Bull. Eur. Ass. Fish Pa tho I. 10 (4), 106, 1990 MORTALI1Y ASSOCIATED WI TH A CYANOBAC TERIAL BLOOM IN FARMED RAINBOW TROUT IN GALICIA (Northwestern, SPAIN) BY A.E. ToRANzo, F. NIETO AND J.L. BARJA Phytoplankton blooms occur in re sponse to a combination of climatic and hydrographic conditions. Important fish losses caused by such blooms have been reported mainly in coastal areas of Cana da (Margolis and Evelyn, 1987), U.K. (Ayres et al., 1982; Bruno et al., 1989) and Norway (Saunders, 1988). However, high levels of ammonia and sulphide, were some of the environmental factors associated with the bloom. Bacteriological and virological analysis were negative. The gill sections from dead fish showed extensive necrosis with separation of the secondary lamellae ep ithelium. Cells of cyanobacteria were ob there are no data about such effects in served in association with the lamellae. fresh water. We report here rapid and significant economic losses of farmed rainbow trout (Oncorhynchus mykiss) at These observations suggest that the mor talities resulted from direct abrasion of tributed to contact with a cyanobacterial the gill structures and therefore, respira tory failure rather than the release of a bloom in North west of Spain. toxin. 1989 Only one type of cyanobacteria was g weight) cultured in present in the water samples and gills. The mortalities occurred in July, among fish (10-12 floating cages in a fresh water reservoir. From a total of 800,000 fish, within 48 h the losses caused by the bloom were ap Following Rippka et al. (1981), it was as signed to the genus Anabaena (Fig. 1 ). Although there are very few options and most of the for safeguarding fish stocks from the af mortalities occurred at night. Reduced fects of blooms, in situ monitoring of the oxygen environmental proximately (2 7°C) 600,000 levels, elevated temperature and organic matter together with conditions can aid alerting fish farmers of their occurence. in Bull. Eur. Ass. Fish Pa tho I. 10 (4 ), 107, 1990 Rl'fere11ccs .'\yn:s. P.A . . Seaton. D.D. and Tctt, P.B. ( 1982). l'la11kton b l onrn .s or economic importance to fishc1 ics i11 U I< water s 1968-1972. Internation al (\1uncil lor the E .x p lo rn t i on of the Sea ( ICl:-.S) CM I lJS2iL:38, 12pp. llru110. D.W, lkar. (i. and Scatc)n, D.D. (1989). lunalitv «ssociatcd with phytoplankton blooms anwng f:nrned Atlantic salmon, Sal mo .salar I. . . i11,'cotla11d. Aquaculture 78, 217-222. MargPlis, l.. and Evelyn, T.P.T. ( 1987). Aspects of disc•asc and parasite problems in cultured sal monicls in Canada with emphasis on the Pacif ic region, the regulatory measures for their control. 111: A. Stcnmark and G. Malmberg (eds.), Parasitt>s and Diseas es in Natural Wa ters and Aquaculture in Nordic Countries. Zoo-Tax Naturhistoriska Riksmuseet, Stock holm, Sweden, pp: 4-19. Rippka, R. Waterbury, J.B. and Stanier, R.Y. ( 1981 ). Provisional generic assignments for cy anobacte1ia in pure cu ltu re . In: M. St ar r, I-!. Stolp, J-l.G. Triiper, A. Balows and H.G. Schle gel (eds.), The Prokaryotes: a han db ook on ha bitats, isolations and identification of bacteria. Springer-Verlag, N.Y. pp: 247-256. Saunders, R.L. (1988). Algal catastrophe in Nor way. World Aquaculture 19, 11-12. Authors' address Departamento de Microbiologfa y Parasitologfa. Facultad de Biologia. Universidad de Santiago. Santiago de Compostela 15706, Spain.