Health survey of selected stocks of commercially exploited Norwegian bivalve molluscs

This paper describes the first systematic health survey of commercially exploited bivalve molluscs in Norway. The main aim of the work was to screen selected stocks of the native flat oyster Ostrea edulis and the introduced Manila clam Ruditapes philippinarum for the presence of pathogenic agents. Samples were also collected from the indigenous clam Ruditapes decussatus and the scallop Pecten maximus, as well as from the introduced Pacific oyster Crassostrea gigas. The indigenous and cultured blue mussel Mytilus edulis and the Iceland scallop Chlamys islandica were not included. No abnormal mortalities were registered, nor were any serious pathogenic agents detected. Some tissue anomalies and some commensals or parasites were reported. These included thigmotrich ciliates on gills and in digestive diverticula, and intracytoplasmic Rickettsiales-like inclusions in epithelia of digestive gland diverticula of oysters, and a cyclopoid Modiolicola like copepod on scallop gills. Polymorphic, presumably inorganic, granulations were observed in connective tissues of oysters. Mature gonads were observed in the Manila clams. Based on the results, some advice on monitoring and management of the stocks is offered