Environmental influences on muscle growth and flesh quality in farmed Atlantic salmon (Salmo salar L.)

Abstract

Chapter 1 - General Introduction : The general introduction sets out the rationale for the study and it's applicability to aquaculture. A description of the life history of the Atlantic salmon {Salmo salar L.) is given, together with a description of the origin and current status of fish farming. The structure and function of fish muscle is explained in addition to its growth, development and inherent plasticity. Finally, the environmental influences on muscle growth in fish are discussed and this is followed by the aims of the thesis. Chapter 2 - Egg incubation temperature influences muscle fibre recruitment during seawater stages of Atlantic salmon (Salmo salar L.) : Eggs of Atlantic salmon {Salmo salar L.) were incubated in either heated or ambient water temperatures. The comparative whole animal and muscle growth performance of the two groups was monitored for 26 months. The number of white muscle fibres per myotome was significantly higher at hatch in the ambient group, resulting in a greater muscle cross sectional area. However, muscle fibre number was almost four-fold higher in the heated group at the SI parr stage relative to the ambient group. Six months following seawater transfer the number of fast white muscle fibres was not significantly different between groups. The 5th and 10th percentile fibre diameters decreased significantly in both groups during winter and spring, indicating a relative increase in the contribution of new fibre recruitment which was correlated with seasonal lows in water temperature and day-length. The results indicate that egg incubation temperature has persistent effects on muscle cellularity throughout the freshwater and early seawater stages of Atlantic salmon. Chapter 3 - Advanced photoperiod treatment affects smoltification and muscle growth in Atlantic salmon (Salmo salar L.) : Juvenile Atlantic salmon {Salmo salar L.) were reared under three different photoperiod regimes: constant light (CL), simulated short winter (SW) and simulated long winter (LW) from July to December 1997. Muscle fibre frequency analysis suggested increased contribution of muscle hypertrophy towards somatic growth in all groups during summer and autumn while increased fibre recruitment was found in the CL and SW groups during winter. The number of white muscle fibres per myotome was linearly related to total white muscle cross-sectional area with no significant differences between groups. This indicates that the SW treatment increased muscle growth but that the relative contribution of hypertrophy to fibre recruitment was not affected by photoperiod treatment at this stage in the life cycle. Chapter 4 - Cage systems and light manipulation influence muscle fibre cellularity and harvest quality of underyearling (0+) Atlantic salmon in seawater : Underyearling (0+) Atlantic salmon {Salmo salar L.) were reared in conventional sea cages and closed pens systems under natural light and additional continuous light for 14 months up to harvest. The comparative muscle cellularity of all groups was assessed in November 1995, together with harvest quality parameters including texture (hardness), percentage lipid and fillet colour. Muscle fibre frequency analysis revealed increased variation in fibre frequency in fish reared under constant light regimes. Two-way analysis of variance showed no significant differences in the 5th, 10th or 50th percentiles between treatment, however a significant effect of light treatment was found on the 95th percentile diameter value. Muscle fibre density (fibres/mm2) was not significantly different between treatments although a within-fish comparison of the steak-section revealed significantly higher fibre densities in the dorsal compared to the lateral epaxial area. Lipid was positively correlated to body mass and regression analysis revealed no significant relationship between muscle fibre density and fillet hardness as measured by texture profile analysis or fillet colour measured by Roche colour card. Chapter 5 - Instrumental measurements of texture and it's relation to muscle fibre density in Atlantic salmon (Salmo salar L.) : Atlantic salmon {Salmo salar L.), l-5kg (pre-gutted body mass) were subjected to a series of standard flesh quality tests along the length of the fillet and the results related to muscle fibre density. The NQC cut had a firmer texture than the Mowi. The NQC cut was also found to be significantly leaner with a lower % lipid than the Mowi region. Significant differences in Hunterlab® colour score were also found with the NQC region having significantly higher readings than those in the Mowi region. Muscle fibre density (MFD) (no. muscle fibres/mm2 muscle cross-sectional area) was found to be significantly higher in the NQC region than the MOWI region. A weak correlation was found between body mass and MFD in the Mowi but not in the NQC region. Muscle fibre density did however, show a positive correlation with maximum shear force. The results indicate that muscle fibre density influences texture in Atlantic salmon. The potential for the manipulation of muscle cellularity and hence flesh quality are briefly discussed. Chapter 6 - General Discussion : The major findings of the thesis are discussed in relation to previous work in addition to possible implications for salmon aquaculture and the scope for future study is evaluated.