328 American Fisheries Society 



mon make their spawning ascent of the rivers. Also the pres- 

 ence of sulphuretted hydrogen gas from the decomposition of 

 organic matter is considered by Shelford as explaining the 

 abandonment by the herring of certain of their former haunts. 

 The influence of these gases can be readily understood, seeing 

 that they are present in air and affect man. Although in the 

 atmosphere differences in their abundance are usually slight, 

 very local, and temporary in nature owing to the equalizing 

 effect of the vigorous air currents, nevertheless in the hydro- 

 sphere they vary considerably in abundance, and such differ- 

 ences are maintained for a considerable time, the currents being 

 comparatively weak. 



Quite different from anything to be found in air are the 

 conditions of alkalinity and acidity which characterize water. 

 They are opposed conditions and between them is a more or 

 less definite neutral ground or point. Water dissociates into 

 hydrogen and hydroxyl (hydrogen plus oxygen) ions. When 

 the former ions are in excess, as when furnished by some dis- 

 solved acid which dissociates in solution giving hydrogen but 

 not hydroxyl ions, the water is said to be acid in reaction; 

 when the hydroxyl ions are in excess, as when furnished by 

 some dissolved and hence dissociated alkali, it is said to be 

 alkaline in reaction ; and when both kinds are in equal amount, 

 it is said to be neutral. The importance of these conditions in 

 their effect on the development of the life in the water has only 

 recently come to be recognized. 



Currents, as we have already indicated, have a very definite 

 effect upon the climate. Their action tends to eliminate dif- 

 ferences, but they are not so uniformly present as to have the 

 general equalizing effect they have in the air. In certain places 

 or at certain seasons there will be equalization of conditions 

 and not at others, depending upon the presence or absence of 

 the currents. To illustrate their effect we may instance the 

 influence of the Gulf Stream on both the water and the air 

 climates of the coast of Europe, and also the mixing of surface 

 and deeper water that occurs in the autumn when the surface 



