with OS, the metabolic conversion of A\\(_x\\) to Ix, or interactions with lipids, respectively. However, the differences in the A\\(_x\\) concentration per unit crude protein at elevated temperature, and changes in energy homeostasis associated with the reduction in A\\(_x\\) concentration at starvation indicated that proteolytic processes could be involved. In order to further the findings of this thesis, the effects of muscle proteolytic processes on pigmentation quality in salmon fillets at elevated temperature warrants further research.