Acute mortalities and sublethal effects on salmon appetence of a phytoplankton bloom dominated by Chaetoceros wighami and a small flagellate species were measured on four salmon farms. The oxygen saturation remained within ambient summer ranges, but Secchi disc readings declined from 9 to 3 m over 7 days. High numbers of C. wighami 4.6 × 105cells 1-1and a small flagellate 1.52 × 106cells 1-1were recorded. Fish were inappetant and lethargic in all four farms, with some fish showing respiratory distress. Mortality occurred asynchronously in farms, from 0.45% to 4.39% of stock, with a total loss of 13,732 fish (44 tonnes). Gills showed severe necrosis with focal hyperplasia and oedematous separation of epithelia. The livers were severely congested with large focal necroses and also necrosis of the gut epithelia. Packed cell volumes and blood chemistry were normal, but urea was elevated. Appetite declined and food intake dropped to 20-79% of guide (0.12-0.55% of body weight). The economic cost was a loss of 170 tonnes of production worth £408,000. Actions to reduce the severest effects of plankton blooms and frequent disruption to appetence are discussed. © 2003 Elsevier Science B.V. All rights reserved.
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