136 SEA FISHERIES 



61 about the year 1535.* Between February and April 

 many fish still in the fry stage of development come up 

 from the open sea towards the lagoons. They remain there 

 for some time ; then from various causes they make ready 

 to leave, principally because of the diminishing tempera- 

 ture and the necessity of spawning. But at this moment 

 the sluices of the dams are shut, and the fish remain 

 prisoners. The weirs were at first made of bundles of 

 reeds, then of banks of beaten earth ; now proper dams 

 are constructed, and "to drive the shoals of fish in a 

 given direction," writes Dr. Schmarda, " or into a given 

 canal, the water is removed by opening sluices made for 

 that purpose, as the fish always swim against the current." 

 Another condition, equally indispensable, is the presence 

 of brackish water in the lagoons, due to the presence of 

 a river, stream, or canal, the odour of fresh water attract- 

 ing the fry. 



One of the most celebrated of these lagoons is that of 

 Comacchio, situated between the mouths of the Po to the 

 north and the city of Ravenna to the south. It is closed 

 by a dike which has four openings communicating with 

 the sea. It is divided into 15 artificial basins, each com- 

 municating on one hand with the fresh water brought 

 from two branches of the delta of the Po, the Volano 

 and the Reno, and on the other hand with the salt water. 

 There are 20 sluices and 80 canals. By means of these 

 arrangements two opposite currents can be induced 

 a current running from the lagoon into the sea, which 



1 The Adriatic is traversed by a current which commences in the 

 Archipelago, follows the coast of Greece and the whole eastern 

 shore, makes the circuit of the Gulf of Venice, and descends along 

 the Italian coast. Its speed amounts to rather less than a third of 

 a mile an hour. The density of Adriatic water is 1*0291 and the 

 salinity 29' 122. The average height of spring tides is 23*8 inches. 



