Effect of lowered oxygen in aquaculture on rainbow trout muscle quality investigated by a proteomic approach

In the last decade there has been a growing understanding of the health benefits of fish consumption. This has lead to an increased interest in studies examining the parameters affecting eating quality of fish from aquaculture. Especially softening of fish muscle is a major problem since it significantly reduces the quality of the major edible part of the fish. Fish in aquaculture are stressed by handling (e.g. sorting, crowding and transportation) prior to slaughter and it is well established that such stress leads to reduced textural quality of the fish. However, it is still unclear which specific stressors actually influence quality and also to what extend. The most likely stressors during handling are the close contact between the fish or changes in environmental parameters such as oxygen, CO2 or ammonia, individually or in combination. As a first step to explore the impact of stressors, a proteome study of the biochemical mechanisms involved in the changes seen in trout muscle following a drop in oxygen level was conducted. In the present study rainbow trout was kept at low densities in tanks, maintaining reduced oxygen (30% of normal oxygen tension), as the only stressor. The fish were sacrificed at different time points (1 hour, 2 hours, 5 hours and 24 hours) after the onset of hypoxia, and muscle samples were taken from each individual fish for proteome analysis. Protein expression profiles of the samples were achieved by 2-dimensional gel electrophoresis. In this way proteins were separated within an apparent range of molecular masses from 10 to 120 kDa and pI values from 4 to 7. Protein spots (31), which individually or in combination with other spots varied according to hypoxia were found by multivariate data analysis (partial least squares regression) on group scaled data (normalised spot volumes) followed by selection of significant spots by jack-knifing. Tandem mass spectrometry was used to identify protein spots of interest. The identified proteins were involved in a number of different biological functions including energy metabolism, hypoxia tolerance, heme transport and protein degradation. The direct effect of these protein changes on the eating quality of the fish need further elucidation, but it clearly shows that lowering of oxygen, alone will affect a number of processes. When compared with already obtained data on protein changes in relation to food quality, this can help pinpointing which changes in the fish muscle are due to hypoxia and which are down to other stressors. This can aid the aquaculture industry when evaluating the type of stressors mostly affecting food quality, allowing an optimisation of rainbow trout handling accordingly.