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Dichotomous Key for Fish Kill Investigations After the initial visual inspection of the scene, an investigator can sometimes make preliminary as- sumptions about the cause of a fish kill. By using a process of elimination based on the evidence at hand, certain types of causes may be highly unlike- ly. A dichotomous key is provided below as an ex- ample of how the thought process might proceed. This key is offered as a tool-not as a definitive reference-for assessing fish kills. Opportunities to use the key to help reach a presumptive conclusion concerning the cause of a fish kill are provided in Chapter 13. Seven case histories are described to help potential investigators test their skill in evalu- ating the information that became available during the on-site investigation. Although the thought process would be the same for ponds, lakes, streams, and estuaries, most of the examples used in prepar- ing the key were taken from data on fish kills in ponds. In streams, where evidence at the site may be transitory because of the flow, the investigator may have to check downstream to attempt to reconstruct the scene. 1. Kill occurred in less than 24 hours ....... 2 1. Not known when kill occurred, or kill continued for longer than 24 hours ...... 16 2. Kill occurred between midnight and sunrise ........................... 3 2. Kill occurred at times other than between midnight and sunrise ....... 8 3. Water dark in color, musty odor, or odor of sour cabbage ....................... 4 3. Water conditions normal in color and odor ................................ 6 4. Some fish alive .................... 5 4. All fish dead ...................... 16 5. Large fish dead, some small fish alive .... 6 5. Small fish dead, some large fish alive .... 18 6. Dissolved oxygen less than 2 ppm .... 7 6. Dissolved oxygen 2 ppm or more .... 9 7. Algal cells absent or dead if present ..... 8 7. Algal cells present and alive ............ 10 8. Dead algal cells abundant ........... .. .... Oxygen depletion due to enrichment 8. Algal cells absent .................. .. Oxygen depletion due to algicidal substance INTERPRETING THE SCENE 17 9. Kill occurred between 9:00 a.m. and 5:00 p.m .............................10 9. Kill occurred at other times as well ...... 23 10. pH above 9.0 ..................... 11 10. pH not above 9.0 .................. 14 11. Dissolved oxygen high, often saturated, or near saturation ..................... 12 11. Dissolved oxygen low or near normal for water temperature recorded ............ 13 12. Heavy bloom of one or more species of blue-green algae ... Toxic algal bloom 12. Heavy bloom of dinoflagellate algae .................... Toxic algal bloom 13. Vegetation dead (appears burned) ....... 14 13. Vegetation normal .................... 15 14. Ammonia levels not high, near zero .. 15 14. Ammonia levels high ................. ............. Anhydrous ammonia spill 15. pH 6.0 to 7.0 ............ Oxygen depletion 15. pH below 6.0 ..... Possible lethal low pH or heavy metal poisoning; possible mine drainage 16. Some fish still alive ................ 17 16. All fish dead ...................... 23 17. Kill size selective ..................... 18 17. Kill not size selective .................. 25 18. Some small fish alive, large fish dead ............................. 6 18. Small fish dead, some large fish alive .............................19 19. Zooplankton and insects alive ........... 7 19. Zooplankton and insects dead ........... 20 20. Algal cells alive ................... 21 20. Algal cells dead or absent ............. ............ Toxic herbicidal substance 21. Fish showing convulsive or aberrant behavior .............................22 21. Fish seemingly normal ................. 24 22. Fins in normal position ............. 23 22. Pectoral fins of fish thrust to extreme forward position ..................... ............ Organophosphate pesticide 23. Kill occurred throughout day .............. ...................... Pesticide poisoning 23. Kill occurred between 9:00 a.m. and 5:00 p.m..... Toxic algal bloom (see also 11) 24. Recent temporary major change in water temperature ................... ........ Temperature kill (as from shut- down of thermal power generating plant or plant exceeding the allowed AT in discharge) I