Yellowtail Kingfish (Seriola lalandi) have been farmed for over a decade in sea cages in the waters of Spencer Gulf, South Australia. Substantial fluctuations of the water temperature in Spencer Gulf occur, reaching 24ºC in summer and dropping below 12ºC in winter. Inclusions of soybean meal in feeds for Yellowtail kingfish are becoming common. High dietary inclusions of soybean meal coupled with low water temperatures are thought to contribute to the development of sub-acute enteritis in the hindgut of Yellowtail kingfish. Prior to this study, the role the mucus layer plays in protecting the underlying mucosa had not been investigated in fish.

In this study, fish were fed increasing dietary inclusion levels of solvent extracted soybean meal (SE SBM), to apparent satiation twice daily, at water temperatures of 18ºC and 22ºC for 34 days. At the conclusion of the study, the intestinal tract was removed, with no fish exhibiting visual features of hindgut inflammation. Samples were collected for histological evaluation, revealing a significant reduction in mucus layer thickness in the hindgut of fish fed increasing dietary inclusion levels of SE SBM. Water temperature had a significant effect on mucin composition. A more profound increase in neutral mucins in the hindgut was observed at 18ºC, while a more profound increase of acidic mucins was evident at 22ºC. Fish fed 20% and 30% dietary inclusions of SE SBM at 18ºC had a significant increase in goblet cell number. Although sub-acute enteritis was not induced in this study, it is evident that the intestinal barrier was compromised. Based on observations from this study, SE SBM inclusion levels in the diet for Yellowtail kingfish of this size range should be restricted to 10% at 18°C and 20% at 22°C. Further studies are required to assess alteration to intestinal morphology and the development of sub-acute enteritis in Yellowtail kingfish reared in colder waters indicative of those experienced during winter months.