44 



Besides the three main causes which periodically 

 entail heavy losses on the valliculturists of Comacchio, 

 there are several others which are either subsidiary and 

 intensifying in their influence or which may be limited 

 in their evil influence to certain sections of the lagoon. 

 Of those latter Dr. E. Feletti mentions* an instance of 

 one valle (Trebba) where percolation through the 

 embankment dividing it from reclaimed land lying at a 

 lower horizon, dangerously reduced the level of the water 

 within, so producing a strong indraft of water from the 

 maritime canal, which had the effect of causing a stam- 

 pede of fishes into the labyrinths. Another cause of loss 

 he traces to the liberation of poisonous gas (sulphuretted 

 hydrogen) from the bed of the various chambers of the 

 labyrinths, when from any cause the fish crowd in in 

 excessive multitudes. Probably this is more or less true 

 and the remedy is obvious that these passages should be 

 cleaned out periodically and mud foul with accumulated 

 organic matter removed. I would point out however 

 that the mere disturbance of a mud bottom, apart 

 altogether from any liberation of asphyxiating gases, is 

 enough to kill many species of fish, mullet in particular. 

 And whenever the labyrinths are choked with fish jostling 

 one another and burrowing excitedly in the mud of the 

 passages, this danger must arise. The factor of poison- 

 ous gas will certainly also be present under such 

 circumstances, and this in combination with mud in 

 suspension may cause a great proportion of the fish to 

 die before it is possible to remove them from the 

 pounds. 



Floods may be guarded against by raising and 

 strengthening the protecting embankments, and loss by 

 percolation implies faulty workmanship. Frost and 

 blizzards aQ-ain are to be fouo-ht and their ill effects 

 minimized by recourse to the device of excavating deep 

 pits and channels in suitable places in the ponds, 

 whereinto the fish may retreat when necessary. Bullo f 

 mentions that these refuge-canals {canale raccoglitore) 

 were first employed in 1863 and that experience has 



* " Intonio all attuale salsedine della acque del Campo Trebba^ ed alia motia 

 tli anguille nelle sue Covole " ; Comacchio, 1909. 

 f Lot. <//., p. 406. 



