322 VICTOR E. SHELFORD AND EDWIN B. POWERS. 



4- Summary. 



We note that on the whole the presence of a quantity of 

 carbon dioxide in the water affected the fishes less than a 

 smaller amount of hydrogen sulfide. The combination of hydro- 

 gen sulfide and carbon dioxide was most rapidly fatal. Since 

 decomposition yields CO2 and consumes oxygen and is accom- 

 panied by the production of hydrogen sulfide which is also accom- 

 panied by the consumption of oxygen, it is reasonable to suppose 

 that on a bottom from which vegetation is absent and decom- 

 position actively takes place a fatal combination of lack of 

 oxygen, and presence of hydrogen sulfide and probably carbon 

 dioxide can develop quickly. 



Considering the fishes tested we note that the herrings were 

 most sensitive. They were sharply marked off from the bottom 

 species which are^ resistant to a marked degree. This resistance 

 is in a very general way associated with the habitat preference 

 of the species. Still the marked resistance of the small cottid 

 is not quite explicable on this or any other basis. 



The importance of factors which kill fishes is greatest in the 

 early stages for two reasons. First the small size of the eggs 

 and embryos makes the ratio between volume and surface 

 smallest and thus any substance in solution will reach all parts 

 of the organism at a more rapid rate. Secondly the inability 

 of the eggs and embryos to move about makes them the easy 

 victims of any adverse conditions that may occur. The eggs 

 of the herring are deposited on the bottom. Nelson mentions 

 rocks only (Marsh and Cobb, '10, p. 46) and rocks are usually 

 swept fairly clear of organic matter and the water well aerated 

 down to the depth of one fathom where the fishes breed. If this 

 means that sandy bottoms of bays are avoided it probably means 

 the avoidance, during the breeding, of water high in hydrogen 

 sulfide (see table) which would be fatal to the eggs and small herring 

 fry to a greater degree than to those studied, which were 6 cm. 

 long. Sensitiveness to hydrogen sulfide is a matter of much 

 importance from the standpoint of the suitability of a given arm 

 of the sea for herring and the influence upon fishes of contamina- 

 tion of the shores with refuse from the land. 



Carbon dioxide is not high in such shallow water on account 



