The 96-hr median tolerance limit (96-hr XL ) ^ representing the 



concentration of a toxicant which will kill 50 per cent of the fish 



in 96 hr, was used as a measure of acute toxicity. In order to 



determine 96-hr TL , several concentrations of the substance being 



tested were prepared using Cultus Lake water, test fish were added, 



and mortalities removed each 2L, hr. The 96-hr TL was determined by 



m '^ 



plotting per cent concentration versus per cent survivors after 96 hr 

 on semilogarithmic paper and drawing a straight line between two 

 points which had survivals greater than 50 per cent, and less than 

 50 per cent, respectively. The concentration which corresponded to 

 50 per cent survival on this line was the median tolerance limit 

 (Henderson and Tarzwell, 1957) . 



Fish were bioassayed at the temperature of the water from which 

 they were taken. Temperature was controlled by standing the aquaria 

 in constant-flox,/ water baths. Smelts were hela overnight, following 

 capture, in aquaria before use in a bioassay. Fry and smolts were 

 not fed during or for 2U hr before a bioassay, All control fish 

 survived short-term bioassays in good condition. 



Thirty-one Day lilxposure 



Flowing-water bioassays with sockeye alevins and fry were 

 performed using concentrations of 0.6, 0.2, 0,1, 0.05 ppm 

 tetrachlorocatechol and 0.2 ppm dichlorocatechol, respectively, in 

 five experimental troughs plus three control troughs. Troughs were 

 covered to exclude light during the alevin stage but were uncovered 

 as the fish advanced to the "swim-up" fry stage. The troughs 

 measured 59.5 in. by 5.25 in. by 7 in. deep with a water detention 

 time of 1.6 hr. In each trough, fish were held in four plastic 

 screen baskets measuring 6 in, by 5.25 in. by 6 in. deep. The first 

 three baskets per trough, each containing 120 fish of identical age 

 and origin, were considered a single group (Group 1). The fourth 

 basket, Group 2, contained 1^0 fish which were less developed than 

 those in Group 1. Temperatures were maintained at 45°F throughout 

 the experiment. 



