A biochemical study of mucus lysozyme, proteins and plasma thyroxine of Atlantic salmon (Salmo salar) during smoltification

Abstract

Abstract Smoltification is a complex developmental phase encompassing hormonal, metabolic and osmoregulatory changes, enabling the Atlantic salmon to migrate from fresh water to sea water. The biological interface between fish and their aqueous environment is a mucus coat composed of biochemically diverse secretions, which have been implicated in respiration, disease resistance and osmoregulation. In this study mucus and blood were sampled from hatchery-reared Atlantic salmon over the period of smoltification. Our aim was to investigate changes in mucus proteins and enzymes and to monitor plasma thyroxine during smoltification. Mucus protein analysis was carried out by sodium dodecyl sulphate (SDS)-PAGE and banding patterns were observed and compared over smoltification. The protein profile results revealed that mucus profiles for individual fish within sample groups were very similar. There were differences in protein profiles between groups sampled on different dates. Significant and consistent changes were observed in the 30- and 14.5-kDa regions of the protein profiles. The intensity of protein bands in the 30-kDa region decreased during the 12-week period. The 14.5-kDa band increased in intensity over time and co-migrated with the lysozyme standard (Hen Egg White Lysozyme, HEWL), which has a molecular mass of 14.2 kDa. Lysozyme, protease and carbonic anhydrase activities were investigated in mucus samples. Lysozyme activity (60–264 IU/ml) was detected in all mucus samples assayed over the period of March to May. There was a significant increase in this activity (264 IU/ml, p p Visual assessment of the salmon showed full smolt characteristics by mid-May. There was an overall increase in plasma thyroxine during the smoltification period. A significant surge in plasma thyroxine (21–36.3 nmol/l, p