SUBLETHAL CONCENTRATIONS OF THE HARMFUL DIATOMS, CHAETOCEROS-CONCAVICORNIS AND C-CONVOLUTUS, INCREASE MORTALITY-RATES OF PENNED PACIFIC SALMON

The harmful marine diatoms, Chaetoceros concavicornis and C convolutus, possess setae with spinules. As these diatoms pass between the primary and secondary lamellae of salmonids, their barbed setae cause many to be retained in the inter-lamellar spaces. There, the barbed setae irritate the goblet cells such that there is an overproduction and accumulation of mucus on the respiratory epithelium. This accumulation of mucus limits dissolved oxygen uptake by the lamellae. At concentrations of approximately 5 cells of harmful Chaetoceros spp. ml-1 seawater or greater, dissolved oxygen uptake may be sufficiently limited that salmonid mortalities occur. At sub-lethal concentrations of between approximately 0.40 and 5 cells of harmful Chaetoceros spp. ml-1, chinook (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) cultured in sea-water net pens displayed increased mortality rates to vibriosis and/or bacterial kidney disease. Under laboratory conditions, the addition of the bacterial pathogen, Vibrio anguillarum, to the surrounding water greatly accelerated the mortality rate of coho salmon exposed to lethal concentrations of C. concavicornis. These results support the hypothesis that sub-lethal concentrations of Chaetoceros spp. may sufficiently disrupt the salmonids' physiology such that they express a disease. The disease would be one to which the salmonid is most prone at the time of exposure to the sub-lethal concentration of harmful Chaetoceros spp.