LOW TOXICITY OF THE BLACK FLY LARVICIDE BACILLUS-THURINGIENSIS VAR ISRAELENSIS TO EARLY STAGES OF BROOK TROUT (SALVELINUS-FONTINALIS), BROWN TROUT (SALMO-TRUTTA), AND STEELHEAD TROUT (ONCORHYNCHUS-MYKISS) FOLLOWING DIRECT AND INDIRECT EXPOSURE

Mortality of brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), and steelhead trout (Oncorhynchus mykiss) from eyed embryo to 82 mm fork length, exposed to Bacillus thuringiensis var. israelensis (B.t.i.) in the laboratory increased when dosages exceeded recommended rates by 12 000x or more. There was generally no toxicity difference between denatured (autoclaved) and nondenatured B.t.i. for all three trout species at all B.t.i. concentrations tested, indicating that mortality was due to formulation components and not B.t.i. toxin. The 48-h LC(50) values for brown and brook trout alevins ranged from 1561 to 2321 ppm for both denatured and nondenatured B.t.i.. Scanning electron micrographs showed particle and mucus accumulation on gill surfaces from fish exposed to 2000 ppm B.t.i. for 4 h. Oxygen and carbon dioxide levels in blood from exposed (4000 ppm B.t.i. for 4 h) and unexposed fish were similar. Brown trout of 43 mm fork length fed excess B.t.i.-killed or live black flies (Cnephia dacotensis) ate similar quantities of each larval type, and both groups experienced similar mortalities and growth rates at 30-d postexposure.