Changes in bacterial abundance and community structure in cage fish-culture caused by water turbulence during feeding

Abstract

Water and sediment quality conditions in and around a cage culture unit located in Wismar Bay (Western Baltic Sea, Germany) were investigated during summer 1994 using bacterial group indicators (Enterobacteriaceae, saprophytic bacteria and Vibrio, determined on selective media). Fish were fed once daily with extruded commercial pellets. Over a period of 8 days samples were taken before and after feeding at different sampling points close to or at the cages. The results showed that a drastic (up to 10 times) increase in different bacterial groups occurred inside the cages 10 min after feeding as well as occasionally in the surrounding water. Additional electron-microscopic observations of the bacterial biofilm on the cage nets revealed that a decrease (in comparison with the original number) in total bacteria on the net materials of 9.5 up to 42.8% occurred after feeding. It was concluded that water mixing caused by frenzied swimming just before and during feeding caused a release of bacteria which were growing vigorously on the surfaces of the cage system. Since the pathogenity and hydrophobicity of bacteria are closely related, this process of redistribution of bacteria into the water column could constitute an important stress factor for fish, which could probably also have a negative effect on fish production.