gill surface may debilitate fish, but no supporting data were given. Many 

 species of freshwater fish can survive for several weeks in highly turbid 

 conditions (European Inland Fisheries Advisory Commission, 1964), indica- 

 ting that compensatory reactions may enable fish to survive despite gill 

 damage. Randall (1970) pointed out that shunt mechanisms are commonly 

 used by fish so that not all of the gill surface is used for respiration. 

 By using the "reserve" surface area, fish may have sufficient functional, 

 but damaged, gas exchange surface to survive prolonged exposure to sus- 

 pended solids. The functional decrease in gill surface area caused by 

 suspended solids also may be offset by compensatory increases in the gas 

 exchange capacity of the blood (Section II). 



IV. EFFECTS OF SUBLETHAL CONCENTRATIONS OF FULLER'S EARTH ON 

 CARBOHYDRATE METABOLISM IN THE HOGCHOKER 



1. Introduction . 



Fish livers contain large quantities of carbohydrate stored as animal 

 starch or glycogen. During periods of starvation or stress increased 

 metabolic demands for energy are met by breaking down liver glycogen into 

 glucose and releasing it into the blood. This section presents the results 

 of experiments determining the rate of glycogen utilization in the hogchoker 

 during exposure to sublethal concentrations of fuller's earth. 



2. Methods. 



The glycogen content in liver samples from hogchokers was determined 

 after the fish had been held for 5 days in either control conditions or 

 in suspensions of 1.24 g l"-^ fuller's earth. Glycogen was extracted from 

 liver tissue by boiling the tissue in 30-percent potassium hydroxide (KOH) , 

 followed by precipitation with 95-percent ethanol (Good, Kramer, and 

 Somogyi, 1933). Quantitative estimates of glycogen concentration were 

 derived colorimetrically using the phenolsulfuric acid technique 

 (Montgomery, 1957). Liver glycogen concentrations were expressed as milli- 

 grams per 100 milligrams (mg 100 mg"-^) of liver tissue. The results were 

 analyzed statistically using "Student's" t-distribution (Snedecor and 

 Cochran, 1967). 



3. Results and Interpretation . 



Liver glycogen content from freshly caught hogchokers was about 15 to 

 17 mg 100 mg"-'^ (Sherk, O'Connor, and Neumann, 1972). Mean glycogen content 

 in hogchoker livers decreased to 15.17 ± 3.6 mg 100 mg"'^ (Table 10) after 

 5 days in control conditions. Fish held in a suspension of 1.24 g 1"-^ 

 fuller's earth had a liver glycogen content of 10.77 ± 3.2 mg 100 mg"-"-, 

 significantly less than the value determined for control fish (p < 0.01, 

 Table 10). 



Similar studies conducted with white perch and striped bass provided 

 no useful data (App. A). Glycogen mobilization rates in these species 

 were so high that the final liver glycogen concentrations in experimental 

 and control fish were below the limits of the analytical procedure. 



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