98 Pub. Puget Sound Biol. Sta. Vol. 2, No. 39 



suppose that some similar form of recovery and treatment will be installed 

 for all of our cities and towns, large and small, in the near future. 



Following a series of experiments on herring performed at the Puget 

 Sound Biological Station in 1914, the writer and Dr. Powers (Shelford 

 & Powers, 1915) concluded that the historical migration of the European 

 herring which had such important economic results as the decline of the 

 Baltic towns of the Hanseatic League, may have been due to the intro- 

 duction of sewage and refuse which caused an increase in the hydrogen ion 

 concentration of sea-water. It was found at that time that the move- 

 ments of these fishes were governed largely by differences in hydrogen ion 

 concentration ; slight increases in hydrogen ion concentrations as com- 

 pared with normal sea-water being uniformly avoided by both herring ' 

 and salmon. Sewage contamination of the sea results in the development 

 of an increase in free carbon dioxide and hydrogen sulphide and the 

 consequent increase in hydrogen ion concentration. This has undoubtedly 

 not only kept important fishes from running on our shores but has also 

 probably decreased the number of many useful animals. The question 

 as to whether this new process with an acid residue will be beneficial or 

 detrimental is one to be determined by a study of the effects of acids 

 on useful fishes and other organisms particularly during the more sensitive 

 periods. In taking up such a study it is not possible to investigate all 

 the various organisms involved or to investigate many of them perhaps 

 during the most sensitive stage. The following points must be kept in 

 mind in such an investigation (Shelford, 1918b): 



1. Animals differ in their resistance to poisons;— is the animal used 

 in making tests one of representative sensitiveness ? 



2. Different stages in the life history of a given species differ in 

 their resistance to poisons ; — what is the most sensitive stage in the life 

 histories of important animals ? 



3. The condition of the sea-water differs at different times of the 

 year; — at what period will the addition of poisonous substances have 

 greatest effect? 



4. Many animals, particularly fishes, recognize dangerous substances 

 and turn back, thus avoiding localities where such substances are found ; — 

 what is the reaction of important food animals to the poisons in question 

 and what concentrations can they distinguish .'' 



5. Effluents often may easily be treated so as greatly to reduce the 

 toxicity; — what methods of treatment will render acids added to sea- 

 water harmless? 



It is not readily possible to conduct an investigation in an ideal 

 manner with reference to all these questions, but they have been kept in 

 mind throughout the work herein described. 



