68 
the river opposite, viz., 89.2 to 97.4 per cent. Such a decrease, on a base . 
of only 150 to 400 pounds per acre as of 1915, evidently leaves a much 
smaller residue in the lakes than remains in the river of the enormously 
greater poundages that were there in and before 1915. Average total- 
poundages in these four lakes, with the exceptions mentioned, in fact 
amounted to only 12 to 16 pounds per acre in 1920 as compared with 
40 pounds and 87 pounds in the channel and 4—7-foot zone of the river 
opposite, and with 5,180 pounds and 2,122 pounds in the corresponding 
river zones in the iormer period. Compared with this enormous falling 
off in yields in the three all-bottom-land lakes and the stagnant portion of 
Quiver, the decrease in poundage of all bottom-animals in the mud-sand- 
and-shell bottom of the Quiver Lake channel amounted to only 65 per 
cent. since 1915, leaving an average of 280 pounds per acre in September, 
1920. As in the river just above Havana, the great part of the decline 
in lake yields of all bottom-animals since 1914—1915 has resulted from 
the destruction of the snails, the percentage of total-weight valuations 
contributed by the small Mollusca dropping during the five or six-year 
period from 97.2 to 33.3 per cent. in Thompson Lake, and in the other 
three lakes, excepting the sandy portions, from 81—93 per cent. to none 
at all. 
ALL SMALL BotroM-ANIMALS, LAKES NORTH OF HAVANA, 
1914—15 anp 1920 
1. Average total valuation, pourids per acre, and percentage by 
weight of average total hauls contributed by snails 
I. Mud bottom, 2 to 6 feet 
Per cent. snails, 
Pounds per acre Per cent. iyi welche 
| decrease, 
191415 
1914—15 1920 to 1920 1914—15 1920 
Liverpool L.* 149 16 3924) 81.8 1 
: ; : : 
Thompson L.* 472 | 12 97.4 97.2 Tepe Sie) 
Dogfish L.+ 397 14 96.7 =! 89.4 0 
Quiver L.+ 388 14 96.3 93.6 0 
(Continued on next page) 
