SECTION 4 



ENDPOINTS IN BIOASSAY 



Lloyd L. Smith, Jr. 



INTRODUCTION 



Bioassay with fish or invertebrates has long been used to determine the 

 suitability of water for aquatic life and the toxicity or deleterious 

 effects of industrial and domestic sewage effluents on aquatic habitat. 

 Various techniques, exposure times, and definitive endpoints of tests to 

 describe effects have been employed. An endpoint in a bioassay is defined 

 for our discussion as a physiological or behavioral response to a specific 

 concentration of a toxicant after a definite period of exposure. In order 

 that data provided on a worldwide basis can be compared and similar conclu- 

 sions derived from similar values, it is essential that a clear understand- 

 ing of the usefulness of various endpoints or indicators of adverse effects 

 be developed. 



The purpose of defining specific endpoints for bioassays is to secure 

 values in terms of milligrams per liter or degrees C which can be trans- 

 lated into an adequate assessment of the effects of a toxicant or effluent 

 on fish or other aquatic species. In order that the value designated for 

 such an endpoint have broad usefulness, it must first have a predictive 

 capability for either acute or long-term effects of a toxicant or effluent 

 on fish and invertebrate populations; second, it must permit comparisons of 

 effects between different species and between different toxicants or other 

 deleterious materials; third, it must be a practical endpoint to observe 

 without unnecessary laboratory sophistication or excessively time-consuming 

 analytical procedures; and fourth, it must be reported in a form that will 

 permit comparison between laboratory and field data. Finally, an endpoint 

 must be selected which is applicable to the particular problem. Unfortun- 

 ately, much bioassay information in the world literature has not been based 

 on careful attention to the points enumerated above. The purpose of this 

 discussion is to elaborate on means of defining meaningful endpoints and 

 interpreting the resultant findings. 



TYPES OF ENDPOINTS 



Four broad types of endpoints have been employed by various investiga- 

 tors at various times. They are: (1) endpoints indicating acute toxicity 

 and resulting in death of the test organism in short-term; (2) endpoints 



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