232 THE SEA FISHERIES 



average from the Sargasso Sea. Similar methods of fishing in 

 tropical and Arctic waters yield strikingly different results. The 

 hauls made in the Bay of Bengal by the Government trawler 

 Golden Crown,''- were far inferior in quantity to those of similar 

 vessels working in the cold waters of Barents Sea, though in both 

 cases virgin grounds were being exploited. What is the reason for 

 this difference ? 



Briefly it is to be sought for in the destruction, or rather the 

 reduction, of nitrogen compounds which goes on in tropic seas. 

 The ultimate source of the nitrogen of organisms is atmospheric 

 air. In natvire the inorganic compounds containing nitrogen are 

 present in three forms, ammonia, nitrates and nitrites. No plant 

 can develop without inorganic nitrogen compounds, and as all 

 animal life is dependent on plants, it follows that all Ufe on the 

 earth or in the sea is ultimately dependent on the presence of these 

 nitrogen compounds. To trace their circulation is, therefore, of 

 some interest. By the action of atmospheric agents the nitrogen 

 compounds of the land are gradually being washed out of the soil, 

 and by means of the rivers into the sea. The replenishment of the 

 soil is carried out by the fixation of nitrogen from the air by means 

 of nitrifying bacteria, 



TiSitr-ve in the sea bacteria which perform the reverse operation, 

 that is, reduce the nitrates, nitrites and ammonia and set free 

 nitrogen from these compounds, these are called denitrifying 

 bacteria. i^ 



Now bacteria flourish best at a certain temperature. In the case 

 of these denitrifying bacteria, the temperature of tropical seas 

 suits them better than that of the colder seas, and consequently 

 the destruction of nitrogen compounds goes on to a much greater 

 extent in the former waters. It is certain that but for the action 

 of these denitrifying bacteria, the waters of the ocean would long 

 ago have been poisoned by excess of nitrogen salts derived from the 

 waste of the land. As a practical instance consider the enormous 

 amount of sewage which is annually turned into the seas around 

 our coasts, most of it in an absolutely crude condition. What 

 happens to this sewage ? 



It is seized upon by these bacteria, and nitrogen compounds are 

 prepared from it. The nitrogen compounds are used up by the 

 planktonic plants, such as the diatoms, these in turn are devoured 

 by the Crustacea of the plankton, such as the Copepoda. The 

 Copepoda are consumed by the herring, and the herring by man. 



1 See Records of the Indian Museum. Vol. VII, 1912. " Observations on the 

 shallow-water fauna of the Bay of Bengal made on the Bengal Fisheries steam- 

 trawler Golden Crown, 1908-9," by J. T. Jenkins. 



