to maintain the recreational and aesthetic values of these forest areas 

 (USDA, 1975). However, streams in forest areas also support important 

 fisheries, particularly trout. Some watersheds of the northeast are 

 drained by major spawning and nursery streams for brook trout [SalvzLLniU 

 fioyitinaZLu) and Atlantic salmon {Salmo i>atoA) . Therefore, the Fish- 

 Pesticide Research Laboratory of the U.S. Fish and Wildlife Service is 

 cooperating with the U.S. Forest Service in investigating the potential 

 toxicological effects of candidate forest insecticides on fish and 

 aquatic invertebrates. The investigations were designed to evaluate 

 changes in toxicity due to various water qualities associated with bio- 

 geographic regions. The investigations also included the toxicity of the 

 candidate insecticides to aquatic invertebrates, to early life stages of 

 brook trout, the possible toxic interaction of insecticide combinations, 

 and the susceptibility of fish containing residues of DDT, or a poly- 

 chlorinated biphenyl (PCB; Aroclor 1254). This paper reports progress in 

 our cooperative research. 



MATERIALS AND METHODS 



Six experimental forest insecticides, Sumithion , carbaryl , Dylox , 

 MatacilQy, Dimi lin® (TH-5U40), and Orthene©were provided by various 

 chemical companies. Stock solutions of the candidate insecticides 

 (technical grade) in reagent grade acetone were prepared immediately be- 

 fore each static test. Stocks of field formulations were prepared by 

 dilution with distilled water. 



Test waters of different chemical characteristics were formulated from 

 deionized water of at least 1 million ohms resistivity by adding reagent 

 grade salts (Marking, 1969). Mineral acids and bases were used to buffer, 

 adjust, and maintain pH (Marking and Dawson, 1972). Various test tempera- 

 tures were controlled by water baths. 



Fish obtained from Federal and State hatcheries were maintained for 2 

 weeks under standard fish cultural care (Brauhn and Schoettger, 1975). 

 They were acclimated to test conditions of temperature and water quality 

 before the experiments and subsequently transferred to test containers 

 about 24 hours before addition of the toxicant (Committee on Methods for 

 Toxicity Tests with Aquatic Organisms, 1975). Fish used in this investi- 

 gation were brook trout and Atlantic salmon. Mature scuds [GammaAuu. 

 p6zu.dotimyia.uu,) and late instar naiads of a stonefly [VoAzAoviaAaiji, 

 catLfiofiyU-ca) were used in the toxicity tests for invertebrates. The in- 

 vertebrates were obtained from wild populations in streams and maintained 

 in the laboratory (Committee on Methods for Toxicity Tests with Aquatic 

 Organisms, 1975). 



Toxicity data were analyzed with the statistical method described by 

 Litchfield and Wilcoxon (1949) to determine the LC50 (concentration pro- 

 ducing 50% mortality) and 95% confidence interval. Estimations of toxic 

 chemical interactions were made with a modification of the methods 

 developed by Marking and Dawson (1975). With this method, toxicities of 

 paired insecticide mixtures were determined in the manner of the indi- 



13 



