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I 18 25 25 27 27 FRED P. MEYER AND ROGER L. HERMAN 24. Normal seasonal change in water temperature ................ Tempera- ture falls below or exceeds thermal toler- ance-e.g., die-off of threadfin shad in cold weather; kill usually restricted to one species Species selectivity evident .............. 26 No species selectivity evident ............. ........ Very high level of a toxic substance 26. Lesions evident on fish ............. 27 26. No lesions on fish .................... .............. Low toxicity or low con- centration of toxic substance (see also 23) Organisms in lesions visible to naked eye. .28 No organisms visible .................. 29 28. Organisms wormlike, attached to external surface of fish ............... ......... Leeches (not a cause of death) 28. Organisms resemble copepods or have jointed body parts ....... Parasitic copepods or isopods (known to kill fish) 29. Lesions not hemorrhagic ............... 30 29. Lesions hemorrhagic ..................... ........... Possible bacterial or viral cause 30. Lesions as small discrete bodies or masses in tissues .................. 31 30. Lesions appear as gray, yellow, or white areas on body .................. ............. Bacterial or fungal cause 31. Lesion or mass filled with cellular material. . ........ Cysts caused by sporozoans, proto- zoans (such as Ichthyophthirius), or helminths 31. Lesion or mass filled with gas .......... 32 32. Bubbles of gas present in gills, fins, and behind eyes ............ Gas bubble disease, due to supersaturation with a gas 32. Odorous gas in large bubbles in necrotic lesions ......................... Bac- terial disease caused by Edwardsiella tarda Chronic exposure to sub- lethal levels of contami- nants may lead to tumors or other adverse effects in surviving fish. Public con- cern is heightened when melanomas, papillomas, and other anomalies, such as those on this black bull- head, are seen on fish.