A Biochemical Study of Mucus Lysomzyme, Proteins and Plasma Thyroxine of Atlanctic Salmon (salmo salar) during Smoltification

Atlantic salmon prepare to migrate from fresh water to sea water during a phase of great physiological change known as smoltification. In the context of stock enhancement programmes and the conservation of Atlantic salmon stocks, the release of smolts from freshwater to sea is of major significance. Accordingly, this study set out to monitor some of the physiological and biochemical changes, which allow the Atlantic salmon to migrate from freshwater to seawater. Our research focused on skin mucus as a possible candidate for the development of a non invasive test and sought to elucidate biochemical or significant protein changes in mucus during parr-smolt transformation. Mucus and blood were sampled from hatchery reared Atlantic salmon during smoltification in 1996 and 1997 to investigate changes in mucus proteins and enzymes and to monitor plasma thyroxine. Mucus protein analysis for both years was carried out by SDS-PAGE. The mucus protein profiles for individual fish within sample groups were found to be very similar, while differences between groups sampled on different dates were observed in both years. Significant and consistent changes were observed in the 30kDA and 14.5kDA regions of the protein profiles. The intensity of protein bands in the 30kDa region decreased during the twelve-week period. The 14.5kDa band increased in intensity over time and co-migrated with the lysozyme standard (Hen Egg While Lysozyme) which has a molecular mass of 14.4kDa. Lysozyme, protease and carbonic anhydrase activities were investigated in mucus samples from 1997. Lysozyme activity (60-264 units/ml) was detected in all mucus samples. A significant increase in lysozyme activity (264 units/ml,p\u3e0.01) and in the soluble protein content (9.2 mg/ml,p\u3e0.0001) of the mucus was observed over the test period and found to correlate with the increase in the visual intensity of the 14.5 kDa band. Carbonic anhydrase activity was not detected in mucus samples. Some apparent esterase activity was observed. Trypsin-like protease activity was not detected by enzyme assay. Post-electrophoretic detection of protease activity did find a high molecular mass protease band in two samples in May (15 May, 29 May). Visual assessment of the salmon showed full smolt characteristics by 1 May in 1996 and by mid-May in 1997. In 1997 plasma thyroxine was monitored over the sampling period and a significant surge in plasma thyroxine (21 to 36.3 nmol/l,p\u3e0.01) was detected at the end of May. The visual intensity of the 14.5kDa mucus protein band also increased significantly during this month, while the 30kDa bands decreased over this period. These findings suggest that the changes in these mucus protein bands warrant further investigation to determine the physiological significance of the changes observed in relation to the smoltification process