498 LIFE IN THE OCEAN. 



try two different ways of ascertainino- the yield. We can weigh the 

 fish taken from a pond, and thus determine the quantity of useful flesh 

 produced per acre per year, or we can find the quantity of organic 

 matter produced in the form of plants in a given body of water in a 

 year. The values found for the yield in flesh and for the production 

 of nutritive substances must have a certain ratio which may be ascer- 

 tained by chemical bacteriological and physical study of the body of 

 water in question from the point of view of its capacity of yield. The 

 product of fish in flesh can be directly determined only in ponds that 

 can be emptied and from which all the fish they contain can be taken. 

 Susta gives extended instructions for doing this in his interesting work 

 on feeding carp. The worst ponds give nearly 11 pounds of carp per 

 year to the acre; but large ponds generally give three or four times as 

 much, while the yield of the small ones is six times as much. Village 

 ponds into which flows manure liquor from farms give a yield running 

 up to twenty times the first number. 



The observations made upon the exploitation of ponds furnish some 

 information concerning the causes of the variations of production. 

 Those ponds into which flow either the water from manured land or 

 the drainage of villages are always better stocked and give a better 

 yield. The introduction of nitrogen compounds has thus here, as in 

 the case of the soil, the effect of augmenting the yield very remark- 

 ably. It has also been found that the yield can be much increased by 

 giving the fish food rich in nitrogen (grains of lupine, etc.). This food 

 is not in all cases directly assimilated. It seems, on the contrary, that 

 it is first taken bj^ larv» of gnats, worms, insects, etc., which after- 

 wards become the prey of the fish, or else that the food, b}^ the inter- 

 vention of certain bacteria undergo a decomposition having the effect 

 of rendering them assimilable bj^ the plants of the pond, these plants 

 being then eaten bj^ small animals which, mixed with microscopic 

 plants, serve in their turn as food for the carp. In any case, experi- 

 ence shows that the conditions of production in a small body of water 

 are by no means unfavorably affected by the presence of manure, but 

 that on the contrary the}^ may be greatly improved in this wa3^ 



The simple and sure process of directly ascertaining the production 

 of fish is, of course, no longer applicable when we come to large lakes 

 or to the sea, because it is then no longer possible to take out all the 

 fish. We have, then, to fall back on the best statistics. According to 

 the catch of the fishermen, Hensen has estimated the annual yield of 

 the Bay of Hela (meaning, presumably, the Putziger Wiek, in the 

 Gulf of Dantzig) at 28.2 pounds per acre (31.6 kilos de poisson pour 

 1 hectare de surface en eau). I have calculated in another way that 

 in the "Haff," at Stettin (I'anse de Stettin), the catch of fish amounts 

 to 90 pounds per acre per year (100 kilos par hectare et par an). On 

 the other hand, Heincke sets down the annual value of the products 



