Recent research has indicated that sequential exposure to 

 two different toxicants may predispose test fish to reduced 

 tolerance of the second toxicant. Dugan (1967) found that 

 exposure to sublethal concentrations of anionic detergents made 

 test goldfish less tolerant when exposed later to the pesticides, 

 Dieldrin and DDT. 



The importance of clilorinated aromatic compounds as toxicants 

 takes on additional significance in light of recent work by 

 Ingols et al. (1966) who found that chlorination of sewage to 

 reduce biochemical oxygen demand (BOD) or odor may produce highly 

 toxic chlorinated compounds which are not easily oxidized by 

 bacteria. Under these circumstances it is possible that chlorinated 

 catechols or other chlorinated aromatic compounds may be formed 

 during chlorination and discharged to waterways. 



The foregoing discussion indicates that the toxicity results 

 reported herein for chlorinated catechols and chlorinated phenols 

 must be applied with care. Caution is especxally necessary since 

 pesticides, and chlorinated organic s from sewage, pulp mills and 

 other industrial operations may coexist or occur in sequence at 

 various times in a watercourse. Therefore, consideration must be 

 given to the combined acute and sublethal effects of toxicants, 

 whatever the various sources, which may exist simultaneously or 

 in sequence in a body of water. 



COiM'GLUSIONS 



1. Young pink salmon had r.bout 35 per cent of the resistance 

 of young sockeye to tetrachlorocatechol. Young pink and sockcyc 

 salmon were about equally tolerant of dichlorocatechol. 



2. Advanced sockeye alevins were more tolerant of 

 tetrachlorocatechol than were freslily hatched alevins, fry or smelts. 



3. Thirty-one day exposure to di- and tetraclalorocatechol did 

 not substantially increase acute toxicity. 



