Effects of the harmful diatom Chaetoceros concavicornis on respiration of rainbow trout Oncorhynchus mykiss

Chaetoceros concavicornis and C. convolutus are harmful diatom species which commonly occur in many temperate coastal marine waters and which can cause finfish mortalities when present at concentrations as low as 5 cells ml-' of seawater. Death of finfish, including salmonids, by these harmful Chaetocerosspp. has been suggested to be caused by: (1) microbial infections of damaged gill tissue; (2) hemorrhage of gill capillaries; or (3) suffocation from excess mucus production at the sites of penetration of the gills by the spines. Our data indicate that the barbed spines of these diatoms damage the physical integrity of the respiratory epithelium of rainbow trout Oncorhynchus mykiss. The trout responds to this physical impairment of gill tissue by producing excessive amounts of mucus. The accumulation of this mucus on and between the secondary lamellae inhibits the gill function of O2 uptake (and likely metabolic waste products release). The hypoxic conditions which result within the fish then cause a cascade of events, including anaerobic metabolism. Trout mortalities occur coincident with severe decreases in O2 concentration and pH as well as increases in lactate and glucose concentrations of the arterial blood.