Occurrence of Listeria monocytogenes in a Finnish rainbow trout fish farm

The occurrence of Listeria monocytogenes was studied in one Finnish fish farm during autumn and winter. In the autumn the fish cultured in the near vicinity of the shore were almost all contaminated with L. monocytogenes (15/20). The samples taken from nets, ropes, sea bottom and seawater contained all L. monocytogenes. The farm succeeded in cleaning the processing facilities well in spite of the incoming L. monocytogenes in raw fish. In a thorough survey no other Listeria than one L. innocua was found from a drain. Soon after the first samples another lot of fish was studied and that lot was free of Listeria. These fishes were cultured further in the sea and were not brought to the shore area at any time. The Listeria contamination was found to be local but extensive in the farming area near the shore. During the winter the sea froze and the water temperature decreased to 4°C below the ice. Due to the freezing the possible Listeria contamination sources coming into the sea were minimal. We found that during winter the occurrence of Listeria clearly decreased in fish, the nets and water were also decontaminated